Designing with the HCPL-4100 and HCPL-4200 Current Loop

Designing with the HCPL-4100

and HCPL-4200 Current Loop

Optocouplers

Application Note 1018

PrefaceAgilent Technologies produces acomprehensive line of opto-couplers which address differentspeed and current gain require-ments for isolation interfacecircuits. New Agilent optocouplerproducts build on the establishedbase of optocoupler technologyand expand with additionalfeatures that make optocouplerseasier to use in special applica-tions. The HCPL-4100 (transmit-ter) and the HCPL-4200 (receiver)optocouplers include specializedcircuits for 20 mA digital currentloop applications. Theseoptocouplers are designed toeasily interface TTL, LSTTL, andCMOS logic systems to currentloop systems. An external currentsource is needed to complete thecurrent loop system.

This application note will assistthe circuit design engineer toachieve maximum performancefrom Agilent Technologies HCPL-4100 and HCPL-4200 current loopoptocouplers. Practical applica-tions for interfacing to and from acurrent loop will be shown. Inaddition, overall current loopsystem aspects and current sourcedesigns will be discussed. Poten-tial applications of current loops,configurations, types of currentsources, and inherent tradeoffs

between data rate and length ofwire line are covered.

IntroductionData transmission betweenelectronic equipment, which arephysically separated by distancesof more than a few feet, can beaccomplished in many ways. Wirelinks, microwave links, and fiberoptic links are among the mostcommon means. Each technologyhas inherent trade-offs in datarate, error rate, simplicity of use,reliability and cost. Of the wiretechniques, a signal can betransmitted as a voltage signal ora current signal. Many popularindustry standards exist forvoltage signal transmission, suchas RS-232-C, RS-422, RS-423(serial data transmission), orIEEE-488 (parallel data trans-mission), etc. When compared tothese voltage signal techniques, acurrent signal or current loop canprovide an excellent alternative.

First, some basic definitions are inorder. A current loop is a loopwhich carries a current, generally20 mA or 60 mA, between elec-tronic equipment via a twistedpair of wires. The loop can beopened and closed by a transmit-ter within equipment connected tothe loop. These interruptions of

current are sensed by otherequipment connected to thecurrent loop (receiver). Theconvention used in this applica-tion note for a MARK, or logic 1,corresponds to a presence of loopcurrent, e.g., 20 mA. A SPACE, orlogic 0, corresponds to an absenceof loop current, e.g., 0 mA. Acurrent loop transmitter orreceiver can be either of twotypes: active (source) or passive(sink). An active transmittersupplies current to the loop. Anyreceivers or other transmitterswithin that loop must then bepassive units which accept thesupplied loop current. Alterna-tively, an active receiver suppliescurrent to passive transmitters orother passive receivers in thecurrent loop.

Current sources used within acurrent loop vary in com-plexity. The simplest currentsource is a resistor and voltagesource. More complex currentsources will contain activeelements or special integratedcircuits to provide constantcurrent under various powersupply and load conditions. Thecompliance voltage of a currentsource is a term which is used toindicate at what level the currentdecreases to zero.

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There are no widely acceptedindustry standards for currentloops, only accepted conventions.Common digital current loops are0-20 mA and 0-60 mA, while theanalog current loop is 4-20 mA.These levels were originallyassociated with digital teleprinterequipment and analog industrialsensors such as thermocouples,strain gauges, etc. The HCPL-4100and HCPL-4200 optocouplersaddress only the 20 mA digitalcurrent loop applications.

Basic Advantages of Current

Loops

In voltage based transmissionlines, a voltage signal atten-uates over distance. A current loopdesign ensures constant currentthrough the loop resulting in nosignal attenuation. Noise immunityis higher with current loop sys-tems. A constant current maintainsa high signal-to-coupled noiseratio. The voltage signal attenua-tion reduces this ratio.

Even with expensive, low capaci-tance cable, RS-232-C wire com-munication over distances greaterthan 60 m (200 ft.) at 20 kBd isimpractical. RS-232-C wire dis-tances are severely limited whencompared to current loop dis-tances at the same operating datarate. Current loop systems employinexpensive twisted pair wirecable. Serial data transmission isaccomplished by a simple two-wire arrangement. Multiplestations can be connected in seriesprovided sufficient compliancevoltage is available. Unlike voltagetechniques, if a fault should openthe loop, permanent loop currentdisruption could be sensed.

Optocouplers can be used incurrent loops to provide opticalisolation of the logic circuits fromthe current loop circuit. This

optical isolation prevents groundpotential differences or commonmode influences from affectingthe current loop performance.Optocoupler construction andsophisticated integrated circuitprocessing achieve excellentcommon mode rejection (10 kV/µs). Electrical independencebetween input and output circuitsof an optocoupler permit differentreference grounds on either sideof the interface.

The maximum data rate of a 20mA current loop system is deter-mined by the characteristics of thetransmitter and receiver and thelength and characteristics of thetransmission line. For shorttransmission lines, less than 40 m(131 ft.), the maximum data rate isapproximately 120 kBd for theHCPL-4100 and approximately 4.5MBd for the HCPL-4200.

The small eight pin dual-in-lineplastic package results in asavings of printed circuit boardspace. Standard automatic inser-tion equipment can be used.

The easy design and implementa-tion of a 20 mA current loop usingthe HCPL-4100 and HCPL-4200permits rapid design, fabrication,and testing.

Current Loop

ApplicationsCurrent loops are used becausethey offer communications linklengths up to 10 km, have highimmunity to errors caused bynoise, and are low cost. Previous20 mA current loop designs wereoften limited to 5 or 10 kBd speedseven for short line lengths. AgilentTechnologies 20 mA current looptransmitters and receivers arecapable of 20 kBd at 400 metres.

In the industrial control industry,current loops are used when thereis a need to communicate digitalinformation. Because of theproliferation of the micro-proces-sor and advances in digital cir-cuitry and software, digital controlof industrial processes is becom-ing widely accepted. Micro, mini,and mainframe computers aredistributed throughout the factoryand need to exchange information.The same considerations applywhere distributed data processingequipment is used in a factoryenvironment for inventory controlor workflow monitoring. AgilentTechnologies 20 mA current loopsare a natural choice for noisyenvironments.

For these applications, 20 mAcurrent loops are desirable be-cause they are resistant to errorscaused by noise, they eliminateground loops, can be used incommunications links up to 10 kmlong, are low cost, and can beeasily installed. No other commoncommunications system can equalthese features.

Agilent Technologies

Current Loop

OptocouplersGeneral Design Considerations

with Optocouplers

When ordinary optocouplers areused as the interface betweenlogic systems and a current loop,and vice versa, careful design mustbe followed. The light emittingdiode (LED) in a current loopreceiver must be protected fromcurrent overstress. A thresholdneeds to be established withhysteresis in order to define theMARK and SPACE conditions andto avoid possible noise effects. Thethreshold circuit should be locatedprior to the optical isolation to



3

prevent the current transfer ratio(CTR) changes within anoptocoupler over time frominfluencing established thresholdlevels. This input protection/threshold circuit should bepowered solely by the current loopto maintain isolation and reducepower supply requirements. Thereceiver output circuit should besimple to interface to TTL orCMOS systems. The total currentloop receiver circuit needs tooccupy minimal board spacewithin loop station equipment. Ofcourse, the design must have goodspeed performance in order tomeet ever increasing speedrequirements.

Similar considerations exist forcurrent loop transmitter design.The transmitter will operate fromeither a TTL or CMOS input datasignal. The output of the transmit-ter should be protected andbuffered to handle the loop cur-rent and compliance voltage levelof the current source. Outputprotection needs to be provided toensure against miswired connec-tion of the current loop to thetransmitter terminals. The outputcircuit must be powered by thecurrent loop source. If powerfailure occurs to the transmitter,the transmitter must remain ON inorder to prevent obstruction ofdata on the current loop. Othersubtle effects such as the type ofcurrent source and terminationused will also influence speedperformance of the loop.

These design considerations havebeen incorporated into the HCPL-4100 and HCPL-4200 to simplifythe circuit designer’s task.

Features Common to the

HCPL-4100 (Transmitter) and

the HCPL-4200 (Receiver)

The Agilent Technologies currentloop optocoupler product familyconsists of two devices: a currentloop transmitter (HCPL-4100) anda current loop receiver (HCPL-4200). Their unique characteristicswill be explained by first dis-cussing the features which arecommon to both devices; thenparticular characteristics of eachdevice will be addressed.

Optical isolation is providedwithin each device to permitelectrical independence of thelogic systems from the currentloop system. Electrical indepen-dence specifically means twoconcepts: 1. signal isolationcapability between the twosystems, 2. insulation capabilitybetween the two systems. Signalisolation is the ability of the deviceto reject common mode signalinterference. The common moderejection capability of both theHCPL-4100 and the HCPL-4200 istypically 10 kV/µs with a specifiedminimum of 1000 V/µs. The deviceinsulation capability is the abilityto withstand a transient potentialdifference stress between the twosystems. The Withstand TestVoltage is 3 kV DC for five sec-onds.

Speed performance of each devicehas a typical propagation delay of0.2 /µs - 0.3 µs with a specifiedmaximum of 1.6 µs over 0°C to70°C, excluding transmission linedelay. Depending upon the loca-tion of the current source within aloop, data rates of 1.5 MBd for 250metres loop length is achievedwith the HCPL-4200 receiver. Upto 75 kBd for 100 metres of looplength with the HCPL-4100 trans-mitter is also achieved.

The transmitter input and receiveroutput are compatible with TTL,LSTTL and CMOS logic levels.Passive current loop switching ormonitoring is performed byintegrated circuits (ICs) withineach device which are poweredfrom an external 20 mA currentsource. No additional design forpower supplies or buffer circuitryis needed for either unit. Currentprotection of up to ±30 mA isprovided in each optocoupler.Specific, simple current sourceswhich can be used are explained ina subsequent section.

The HCPL-4100 20 mA currentloop transmitter and HCPL-420020 mA current loop receiverincorporate these features in eightpin dual-in-line plastic packages.The eight pin dual-in-line packageallows for automatic insertioncapability and conserves printedcircuit board space.

Features of the HCPL-4100

Transmitter

The output integrated circuit ofthe HCPL-4100 transmitter directlycontrols a recommended 20 mAloop current. The minimumbreakdown voltage of this IC,which determines the uppercompliance voltage limit of thecurrent source, is 27 V DC. At arecommended loop current of 20mA, the MARK state voltage dropacross the output terminals istypically 2.35 V DC. The SPACEstate current level is typically 1.1mA (maximum 2 mA) whichpowers the output integratedcircuit. Should power fail to theinput IC of the HCPL-4100 trans-mitter, the output IC will remain inthe MARK state. This importantfeature is needed in multidropcurrent loop applications. Aminimum capacitive loading of1000 pF is required for the outputIC (loop side) of the transmitter.



4

This guarantees stability of the ICunder all possible load conditions.In general, an external 1000 pFcapacitor can be omitted fortwisted wire cable lengths greaterthan 30 metres (100 ft.). If there isany possibility of using a wire looplength less than 30 metres (100 ft.),the 1000 pF capacitor must beconnected. Good engineeringpractice is to use a bypassingcapacitor (0.1 µF) from VCC toground for the input IC of thetransmitter. Data from directlyconnected TTL, LSTTL or CMOSunits to the input of the HCPL-4100 transmitter will properlyoperate this device. A blockdiagram of the HCPL-4100 is givenin Figure 1 and transfer character-istic is illustrated in Figure 2.

Features of the HCPL-4200

Receiver

The HCPL-4200 receiver input ICdirectly monitors the 20 mA loopcurrent. Power for this IC issupplied by the current loop fromthe external current source. Loopcurrent level detection is per-formed prior to the optical isola-tion.

Specified current thresholds are aminimum of 12 mA for MARK stateand a maximum of 3 mA forSPACE state. The typical thresholdfor MARK state is 6.8 mA with 0.8mA hysteresis. A guaranteedswitching threshold level for loopcurrent provides additionalcommon mode and differentialmode noise immunity, and pre-vents possible current transferratio (CTR) changes from affectingloop current threshold levels. Atthe recommended loop current of20 mA, the MARK state voltagedrop across the input terminals istypically 2.52 V DC.

VO

ICC

VCC

VI

II

IO

IO

1+

1–4

*CURRENT LOOP CONVENTION – H = MARK: IO ≥ 12 mA, L = SPACE: IO ≤ 2 mA.

+

–

SHIELD

TRUTH TABLE

(POSITIVE LOGIC)*

3

8

6

GND5

VI VCC IO

H ON HL ON LH OFF HL OFF H

Figure 1. Block Diagram and Truth Table for HCPL-4100 Optocoupler

The output integrated circuit ofthe HCPL-4200 receiver can have avoltage output (VO) greater thanthe power supply voltage (VCC).The absolute maximum voltagelevel for VO and VCC is 20 V DC.A third state (high impedance)output enable feature is availablefor use with buss interface appli-cations or special inhibit func-tions. The output will remain at a

high impedance level (third state)if VCC is zero volts. A blockdiagram of the HCPL-4200 is givenin Figure 3 and the transfercharacteristic is illustrated inFigure 4. Conventional TTL andLSTTL logic devices can bedirectly connected to the output ofthe HCPL-4200 unit. A standardCMOS interface for the HCPL-4200receiver is given in Figure 5.

25

20

15

10

5

00 1 2 3 4 20

VIN – V

I OU

T –

mA

Figure 2. HCPL-4100 Typical Transfer

Characteristic. Output Loop Current

vs. Input Voltage. MARK State Loop

Current is Provided by External

Current Source



5

Figure 3. Block Diagram and Truth Table for HCPL-4200 Optocoupler

Figure 5. Recommended Interface from HCPL-4200 Receiver to CMOS Circuit

II

*CURRENT LOOP CONVENTION – H = MARK: II ≥ 12 mA, L = SPACE: II ≤ 3 mA, Z = OFF (HIGH IMPEDANCE) STATE.

TRUTH TABLE

(POSITIVE LOGIC)*

II VE VO

H H ZL H ZH L HL L L

VO

VE

ICC

VCC

GND

IO

IE

5

SHIELD

8

7

6

VI

1+

1–

–

–

1

2

4

3

2

1

00 5 10 15 20

II – INPUT CURRENT – mA

VO

– O

UT

PU

T V

OL

TA

GE

– V

SPACESTATE

MARKSTATE

TA = 25° CVCC = 4.5 V

ISI (MAX) = 3.0 mA IMI (MIN.) = 12 mA

IOH = -2.6 mA

IOH = 6.4 mA

IHYS

Figure 4. HCPL-4200 Typical Transfer

Characteristic. Output Voltage vs. Input

Loop Current

8

HCPL-4200

7

6

5

2

ILOOP

RL

1

2

CMOSDATA

OUTPUT

VCC2(4.5 TO 20 V)

VCC2 RL

5 V 1.1 kΩ10 V 2.37 kΩ15 V 3.83 kΩ20 V 5.11 kΩ

Current Loop

Configurations

Current loop configurations can bearranged in three basic ways:simplex, half duplex and fullduplex. Two variations withineach configuration can exist, i.e.,point-to-point or multidropconnections. Definitions for theseterms, recommended interfacecircuits, performance trade-offsand measured performance datawill be discussed and illustratedwithin this section.

Simplex

The most fundamental loopconfiguration is simplex point-to-point. The data flows in onlyone direction from the transmitter

over the current loop to theremote receiver. This configura-tion is shown in Figures 6a and 6b.Figure 6a illustrates a non-iso-lated, active current loop transmit-ter used with an isolated HCPL-4200 receiver. Figure 6b shows anisolated HCPL-4100 current looptransmitter used with a non-isolated, active receiver. A pos-sible point-to-point application isto use a 20 mA current loop forcommunication between a com-puter and remote printer. Thesimplex topology can be expandedfrom point-to-point to multidrop.In a multidrop system, a numberof receivers are in series on theloop which contains only onetransmitter as illustrated in Figure

6c. A configuration with multipletransmitters and one receiver ispossible as well. However, thepriority of the operating transmit-ter must be established in thatcase.

Undesired ground loops areeliminated and common moderejection is increased by providingelectrical isolation of the currentloop. This isolation can be pro-vided at either the receiver ortransmitter end of the loop as illus-trated in Figures 6a and 6b respec-tively. The opposite end need notbe isolated. The non-isolated unitwill supply the current (active) forthis simplex configuration. Acircuit diagram of a non-isolated,



6

active transmitter connected tothe HCPL-4200 receiver (Figure6a) is given in Figure 7. The datarate performance versus the lengthof the current loop for this circuitis given in Figure 8.

c. SIMPLEX, MULTIDROP WITH ISOLATED RECEIVERS

Figure 6. Simplex Current Loop Configurations: Point-to-Point with Current Source Located at: a. Transmitter, b. Receiver,

c. Multidrop

The circuit for the alternatesimplex arrangement of a non-isolated, active receiver connectedto the HCPL-4100 transmitter isgiven in Figure 9. The trade-off ofdata rate versus loop length and

compliance voltage is illustrated inFigure 10.

Comparing the data rate perfor-mance for each circuit shows thatthe active, non-isolated transmitter

DATA DATA

RCVR

HCPL-4200

20 mA

XMTR

RCVR

HCPL-4200

RCVR

HCPL-4200

DATA

RCVR

HCPL-4200

RCVR

HCPL-4200DATA

ISOLATEDSTATION

ISOLATEDSTATION

ISOLATEDSTATION

ISOLATEDSTATION

ISOLATEDSTATION

NON-ISOLATEDSTATION

DATA DATA

20 mA

XMTRDATA

NON-ISOLATEDSTATION

NON-ISOLATEDSTATION

RCVR

HCPL-4200DATA

a. SIMPLEX, POINT-TO-POINT WITH ISOLATED RECEIVER

b. SIMPLEX, POINT-TO-POINT WITH ISOLATED TRANSMITTER

XMTRHCPL-4100DATA

ISOLATEDSTATION

NON-ISOLATEDSTATION

RCVR DATA

20 mA



7

51.1 Ω

0.01µF

1

R2

R3

2N3740

HLMP-3000

TRUTH TABLE(POSITIVE LOGIC)

VCC = 5 V DC – 27 V DC

7407

5 V

VIN

ILOOP8

HCPL-4200

7

6

5

2

1

2

VOUT

5 V

VCC VDC R2 kΩ R3 Ω

5 1.0 82.510 2.15 23715 3.16 38324 5.62 68127 6.19 750

VIN ILOOP VOUT

H 20 mA HL 0 mA L

LLOOP = 20 mA

L

Figure 7. Recommended Non-Isolated Active Transmitter with HCPL-4200 Isolated Receiver for Simplex Point-to-Point

20 mA Current Loop

10,000

1,000

100

10

1

DA

TA

RA

TE

– k

BA

UD

10 100L – LOOP LENGTH (ONE DIRECTION) – METRES

1,000 10,000

TA = 25° CILOOP = 20 mA

10% DISTORTIONDATA RATE

25% DISTORTIONDATA RATE

Figure 9. Recommended Non-Isolated Active Receiver with HCPL-4100 Isolated Transmitter for Simplex Point-to-Point

20 mA Current Loop

Figure 8. Typical Data Rate vs. Distance for

Circuit of Figure 7

1000

100

10

1.0

0.1

DA

TA

RA

TE

– k

BA

UD

10 100L – LOOP LENGTH (ONE DIRECTION) – METRES

1,000 10,000

TA = 25° C, ILOOP = 20 mA

25% DISTORTION DATA RATE10% DISTORTION DATA RATEMAXIMUM DATA RATE (< 25% DISTORTION)

10 V DC

15 V DC

24 V DC

VCC

Figure 10. Typical Data Rate vs. Distance and

Supply Voltage for Circuit of Figure 9

0.01µF

1 kΩ

5.6 kΩ

10 kΩ

51.1Ω

RTH75 Ω

4

L

3

8

3

4 1N914

ALTERNATIVECURRENTSOURCE

(6 V DC – 27 V DC)

HLMP-3000

VTH

RS

RT

VCC

VOUT

ILOOP

5 V dc5 V dc

VIN

OPTIONALTERMINATION

1000 pF


VIN ILOOP VOUT

H 20 mA HL ≤2 mA L

2

2

2

2

1

–

+LM311

2N3740

8

6

5

HCPL-4100

1

7



8

configuration is faster by a factorof 11 at 1000 metres (3281 ft.) ofloop distance.

The design and location of thecurrent source determine looplength and data rate performance.In both simplex configurations,the current source compliancevoltage can be much lower thanthe HCPL-4100 device maximumof 27 V. However, limitation on thelength of the loop (DC resistance)and data rate capability woulddictate a minimum VCC which canbe used for the current source.With a 24 V ±10% power supply,multidrop applications for verylong loop lengths (>3 km) can bedone. Device performance as afunction of current source designis covered in a later section.

DC Performance

By summing the MARK statevoltage drops around the currentloop, a basic equation is derivedfor DC performance for the loop.This is expressed as the maximumdistance over which a current loopcan be operated.

WhereL = Length of wire in one directionVCOMP = Compliance voltage ofcurrent source = (VCC - VSAT) [1]

(2)VCC = Supply voltage for currentsource;VSAT = Saturation voltage ofcurrent sourceILOOP = Operating loop currentRLINE = DC resistance of wire perlengthΣVXMTR = Summation of voltage MARK drops across each non-isolated and isolated transmitter

VCOMP – ΣVRCVR – ΣVXMTR

2 ILOOP RLINEL =

(1)

MARK MARK

current loop terminals whenMARK state current flows.[2]

ΣVRCVR = Summation of voltage MARK drops across each non-isolated and isolated receivercurrent loop terminals whenMARK state current flows.[2]

Notes:1. VCOMP must be less than thebreakdown voltage rating of thetransmitter unit in SPACE state.

2. The non-isolated unit willsupply loop current. If two ormore non-isolated units are used,then only one of these units needsto supply loop current.

This equation can be solved forany particular parameter ofinterest if the other remainingparameters are known. Forexample, Equations (1) and (2)can be solved for minimum powersupply voltage, VCC, which can beused for current source of Figure 7for the following conditions.

VCOMP = ΣVRCVR + ΣVXMTR

(3)

MARK MARK

+ 2L ILOOP RLINE

Where,ΣVRCVR = 2.76 V (One isolated

MARK receiver)ΣVXMTR = 0 V (Non-isolated MARK transmitter voltage dropis incorporated in VCOMP)ILOOP = 22 mA (20 mA + 10%),RLINE = 0.053 Ω/m (AWG No. 22),L = 305 mVCOMP = 2.76 V + 0 V + 2(305 m)(0.022 A) (0.053 Ω/m)

= 3.47 V

and using equation (2)

VCC = VCOMP + VSAT;where VSAT = 1.5 V

= 3.47 V + 1.5 VVCC = 4.97 V

For the listed conditions, thesecalculations are for the worst casetemperature and unit to unitvariations.

To address device voltage break-down concerns, the current sourcecompliance level for the currentloop and the SPACE state currentneed to be known. The SPACEstate current is determined by thetransmitter when it is in the OFFstate. SPACE state current will notbe necessarily equal to zero.Hence, when a current loopreceiver is designed, the thresholdmust be set at a proper level toavoid detecting the SPACE statecurrent as well as to provide anoise margin. With the HCPL-4200receiver, SPACE state current of3.0 mA or less is allowed. Should alarger noise current margin bedesired for SPACE state, the max-imum SPACE state current of theHCPL-4200 can be raised effec-tively by shunting the input with aresistor. Actual device currentthreshold does not change; onlythe loop current level required toreach SPACE state thresholdincreases. Use of a shuntingresistor will reduce the noisecurrent margin in the MARK statefor the HCPL-4200.

Should the HCPL-4100 transmitterbe used, the maximum SPACEstate current is 2.0 mA. For halfduplex applications, a noisemargin of 1.0 mA is specified forSPACE state with the combinationof the HCPL-4100/-4200 units.

AC Performance

AC performance of a current loopis influenced by many factors.Typical data rate capability for thesimplex configurations shown inFigures 7 and 9 are shown in



9

Figures 8 and 10 respectively. The10% (25%) distortion data rate isdefined as the rate at which 10%(25%) distortion occurs to theoutput bit interval with respect tothe reference input bit interval.This is expressed by equation (4).

Distortion Data Rate

Where D = % distortion and tBIT isbit time of the input signal when Ddistortion is present in the outputbit interval.

A square wave as well as a 16 bitdata stream was used to “exercise”the current loop system. In bothsimplex point-to-point configura-tions, the input data rate wasadjusted to result in a 10% (25%)distortion to the output signal foreither an input square wave or a 16bit pattern.

The 16 bit pattern was used toprovide two propagation delay testconditions for the current loop.One part of the 16 bit waveformtests for data rate distortion

fD = (4)1tBIT

affected by a long duration ineither a MARK or SPACE stateprior to state change. The otherpart of the 16 bit waveform testsfor effects on data rate distortionvia short durations in either aMARK or SPACE state prior to astate change. In both simplexconfigurations, Figures 7 or 9, thetransition from either a short or along duration MARK state to aSPACE state affects the data ratedistortion the most. The 16 bitexerciser circuit diagram and bitstream waveform are shown inAppendix A.

In the description of the ACperformance that follows, anapproximation has been made tosimplify the analysis. The trans-mission line is treated as a lumpedcapacitance. A more completeanalysis of AC performance wouldrequire rigorous treatment viatransmission line theory.

Non-isolated Active Transmit-

ter with the HCPL-4200 Re-

ceiver

The typical waveforms for thenon-isolated active transmitterused with the HCPL-4200 (Figure

11) are shown in Figures 12 and13. The loop length is 305 metres(1000 ft.). Figure 12 shows 10%distortion when the input signal isa 16 bit pattern. The total propa-gation delay shown in Figure 12for a SPACE to MARK transitionis 2.4 µs. The propagation delay ofthe transmitter and receiver is 0.3µs. Hence, the propagation delayfor the transmission line is 2.1 µs.Current and voltage waveformsfor the current loop at the re-ceiver end are shown in Figure 13for a 10% distortion of data to aninput square wave. Notice theloop current waveform delay of2.4 µs before VO follows VIN. Theimportant parameter to observe isloop current. When the loopcurrent level exceeds the HCPL-4200 current threshold, VOchanges state. Loop voltage at thereceiver only follows the currentlevel and changes by approxi-mately one volt. This simplexconfiguration provides the bestspeed performance.

ILOOP

51.1 Ω

0.01µF

11

5.62 kΩ

681 Ω

2N3740

HLMP-3000


VCC = 24 V DC

7407

5 V DC

VIN

ILOOP

VIN ILOOP VOUT

H 20 mA HL 0 mA L

16 BITWORD

GENERATOR

SQUAREWAVE

GENERATOR

L

10 Ω

8

HCPL-4200

7

6

5

2

VL VOUT

5 V DC1

2

Figure 11. Data Rate Test Circuit for Non-Isolated Active Transmitter to Isolated HCPL-4200 Receiver



10

Figure 12. Timing Comparision of Output Signal (VO) to

Input Signal (VIN) using 16-bit word at 10% Distortion

with 305 m (1000 ft.) Loop Distance. Reference Figure 11.

Figure 13. Characteristic Waveform Performance of

Circuit in Figure 11 with 305 m (1000 ft.) Loop

Distance, Square Wave Input and Output Distortion of

10%. a. Input Voltage (VIN), b. Receiver Load Voltage

(VL), c. Loop Current (ILOOP), d. Output Voltage (VO)

Non-Isolated Active

Receiver with the HCPL-4100

Transmitter

The typical waveforms for thenon-isolated active receiver usedwith the HCPL-4100 (Figure 14)are shown in Figures 15 and 16.The loop length is 305 metres(1000 ft.). Figure 15 shows 10%distortion when the input signal isa 16 bit pattern. Figure 16 illus-trates a 10% distortion for aninput square wave. Recall thatcurrent level determines theswitching point, not the voltagelevel.

At the input low to high statechange, the loop current changesmomentarily to the MARK stateshort circuit output current level,ISC, (typically 85 mA). A currentspike appears at the receiver endafter the inherent transmissionline propagation delay of approxi-mately 2 µs, as expanded inFigure 17. The spike is caused bysudden discharge of cable capaci-tance. However, the loop currentrapidly decreases from the ISClevel to a 20 mA level when linedischarge is complete. Timeduration of ISC can be calculatedby referring to the output powerdissipation calculations for the

HCPL-4100 in Appendix B.

The non-isolated receiverswitched states when loop currentcrossed approximately 10 mAlevel. Yet, the loop voltage tookapproximately 10 µs to changefrom a current source voltagecompliance level of 22.5 V to aMARK state voltage of 2.4 V at20 mA. This loop voltage charac-teristic does not limit the turn onspeed performance.

At the input signal high to lowtransition, the HCPL-4100 trans-mitter turns off after the devicepropagation delay. However, theloop current at the receiver doesnot change from 20 mA untilapproximately 30 µs later. This isdue to the current source chargingthe line capacitance at a fixed rateof 20 mA from essentially a MARKstate voltage level (2.4 V) to thecompliance level of the currentsource. Only after the compliancevoltage is reached does the loopcurrent fall below the 10 mAthreshold of the receiver. Asbefore, the current level at thereceiver determines the switchingtime. In this example, the compli-ance voltage of the current sourceinfluences the turn off delay of the

loop. This dependence is graphi-cally illustrated in Figure 10.

General Factors Affecting Data

Rate

Three factors which limit datarate and their correspondingtrade-offs are:

1. Loop length: The length of theline will limit the speed perfor-mance of a current loop. Thisresults mainly from the totalcapacitive loading effect of thecable upon the transmitter andreceiver. The data rate versuslength of line for the two simplexpoint-to-point configurations isillustrated in Figures 8 and 10.Shielded cable provides morenoise immunity but, in general,the capacitance per unit lengthwill be larger than unshieldedcable. The characteristics of thecable used, such as wire size,twist, insulation material, etc. allinfluence the maximum data rate.(The characteristics of the cableused in the text applications arelisted in Appendix C.)

2. Current Source: The location,type and compliance voltage ofthe current source used within acurrent loop can affect the system



11

VL

1

VIN

16 BITWORD

GENERATOR

SQUAREWAVE

GENERATOR

L

5 V DC

HCPL-4100

4

3

OPTIONALTERMINATION

8

6

5

1000 pF

RT


VIN ILOOP VOUT

H 20 mA HL ≤2 mA L

0.01µF 1 kΩ10 kΩ

5.6 kΩ

51.1Ω

75 Ω

2

2

2

LM311–

++

–

14

83

2

7

1N914

2N3740

ALTERNATIVECURRENTSOURCE

24 V DC

HLMP-3000

VTH

RS

VOUT

ILOOP

5 V DC

RTH

Figure 14. Data Rate Test Circuit for Isolated HCPL-4100 Transmitter to Non-Isolated Active Receiver

Figure 15. Timing Comparison of Output Signal (VO) to

Input Signal (VIN) Using 16-bit Word at 10% Distortion

with 305 m (1000 ft.) Loop Distance. Reference Figure

14.

Figure 16. Characteristic Waveform Performance of

Circuit in Figure 14 with 305 m (1000 ft.) Loop

Distance, Square Wave Input and Output Distortion

of 10%. a. Input Voltage (VIN), b. Transmitter Load

Voltage (VL), c. Loop Current (ILOOP), d. Output

Voltage (VO)

Figure 17. Expanded Waveform Response of Figure 16.

Displayed are details of ISC Magnitude and Duration at

Receiver as well as Transmitter Output Voltage (VL)



12

data rate. The primary limitationon data rate is the location of thecurrent source within the loop.When the current source is locatedat the transmitter end of the looprather than at the receiver end, themaximum data rate is much higherfor a given loop length. This effectcan be seen when comparingdata in Figure 8 to Figure 10. Themain reason for the lower datarate capability shown in Figure 10is that the current source mustcharge the cable capacitance tothe compliance voltage of thecurrent source. The speedachieved in Figure 8 resultsbecause cable voltage changes bya small amount between theMARK and SPACE states. Practicalapplications of the HCPL-4100 andHCPL-4200 optocouplers generallywill require the current source tobe located at the non-isolated endof the loop. For best speed perfor-mance, an active non-isolatedtransmitter would be used with theHCPL-4200 receiver. However, infull duplex applications (two sim-plex loops), the non-isolated endsshould be at a common location.Hence, overall bidirectionalsystem data rates would be limitedby the slowest loop, i.e., the onewhich contained a non-isolated,active receiver. More discussion offull and half duplex applications isfound in subsequent sections.

The type of current source canaffect speed performance. Asimple resistor, when used with avoltage source at the non-isolatedloop end, can set the loop currentlevel. Improper source impedanceor termination of the loop causesreflections which distort dataresulting in a lower data ratecapability. Termination of acurrent loop with an impedanceequivalent to the characteristicimpedance of the line can improvethe data rate capability by approxi-

mately 20%. Termination resis-tance does reduce part of thevoltage available for operatingstation(s) on the loop.

An active element current sourcecan be controlled by currentsteering which allows for a higherspeed of operation while providingconsistently high on and offoutput impedance. Less variationin current source output imped-ance can help to reduce multiplesignal reflections from occurring.Active element current sourcesare described in the CurrentSources section of this applicationnote.

The compliance voltage caninfluence the speed performanceof the non-isolated, active receivercircuit shown in Figure 9. Datarate dependency upon the compli-ance voltage is shown in Figure10. Reducing the current sourcesupply voltage from 24 V DC to10 V DC for a simplex point-to-point current loop configurationincreases the data rate by a factorof 3.5 in a 1000 metre (3281 ft.)loop. The reason for the improve-ment is that less time is requiredto charge the transmission line tothe lower compliance voltage levelat a constant charge rate. How-ever, the opposite simplex ar-rangement of a non-isolated,active transmitter, Figure 7, has nodata rate dependency on compli-ance voltage as illustrated inFigure 8. This is the result of theHCPL-4200 performing currentthreshold detection, not voltagelevel detection.

The maximum usable data rate fora given loop length and configura-tion is usually determined by thepropagation delay differences ofthe current loop and the amountof skew that is allowed in theuser’s application. Propagation

delay skew is |tPLH - tPHL|. Factorswhich contribute to the systempropagation delay skew are thecurrent source compliance voltagelevel, the current source location,the inherent propagation delayskew of the current source, as wellas the optocoupler propagationdelay skew. Detailed informationon factors which affect distortionof data are discussed in the subse-quent Half Duplex section.

3. Device Performance: Thepropagation delay of the transmit-ter and the receiver can influencedata rates for short loop lengths(<100 metres) in high speedapplications (>1 MBd). Generally,current loops are designed tooperate over much greater dis-tances than 100 metres. Conse-quently, the cable capacitancedominates the data rate limita-tions.

Another device performanceconsideration is the power dissi-pated in the HCPL-4100 transmit-ter. An example of power dissipa-tion calculation for the HCPL-4100is given in Appendix B. Deratingmay be required when operatingthe unit at large VCC voltage.

Optional Device Protection

Considerations

Specific end product applicationsmay require additional deviceprotection. A bridge rectifier canbe included in order to removeloop polarity connection concerns.Current buffering can be designedfor loop currents greater than 30mA. Possibility of damage due toelectrostatic discharge to the cableor interconnection points can besignificantly reduced by the use ofenergy absorbing devices, such asTransZorbs® or metal oxidevaristors (MOV).



13

Full Duplex

A common and useful extension ofthe simplex configuration is thefull duplex configuration. Fullduplex communication is definedas the simultaneous, bidirectionaltransmission of informationbetween local and remote units.The block diagram in Figure 18shows this point-to-point configu-ration.

Essentially, a full duplex arrange-ment is two point-to-point simplexloops (a four wire system) usedsimultaneously for back and forthdata flow. A common current loopapplication for full duplex configu-ration is connecting remoteterminals to an I/O interfacesection of a central computersystem.

Implementation of optical isola-tion within a full duplex currentloop system should be done at onecommon end of the loops. Twoseparate, active, non-isolated unitsare located at the other end of theloops. Figure 18 illustrates thesecomments. Providing isolation atone common end will significantlyreduce common mode couplingbetween the two loops. In most

cases, the full duplex data rate islimited by the loop containing theHCPL-4100 and the non-isolatedactive receiver. This limitation canbe circumvented by using anisolated current source at theHCPL-4100 end of the loop and anon-isolated passive receiver. Theisolated current source increasesthe cost of the data link, but thisincreased cost may be justified bythe speed improvement obtained.All information which is describedin the Simplex section for theperformance of a simplex point-to-point current loop configurationwill apply to the full duplexapplications as well.

Half Duplex

A half duplex current loop pro-vides non-simultaneous, bidirec-tional data flow between remoteand local equipment. This configu-ration utilizes a simpler two wireinterconnection, compared to thefour wire arrangement of the fullduplex case. The half duplexsystem can easily be expandedfrom a point-to-point applicationto a multi-drop application withmultiple transmit and receivestations along the loop. The blockdiagrams of Figure 19 demon-

strate the two cases of point-to-point and multidrop for halfduplex.

A characteristic of the half duplexconfiguration is the need toestablish priority of transmitters inthe current loop whether the loopis point-to-point or multidrop. Pro-tocol in the information beingtransferred will ensure that onlyone transmitter is sending data atany given time. Typical usage of ahalf duplex current loop parallelssimilar applications for the fullduplex case.

DC Performance

As Figure 19 illustrates, isolationof the current loop from logicsystems is easily implemented atone end of the loop by the use ofthe combination of the HCPL-4100transmitter and the HCPL-4200receiver. Significant reduction ofcommon mode influences on thecurrent loop, elimination ofground potential differences andspecified loop current noiseimmunity are achieved by the useof these complementaryoptocouplers. For this combina-tion of optocouplers, the loopcurrent noise immunity is a

20 mA

20 mA

XMTRDATA

NON-ISOLATEDSTATION ISOLATED STATION

RCVR

HCPL-4200DATA

RCVRDATA

XMTR

HCPL-4100 DATA

Figure 18. Full Duplex Point-to-Point Current Loop System Configuration



14

Figure 19. Half Duplex Current Loop System Configurations for: a. Point-to-Point, b. Multidrop

minimum of 1 mA in the SPACEstate and 8 mA in the MARK state.A non-isolated active transceiver(transmitter and receiver) withone current source operates at theopposite end of the current loop.

The maximum compliance voltageof the current source is limited bythe breakdown voltage capabilityof any transmitter in the halfduplex loop. The isolated HCPL-4100 transmitter has a maximumoperating voltage of 27 V DC.However, in multidrop applica-tions, each isolated HCPL-4200

receiver will share a portion of thecompliance voltage during theSPACE state, worst case 0.9 V DCat a minimum SPACE state currentof 0.5 mA. Also, each isolatedconducting HCPL-4100 transmitterwill share a portion of the compli-ance voltage during SPACE stateas well. Worst case, this is 0.95 VDC at 0.5 mA. For example, withfour pairs of HCPL-4100 andHCPL-4200 units being used, anadditional worst case 6.5 V DC canbe added to increase the maxi-mum compliance voltage valuefrom 27 V DC to 33.5 V DC for the

current source. The additional 6.5V DC is calculated on the basis ofone of the HCPL-4100 transmittersinterrupting 22 mA loop current at200 metre station intervals. Thisincrease in the compliance voltagelevel can allow another isolatedstation to be used. Minimum VCCrequired for the current source ofFigure 20(b) with five isolatedstations in the MARK conditionwith worst case parameterspreviously used yields 33.2 V DC. Ahigher compliance voltage canallow longer loop lengths but datarate will be slower. Precautions

20 mAXMTRDATA


RCVRHCPL-4200 DATA

RCVRDATAXMTR

HCPL-4100DATA

20 mAXMTRDATA

DATA DATA


ISOLATEDSTATION

RCVRHCPL-4200

DATA

RCVRDATAXMTR

HCPL-4100 DATA

RCVRHCPL-4200

XMTRHCPL-4100

DATA DATA

ISOLATEDSTATION

XMTRHCPL-4100

RCVRHCPL-4200

(a) POINT TO POINT

(b) MULTIDROP



15

must be taken to prevent theremoval of any stations from theloop in order to avoid an excessivecompliance voltage from over-stressing any transmitter remain-ing in the loop. A design compro-mise has to be determined for thebenefits achieved at differentcompliance voltages. Equation (1)from the Simplex section can beused to determine any DC param-eter in a half duplex application.

A specific active, non-isolatedtransceiver which can be used inpoint-to-point and multidropapplications with the HCPL-4100

1

11

1

3 214

5 V8

76556

8

5 VHCPL-4100

+ –HCPL-4200

+ –

DATAIN

ISOLATED T/R NO. 1 ISOLATED T/R NO. 2 ISOLATED T/R NO. 3 ISOLATED T/R NO. 4

DATAOUT

1000 pF

2

22

2

3 214

5 V8

76556

8

5 VHCPL-4100

+ –HCPL-4200

+ –

DATAIN

DATAOUT

1000 pF

3

33

3

3 214

5 V8

76556

8

5 VHCPL-4100

+ –HCPL-4200

+ –

DATAIN

DATAOUT

1000 pF

44

4

4

3 214

8

76556

85 V HCPL-

4100

+ –HCPL-4200

+ –

DATAIN

DATAOUT

1000 pF

(b)

(a)

L

L

L L

DEARBORN TWISTEDPAIR WIRE (NO. 862205)

NON-ISOLATED T/R

0.01 µF

5.62 k Ω

1 k Ω

681 Ω

51.1 Ω 2N37401/6 7407

VCC2 = 24 V

12

HLMP-3000

145 V

13

7

1N914

8

5 V

DATA IN DATA OUT

7 4

3 2

LM311 – +10 kΩ

100 Ω

Figure 20. Circuit Schematic of Half Duplex Configurations: a. Point-toPoint, b. Multidrop

and HCPL-4200 current loopoptocouplers is shown inFigure 20.

AC Performance

Point-to-Point

The corresponding data rateperformance versus loop dis-tance and direction of data isgiven in Figure 21 for the halfduplex point-to-point application.Definitions for 10% and 25%distortion data rate are givenunder the Simplex Configurationsection.

Transmission of data from anisolated T/R (transmitter andreceiver pair) to an active non-isolated T/R limits the half duplexdata rate performance. Thislimitation is due to slower speed ofa current loop when using anisolated transmitter. A lowercurrent source compliance voltagewill improve the data rate of anisolated T/R to a non-isolated T/Ras discussed in the Simplexsection.



16

Multidrop

The data rate performance for halfduplex multidrop applications, asillustrated in Figure 20(b), issummarized in Tables 1 and 2.These tables compare data rateversus the loop distance betweenstations, total length of loop, direc-tion of data, distortion data rateand data format. Table 1 illustratesat fixed distortion (10% or 25%)how data rate can be different atdifferent stations along the loop.Table 2 shows the pulse widthdistortion that results when themultidrop loop is operated atseveral fixed data rates.

Comparision of data rates forpoint-to-point (half duplex) andmultidrop configurations can

10 M

1 M

100 k

10 k

1 k

10010 100 1000 10,000

L – LOOP LENGTH (ONE DIRECTION) – METRES

DA

TA

RA

TE

– B

AU

D

25% DISTORTION DATA RATE

10% DISTORTION DATA RATE

MAXIMUM DATA RATE (≤ 25% DISTORTION)

NON – ISOLATED T/R

ISOLATEDT/R

NO. 1

NON – ISOLATED T/R

ISO

LATE

DT/R

NO

. 1

TA = 25° C

ILOOP = 20 mA

VCC2 = 24 V DC

Figure 21. Half Duplex Point-to-Point (Figure 20a) Data

Rate vs. Loop Length and Data Direction

appear contradictory. Datapresented in Figure 21 and Table 1illustrate this at a 25% distortionon a 915 m (3000 ft.) loop. Acomparison shows the influenceof multiple stations upon the datarate for a fixed loop length. In thepoint-to-point case, the data rate is62.5 kBd from an active, non-isolated T/R to the isolated T/RNo. 1. In the multidrop case, non-isolated T/R to the isolated T/RNo. 3, data rate in the samedirection is 25 kBd. As expected,the data rate in point-to-point isfaster because the current sourceonly needs to charge the line to alower sum (≅1/2) of MARK statevoltages than in the multiplestation case. However, in theopposite direction of isolated T/R

No. 1 to active, non-isolated T/R,the point-to-point data rate is 3.33kBd while the multidrop data rateis 6.25 kBd. In this instance,multidrop is 2X faster than thepoint-to-point case over the samedistance, contrary to “expected”results. The reason for this speedimprovement results from the factthat when T/R No. 3 interrupts theloop current, the current sourcewill continue to charge the loopuntil the compliance voltage isreached causing the currentsource to shut down. Fortunately,the voltage difference between thesum of all MARK state voltagedrops and compliance voltage issmall due to the total of fourstations in the loop. Hence, a smallvoltage excursion requires consid-erably less charging time than ifthis charging occurred over alarger voltage excursion.

In addition, with the usage ofmultiple stations at a constant datarate, the data distortion can varyfrom station to station. Thisdistortion varies with respect tothe transmitting station. As anillustration from Table 2, the 915 m(3000 ft.) example with threeremote stations [i.e., the iso-lated T/R No. 3 transmitting at 6.25kBd to successive 305 m (1000 ft.)stations of T/R No. 2, T/R No. 1and an active non-isolated trans-ceiver] yields distortion levels of19%, 31%, 25% respectively. Largerdistortion at an interim stationthan at a station located at an endof a loop is due to differentthresholds for each station.Identical current threshold levelsat each station will provideprogressively increasing distortionlevels at each station along theloop. Also, reflection of signalsbetween stations can account forinconsistent distortion levels atvarious stations on a loop system.The designer will be limited to a



17

data rate that results in acceptabledistortion for each station.

In general, data rate limitations inmultidrop applications for datawhich flows from active non-isolated station to isolated stationswill be caused by propagationdelay time of the line and thecharging or discharging time for

the total line capacitance. Charg-ing and discharging the line occursover a voltage level which is equalto the difference between the sumof all MARK state voltages and thesum of all SPACE state voltageswhich are presented at the activenon-isolated loop connectionpoints. Data rate limitations in theopposite direction (isolated units

to an active, non-isolated unit) arelimited by propagation delay timeof the line and by the time tocharge total line capacitance overa voltage equal to the differencebetween the sum of MARK statevoltages and compliance voltage ofthe current source.

Table 1. Constant % Distortion, Variable Data Rate for Half Duplex

Multidrop Configuration

L LT

Length between Total Data

Data In Data Out Data Input and Loop Rate % Comments

Data Output Points Length kBd Distortion

m ft. m (ft.)

Active T/R #4 122 400 250 25% Input Signal:Non-Isolated T/R #3 92 300 370 Square WaveTransceiver T/R #2 61 200 370

T/R #1 31 100 250 Current Source:20 mA/24 V DC

T/R #4 122 400 125 10%T/R #3 92 300 167T/R #2 61 200 122 167 Half DuplexT/R #1 31 100 (400) 125 Current Loop

Isolated Non-Iso. T/R 122 400 62.5 25% CurcuitT/R #4 T/R #1 92 300 62.5 Schematic:

T/R #2 61 200 82.5 Figure 20bT/R #3 31 100 100

Non-Iso. T/R 122 400 25 10%T/R #1 92 300 25T/R #2 61 200 33.3T/R #3 31 100 40

Active T/R #3 915 3000 25 25%Non-Isolated T/R #2 610 2000 33.3Transceiver T/R #1 305 1000 25

T/R #3 915 3000 12.5 10%T/R #2 610 2000 915 16.7T/R #1 305 1000 (3000) 12.5

Isolated Non-Iso. T/R 915 3000 6.25 25%T/R #3 T/R #1 610 2000 5.0

T/R #2 305 1000 8.25

Non-Iso. T/R 915 3000 2.0 10%T/R #1 610 2000 2.0T/R #2 305 1000 3.33



Table 2. Constant Data Rate, Variable % Distortion for Half Duplex

Multidrop Configuration

L LT

Length between Total Data

Data In Data Out Data Input and Loop Rate % Comments

Data Output Points Length kBd Distortion

m ft. m (ft.)

Active T/R #4 122 400 250 25% Input Signal:Non-Isolated T/R #3 92 300 250 30% Square WaveTransceiver T/R #2 61 200 250 15%

T/R #1 31 100 250 25% Current Source:20 mA/24 V DC

T/R #4 122 400 125 10%T/R #3 92 300 125 6%T/R #2 61 200 122 125 7.5% Half DuplexT/R #1 31 100 (400) 125 12% Current Loop

Isolated Non-Iso. T/R 122 400 62.5 25% CurcuitT/R #4 T/R #1 92 300 62.5 25% Schematic:

T/R #2 61 200 62.5 19% Figure 20bT/R #3 31 100 62.5 6%

Non-Iso. T/R 122 400 25 10%T/R #1 92 300 25 10%T/R #2 61 200 25 7.5%T/R #3 31 100 25 7.5%

Active T/R #3 915 3000 25 25%Non-Isolated T/R #2 610 2000 25 17.5%Transceiver T/R #1 305 1000 25 25%

T/R #3 915 3000 12.5 10%T/R #2 610 2000 915 12.5 7.5%T/R #1 305 1000 (3000) 12.5 12%

Isolated Non-Iso. T/R 915 3000 6.25 25%T/R #3 T/R #1 610 2000 6.25 31%

T/R #2 305 1000 6.25 19%

Non-Iso. T/R 915 3000 2.0 10%T/R #1 610 2000 2.0 8%T/R #2 305 1000 2.0 12%

18



19

Current Sources

Resistor

Current sources used for currentloop systems can vary in perfor-mance, complexity and cost. Themost elementary and inexpensivetechnique for supplying loopcurrent is to use a series resistorbetween the non-isolated stationpower supply and the loop. Theresistance value (RS) to use is afunction of the desired loopcurrent (ILOOP), of the type ofcable (resistance per length,RLINE), the loop length (L), thenumber of stations on the loopwith their respective MARK stateterminal voltages (ΣVMARK) andthe power supply voltage (VCC) forthe non-isolated station. Equation(5) determines the resistor value.The simplicity of a single resistoryielding a 20 mA loop current isachieved at the expense of requir-ing that the loop configurationcannot be changed withoutchanging the resistor value.

Speed performance of a loopsystem using a current settingresistor is slower than with anactive current source. This differ-ence arises from an exponentialcurrent response versus a linearcurrent response associated withan active current source. For agiven current threshold level, thelinear response will charge acapacitive line quicker than anexponential response. When acurrent setting resistor is used,termination resistance can beemployed at the isolated stationsin order to reduce somewhat thesignal reflections along a loop. Upto a 20% speed improvement canbe obtained with termination res-

VCC – (ILOOP) (RLINE)2L – ΣVMARK

RS =(5)

ILOOP

istance equal to characteristic lineimpedance. However, with the useof termination resistance, allow-ance must be made for the addi-tional MARK state voltage drop ofthe terminating resistor withrespect to the available com-pliance range of the current loop.

LED/Transistor

The recommended current sourceto use is a simple, relativelyinexpensive, active constantcurrent source shown in Figure 22.The LED provides a stable voltagereference (VF) for the transistorbase and also helps to compensatethe base-emitter junction voltage(VBE) changes with temperature.This current source is fairlyindependent of the VCC level if theLED bias current (IF) is set so thatthe VF only changes slightly withlarge variations in IF (≅ 60 mV/decade of IF). Regulation of 10% inoutput current can easily beachieved over a VCC range of 5 VDC - 27 V DC.

+VCC+VCC

+

IO

Q1

Q2

VBE

VF

VF

VBE

R

R =1 mA

VCC + VEE – VF

R

-VEE -VEE

VEE

VCC

RE

RE

HLMP-3000

OR

EQUIVALENT

HLMP-3000

OR

EQUIVALENT

IOIF

IF

IO =

P = POWER IN Q1 OR Q2

= IO (VCC – IO RE – Σ VMARK)

VF ≅ 1.5 V AT IF = 1 mA

RE

VF – VBE

Figure 22. Recommended Constant Current Source

When large VCC is used with lowMARK state voltage conditions, theuse of a transistor with goodthermal conduction to the ambientenvironment will minimize thevariation in the source currentbecause of the variation of VBEwith temperature. This state is thelarge power dissipation condition.

This current source can be easilycurrent steered for fast switching.Current steering is illustrated inFigure 7. For applications of thiscurrent source at high data ratesover short loop distances it isimportant that the difference inpropagation delays from on-to-offand off-to-on be made as small aspossible. Status indication isconveniently given by the LED.

Specialized Integrated Circuits

More complicated current sourceswhich are usable in current loopscan be current mirrors or special-ized integrated circuits. A currentmirror displayed in Figure 23



20

requires the use of a matched pairof transistors in order to performwell.

In general, commercially availablematched pairs are not designed foroutput collector currents greaterthan 10 mA. Additional compo-nents which are needed to obtain20 mA results in greater complex-ity and higher cost with less reli-ability. In order to reduce powersupply noise influences andimprove thermal stability, signifi-

cant transistor emitter degenera-tion resistance must be used. Thisdegeneration sacrifices theavailable voltage range betweenthe total MARK state voltage andthe compliance voltage of thecurrent mirror.

In Figure 23(c), the LM234 inte-grated circuit current source canbe used. More components arerequired for temperature compen-sation to provide a proper currentlevel and regulation over a wide

temperature range as well as toprevent possible oscillations ofthis active device. In addition,switching characteristics of theLM234 must be considered withregard to the desired loop speedperformance. Over all, the currentsource provides good regulation(±3%) for current loop usage, butthe cost of additional componentsand space in an input-outputmodule may detract from thedesign convenience of this device.

RE

R

(a) (b) (c)

GND

VBE

–VEE

+VCC

IO

RERE

RE

R

LM234CURRENT

REGULATORGND

VBE

+VCC

–VEE

IO

1N914

V+

2N2905A

R1 ≅ 10 RSET = 68 Ω

RSET = 6.8 Ω

IO

IIN

V–

33 k Ω

0.001 µF

VIN

RE =∆I

∆T (∆ VBE/∆T)

∆VBE ∆VBE2 – ∆VBE1

R + RE

VCC + VEE – VBEIO ≅

∆T = TEMPERATURE CHANGE FROM ROOM TEMPERATURE IN °C∆I = ACCEPTABLE DEVIATION FROM IO IN mA

∆T ∆T=

;

Figure 23. Specialized Integrated Circuit Current Sources. Current Mirrors: a. PNP, b. NPN. Current Regulator:

c. LM234 plus Components



21

Appendix A.

16-Bit TTL Data Exerciser

Circuit

The purpose of this circuit is toprovide long (6 bits) and short (1bit) durations of the logic one andlogic zero states in different timesequences. Figure 24 illustratesthe 16-bit TTL output waveform(QA).

Appendix B.

HCPL-4100 Power Dissipation

Calculations

The power dissipation calculationfor the HCPL-4100 transmitter ismade using the standard methodfor determining the input powerdissipation. Calculation of theoutput power dissipation musttake into account power con-sumed in three different operatingconditions. These conditions are:1. power dissipated duringtransition from SPACE to MARKstate, 2. power dissipated inMARK state, and 3. power dissi-pated in SPACE state. The worstcase average power dissipated

DATA IN

6 CLK2

74LS197

92

1

132

12

VCC

QC

QB

QD

12

1110

9

8

74LS10

3

45

6

1

8

13

5DATA OUT

LOAD

CLK1

CLEAR

QA

DATA IN(CLK 2)

DATA OUT(QA)

Figure 24. 16-bit TTL Data Exerciser Circuit and Corresponding Waveforms

over two bit intervals should becalculated. The average outputpower dissipation is calculatedfrom the following formula:

wherePO = Average output powerdissipated during a worst casetime interval.PSM = Power dissipated duringSPACE to MARK transition

PSMtSM + PMtM + PStSPO =

(6)

2 tBIT

ISC = MARK state short circuitoutput currentVCOMP = Compliance voltage ofcurrent sourceVMO = MARK state outputvoltagePM = Power dissipated duringMARK state

= VMOILOOP (8)

ILOOP = Loop Current

ISC (VCOMP + VMO=

2(7)

PS = Power dissipated duringSPACE state

= VCOMPISO (9)

ISO = SPACE state outputcurrenttSM = Duration time of powerdissipated during SPACE to MARKtransition

CL (VCOMP – VMO)= (10)

ISC

CL = Total load capacitancetM = Duration time of MARKstate

tS = Duration time of SPACEstate

tBIT = Time of bit interval

These formulas are based upon anassumption of a linear discharge ofa capacitive load.

Using equations (6) through (10),the worst case output powerdissipated in the HCPL-4100



22

transmitter can be calculated forthe following operating conditions.

L = 200 metres (656 ft.)CL = 36,000 pF (180 pF/m)

VCOMP = 23.5 V DC (VCC = 24 VDC)

ILOOP = 22 mA (20 mA + 10%)

ISC = 85 mA

TA = 70°C

CMOS LOGIC (15 V ± 5%)

HCPL-4100 Power DissipationLimits at 70°CPIN = 208 mW,

PTOTAL = 283.5 mW

VMO = 2.75 V at 22 mA

ISO= 2 mA

tBIT= 104.1 µs

(9600 Baud, NRZ* alternatingMARK and SPACE states.Reference Figure 25.)

*Non Return to Zero data format.

D = 25% Distortion

Calculation of maximum inputpower to the HCPL-4100 is basedupon linear interpolation ofmaximum ICC and IIH.

PIN = VCCICC + VIHIIH (11)

= (15.8 V) (12.6 mA) +(15.8 V) (0.206 mA)

= 202.3 mW. < 208 mWmax

INPUT DATA

OUTPUT POWER

MARK

SPACE

PM

tM tS

2 tBIT

PSM

tSM

PS

Figure 25. Output Power Dissipation over Two Bit Intervals in HCPL-4100

Transmitter

Consequently, maximum deratedoutput power is

PO = PTOTAL - PIN (12)

= 81.2 mW

Applying given values to equations(7), (8), (9), and (10)results in

PSM = 1115.6 mWPM = 60.5 mWPS = 47 mWtSM = 8.8 µs

Equation (6) yields average outputpower per bit interval ofPO = 98.4 mW > 81.2 mW max

To circumvent this excess outputpower dissipation condition,VCC ofcurrent source can be reducedfrom 24 V DC to 15V DC. Recalcu-lation with VCC = 15 V DC yields

PSM = 733.1 mW

PM = 60.5 mW

PS = 29 mW

tSM = 5.0 µs

PO = 60.9 mW < 81.2 mW max

The major compromises whichwould be given up are:a. maximum loop length is reducedby one half, andb. less capability for multidropapplications. However, data ratecan be doubled to 20 kBd at VCC =15 V DC over 200 metres.

Appendix C.

Cable Data

The cable which was usedthroughout this application notecontained five pairs of unshielded,twisted, 22 AWG wire (DearbornNo. 862205). Typical measuredproperties of this cable are listed



23

below. Other manufacturersprovide cable with similar charac-teristics (Belden No. 9745). Con-sult cable manufacturer catalogsfor detailed information.

ZO = 75 Ω, CharacteristicImpedance - line to line

RLINE = 52.91 Ω/1000 m, DCResistance - single conductor

C = 174 pF/m, Capacitance -line to line

tDELAY = 5.9 nsec/m

Appendix D.

List of Parameters

CL = Total load capacitance

fD = Data rate at which D%output pulse distortion occurs

ILOOP = Operating loop current

ISC = MARK state short circuitoutput current

ISO = SPACE state outputcurrent

L = Length of wire in onedirection

PS = Power dissipated duringSPACE state

PO = Average output powerdissipated over a time interval

PSM = Power dissipated duringSPACE to MARK transition

PM = Power dissipated duringMARK state

RLINE = DC resistance of wireper length

RS = Current setting resistorfrom non-isolated power supply

tBIT = Time of bit interval

tS = Duration time of SPACEstate

tM = Duration time of MARKstate

tSM = Duration time of SPACEto MARK transition

VCC = Power supply voltage

VCOMP = Compliance voltage ofcurrent source

VMO = MARK state outputvoltage

VRCVR = Voltage across currentMARK loop receiver

terminals when MARK statecurrent flows

VSAT = Saturation voltage ofcurrent source

VXMTR =Voltage across currentMARK loop transmitterterminals when MARK statecurrent flows

Appendix E.

References

1. Optoelectronics / Fiber Optics

Application Manual, AgilentTechnologies, Second Edition,McGraw-Hill, New York, 1981, pp.3.1-3.77, 10.1-10.8.

2. True, Kenneth M., The Interface

Handbook – Line Drivers and

Receivers, Fairchild Semiconduc-tor Components Group, MountainView, CA., 1975, pp. 1-1 – 5-12.

3. McNamara, John E., Technical

Aspects of Data Communications,

Second Edition, Digital EquipmentCorporation, Bedford, MA., 1982,pp. 13-21, 233-242.

4. Larsen, David G., Ronz, Peter R.,Titus, Jonathan A., “INWAS:Interfacing with AsynchronousSerial Mode,” IEEE Transactions

on Industrial Electronics and

Control Instrumentation, Vol.IECI-24, No. 1, February 1977, pp.2-10.

5. Agilent 13266A Current Loop

Converter User’s Manual,Agilent Technologies, Palo Alto,CA 1979



www.semiconductor.agilent.com

Data subject to change.Copyright © 1999 Agilent Technologies, Inc.5953-9359 (11/99)



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Documents

Designing with the HCPL-4100 and HCPL-4200 Current Loop