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ideasdesign

Edited by Bill Travis

Sensitive systems, such as thosein aircraft, must withstand faultconditions, thereby avoiding

component and system damage, be-cause a sensor failure could cause acatastrophic event to occur.A channelprotector, comprising two n-channel

MOSFETs connected in series with ap-channel MOSFET, can protect sen-sitive components from voltage tran-sients in the signal path, whether ornot the power supplies are present(Figure 1). The channel protectoracts as series resistor during normaloperation. If the input exceeds the pow-er-supply voltages, one of the MOSFETsturns off, clamping the output within thesupply rails, thus protecting the circuitryin the event of overvoltage or supply-lossconditions. Because channel protectors

work regardless of the presence of thesupplies, they are also ideal for applica-

tions in which correct power sequencingcannot be guaranteed and for hot-inser-tion rack systems. Figure 2 shows anADG465 channel protector with an inputsignal that exceeds the power-supply volt-age. The protector clamps the output sig-

nal, protecting the sensitive componentsthat follow the channel protector.

When a fault condition occurs, thevoltage on the input of the channelprotector exceeds a voltage set by thesupply-rail voltage minus the MOS-FETs threshold voltage.For a positiveovervoltage, this voltage is V

DD

VTN

, where VTN

is the threshold volt-

age of the NMOS transistor (typical-ly,1.5V).In the case of a negative over-voltage, the voltage is V

SSV

TP, where

VTP

is the threshold voltage of thePMOS device (typically,2V).Whenthe input of the channel protector ex-ceeds either of these voltages, the pro-

tector clamps the output within them.These devices offer bidirectional fault andovervoltage protection,so you can use theinputs or outputs interchangeably.Figure3 shows the voltages and MOSFET statesfor a positive-overvoltage event.

The output load limits the currentduring the fault condition to V

CLAMP/R

L

(Figure 4). If the supplies are off, theprotector limits the fault current tonanoamps. Figure 5 shows how you canuse the ADG466 channel protector toprotect the sensitive inputs of an instru-mentation amp from a sensor fault. Inapplications that require a multiplexer in

Data-acquisition system uses fault protectionCatherine Redmond, Analog Devices, Limerick, Ireland

NMOS

VDD

NMOS

PMOS

PMOS

VDDVSS

VSS

VIN VOUT

VOUT

VD1

VDD

VDD

VS1

VSS

ADG465VIN

VDD

OUTPUT CLAMPED

AT VDD

1.5V

F igure 2

The channel protector clamps overvoltage transients to a safe level.

NMOS NMOSPMOSPOSITIVEOVERVOLTAGE

(20V)

SATURATED NONSATURATED NONSATURATED

VDDVTN13.5V

VDD15V

VDD15V

VSS15V

NOTE: VTN = NMOS-THRESHOLD VOLTAGE (1.5V).

F igure 3

The voltages and MOSFET states appear like this during a positive-overvoltage event.

A channel protector can protect sensi-

tive circuitry from voltage transients.

Data-acquisition system

uses fault protection......................................69

Take steps to reduce

antiresonance in decoupling ......................70

Precision level shifter has

excellent CMRR ..............................................72

Celsius-to-digital thermometer works

with remote sensor........................................74

Quasiresonant converter uses

a simple CMOS IC..........................................74

Simple circuit serves

as milliohmmeter ..........................................78

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addition to channel protection,you canuse the ADG439F fault-protected, four-channel analog multiplexer

(Figure 6). These multiplex-ers use a series n-channel, p-channel,n-channel MOSFET connection. Duringfault conditions, the inputs or outputsappear as open circuits, protecting thesensor or signal source as well as theoutput circuitry.

SENSOR 1

REFERENCE

+

IN AMP

ANALOG OUT

TO ACTUATOR

ADG466

ADC

DAC

DSP

F igure 5

In this circuit, the ADG466 channel protector guards the sensi-

tive inputs of an instrumentation amplifier from a sensor fault.

SENSOR 1

SENSOR 4

REFERENCE

+

IN AMP

ANALOG OUT

TO ACTUATOR

ADG439F

ADC

DAC

DSP

F igure 6

A multiplexer in a data-acquisition system protects the signal source as well

as the output circuitry.

OVERVOLTAGEOPERATION

(SATURATED)

NONSATURATEDOPERATION

PMOS

V

NMOS

N+ N+ N+EFFECTIVE

SPACE-CHARGEREGION

N-CHANNEL

VD VDD15V

VT = 1.5V

VG VT =13.5V

13.5V

P

VG VS

IOUT

RL VCLAMP

20V

Figure 4

The output load limits the current to VCLAMP

/RLduring a fault condition.

To maintain power integrity on pcboards, you need multiple capaci-tors to decouple the power-distri-

bution system. A typical configurationmight comprise five capacitors connect-ed in parallel between the power and theground traces or planes. To providebroadband decoupling per-formance, assume the indi-vidual values of the capacitors are 470,1, 10, 100, and 220 nF (Figure 1). Thisparallel network provides 801-nF total

capacitance to the power-distributionsystem. If you measure each capacitor

with a vector-network analyzer, you canidentify each capacitors SRF (self-res-onant frequency). Figure 2 is a plot ofeach capacitors SRF, as well as the SRFof the overall parallel connection. EachSRF can cause antiresonance in the par-allel decoupling configuration.Theantiresonance occurs when one ca-

pacitor is still capacitive, while anotherhas become inductive.

470 nF

0.801 F

TOTAL

1 nF 10 nF 100 nF 220 nF

POWER

GROUND

F igure 1

Take steps to reduce antiresonance in decouplingDale Sanders, X2Y Attenuators, LLC, Farmington Hills, MI

A typical decoupling configuration uses several

multilayer-ceramic capacitors connected in

parallel.

400 nF(801 nF TOTAL)

POWER

RETURN

G1 G2

B

A

F igure 3

A 400-nF X2Y capacitor yields a total decou-pling capacitance of 800 nF.

10

0

10

20

30

40

50

60

70

80

VOLTAGE

(dBV)

10 kHz 10 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz

FIVE STANDARD CAPACITORS: 470, 1, 10, 100,220-nF (801-nF TOTAL CAPACITANCE)

ONE STANDARD 1206 10-nF CAPACITOR

ONE STANDARD 1206 220-nF CAPACITORBOARD S21




Figure 2

Measurements with a vector-network analyzer reveal undesirable antiresonance effects.

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A way to considerably reduce the an-tiresonance effects is to use a single 400-nF X2Y capacitor for decoupling. (Ca-

pacitors using X2Y technology areavailable, for example, from JohansonDielectrics (www.johansondielectrics.com). You measure the capacitance rat-ing for an X2Y componentfrom line to ground; in oth-er words, from an A or a B terminal toeither of the G1 or G2 terminals in Fig-ure 3. So, the total capacitance a 400-nFX2Y component supplies, connected asin Figure 3 would be double the capac-itance rating, or 800 nF. Figure 4 showsthat a single X2Y capacitor with the

same total capacitance as in Figure 1provides the same broadband decou-

pling as the standard decoupling con-figuration but without the antireso-

nance effects. In addition, because X2Ycomponents come in the same package

sizes as standard capacitors (1812, 1210,1206, 0805, and 0603), the use of X2Y

components saves pc-board space andreduces layout complexity.

10

0

10

20

3040

50

60

70

80

VOLTAGE

(dBV)

10 kHz 10 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz

FIVE STANDARD CAPACITORS: 470, 1, 10, 100,220nF (801-nF TOTAL CAPACITANCE)

BOARD S211 X2Y 1206 400 nF

F igure 4

The single X2Y decoupling capacitor displays no antiresonance effects.

Most designers makelevel shifters withop amps and 1%-

tolerance discrete resis-tors. Discrete-resistormismatching limits theop amps CMMR (com-mon-mode rejection ra-tio) to 40 dB, so you can-not use op amps incircuits that require highCMRR. Differential am-plifiers contain precisionmatched internal resis-tors, so ICs such as theINA133 can readily

achieve CMRRs of ap-proximately 90 dB. Theycan offer such highCMRR by trim-ming internal matchedresistors. Assume thateach input in the circuit ofFigure 1 has an associated noise voltage(V

N1, V

N2, and V

NREF). The transfer func-

tion of the amplifier circuit isV

OUT(V

REFV

NREF)(V

IN 2V

N2)

(VIN1V

N1). Note that the reference volt-

age shifts the output signal, either single

or differential.Once this level shifting oc-curs, you can turn your attention to the

noise cancellation. Careful cabling anddifferentially coupling the signal into thedifferential amplifiers inputs force thenoise on the signal inputs to be equal(V

N1V

N2). The input noise is a com-

mon-mode signal,so the differential am-

plifier rejects it to the best of its ability(nominally, 90 dB). Now, V

OUT

VIN2V

IN1V

REFV

NREF.

Now, you need to elim-inate the reference noise

to obtain a clean level-shifted signal. You couldconnect the X end of C

1to

ground to shunt the ref-erence noise to ground,but this solution may beineffective because thesource impedance of thereference is low. When,however,you connect theX end of C

1to the V

IN1sig-

nal source,the differentialamplifier acts as a lowpass

filter and rejects the refer-ence noise. This circuitkeeps the input imped-ance of the differentialamplifier low (approxi-mately 25 k for theINA133) to facilitate

matching.Thus,you must keep the signalsource impedance low to prevent gain er-rors. The source impedance should beless than 1/1000 the input impedance tominimize gain error. If this situationdoesnt occur naturally, then it is best to

buffer the inputs.

Precision level shifter has excellent CMRRRonald Mancini, Texas Instruments, Bushnell, FL

VIN1

VIN2

VOUT

VREF

C1

25k 25k SENSE

25k

25k

X

+V

+IN

V

IN

INA133

_

+

Figure 1

C1

allows the level shifter to act as a lowpass filter that rejects the reference noise.

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You can use a single-supply system to pre-cisely measure the

temperature at a remotelocation with less than1C error over a 0 to100C range (Figure 1).The circuit includes T

1, a

low-cost AD590 tempera-ture sensor; IC

1, an

AD8541 rail-to-rail am-plifier; four resistors; atrimming potentiometer;and an ADC. You canomit the ADC if you needan analog output. Youcould replace the trim-ming potentiometer withan AD8400 or AD5273digital potentiometer for easier calibra-tion. The feedback resistor, R

F, should be

a precision resistor to minimize the scale-factor error, but the accuracy of the re-

maining resistors is not critical. You canchoose the grade of the AD590 sensor toachieve the required accuracy.

The AD590 provides an output currentproportional to absolute temperature (1A/K).In this application, the circuit off-sets and scales the output to provide afull-scale range of 0 to 5V with a scale fac-tor of 50 mV/C over the chosen tem-perature range of 0Cthe freezingpoint of waterto 100C, the boilingpoint of water. The AD8541 is a low-cost,

low-power, rail-to-rail operational am-plifier.It has a high common-mode volt-age range and extremely low bias cur-rents. You can calibrate out its 1-mV

typical offset, the resistor,and AD590 er-rors. The output swing of the amplifieris 25 mV to 4.965V with a single 5V pow-er supply, limiting the output by about0.5C on either end.

This circuit can derive its power froma single 5V power supply. The output ofthe AD590 varies from 273.15 to 373.15A as the temperature varies from 0 to100C. The positive input of the AD8541has an offset of 4V to provide sufficientheadroom for the AD590. The series

combination of R1

and R2

develops a 1V drop, andyou adjust R

2to provide a

nominal current of 353.15A. Thus, the currentthrough the feedback re-sistor, R

F, varies from80

to20 A as the temper-ature varies from 0 to100C. The voltage across

this resistor varies from4 to1V. The 4V offsetcauses the output voltageof the amplifier to varyfrom 0 to 5V.

To guarantee the accu-racy of 1C throughoutthe range,you need to per-form a calibration proce-

dure. At a known temperature, such as25C, adjust trimming potentiometer R

2

to obtain the desired voltage at the out-put of the amplifier, 1.250V, or the de-

sired code at the output of the ADC,400H.Once you perform the calibration,

you can calculate the temperature in Cel-sius at any measured point inside therange by multiplying the output voltageby 20. Because the sensor has a currentoutput, it is immune to voltage-noisepickup and voltage drops in the signalleads; you can thus use it at a remote lo-cation. You should use a twisted-pair orshielded cable.

AD8541

AD590

IC1

R4

R3

R1RF

AD7476

50kR21k

2k50k

200k

OUT DIGITAL-DATA

OUTPUT

GND

VDD

VIN

LONGWIRES

+T1

_

+

Celsius-to-digital thermometer

works with remote sensorElana Lian and Chau Tran, Analog Devices, Wilmington, MA

This system precisely measures temperature at a remote

location, with less than 1C error over a 0 to 100C range.

Figure 1

Figure 1 shows a flyback power sup-ply that has low noise and uses a sim-ple CMOS 4093 IC for its control.

The electrical noise of a converter arisesmainly when current switches on. Dioderecovery and charging parasitic capaci-tances create high di/dt,which is the maincause of noise. The converter in Figure 1

(pg 76) has a low noise level, because itslowly switches current on at nearly zero

voltage. The converter works in theboundary between discontinuous andcontinuous mode and switches on whenthe drain voltage is at its lowest value. Toavoid working with low gate voltages,which would cause excessive MOSFETlosses, ZD

1conducts and enables the in-

put gate of the 4093 when the voltage is

high enough. When the supply starts, theauxiliary nonisolated winding through D

3

keeps the gate input high. When theMOSFET is on, current increases linearlyuntil the base of Q

5starts to conduct,and

this transistor turns the MOSFET off.Theflyback operation then starts,and the pri-mary energy charges the output capaci-tors. During this phase of operation, D

5

and R6keep Q

5conducting and the MOS-

FET off.When the energy has discharged,

Quasiresonant converter uses a simple CMOS ICFrancesc Casanellas, Aiguafreda, Spain

(continued on pg 78)

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C2100 F

C11 F

C647 pF

R24.7k

R122k

R68.2k

R154.7k

R12470k

R10470

OC1

4N35

R9

1kC9

1 nF

R7

1k

R16

100

R510k

ZD315V

ZD14.7V

C533 F

Q1

Q3

Q4

D3

D4

D5

D1

D2

D6

Q2

114

7

23

2222

2907

Q5

2369

4093

2369

8

910

12

1311

5

614

R8

C3

C4

C7

R3

R4

VCC

VOUT

R11

ZD2

TL431

R13

R17

R14

C8

C10

+

+

Figure 1

Using a simple CMOS IC, this flyback power-supply circuit exhibits extremely low noise.

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D5stops conducting, as do the secondary

diodes, so no recovery problems exist.

The time constant of R5 and C5 keepsthe MOSFET off for a while. The outputcapacitance of the MOSFET plus theparasitic capacitance of the primary res-onate with the primary inductance andthe voltage decreases.R

5and C

5allow the

MOSFET to turn on when the voltage

has reached the minimum value. Thevalues are valid only for this case. Thecircuit ofFigure 1 not only minimizes

turn-on losses, but also reduces electri-cal noise. Voltage regulation uses tradi-tional techniques, using a TL431. Theoptocoupler current adds to the shuntcurrent. Because the MOSFET turns onwhen current is zero, the gate resistormay be high, so parasitic capacitances

charge slowly, further reducing switch-ing noise. The circuit around Q

4is op-

tional; you can use it in most power sup-

plies. It kills the current glitch when Q3turns on. It is more effective than theusual RC circuit,and it allows a low dutycycle at low loads. Note that many of thecomponent values in Figure 1 are un-designated; you should determine thesevalues to fit the application.

When I was recently debugging adesign, I discovered that a shortcircuit existed from a ground

plane to a power plane.I did not have ac-cess to a milliohmmeter or an equivalenttester for locating this type of short cir-cuit.So, I logged onto the Internet to findan easily constructible milliohmmeter. I

found the answer in a manufacturersdata sheet,which outlined the basic four-wire method of making low-resistancemeasurements. The method uses a volt-age-reference IC as the input stage for acontrolled constant-current source. Aquick dig in the old component bucketrevealed a supply of LM317 variable-volt-

age regulators. These ICs provide 1.25Vbetween their VOUT

and VADJ

terminals, aconstant voltage to attack the constant-current problem. The other problem toattack was the output-voltage range ofthe constant-current source. The circuitI was working on used a 3.3V supply, soI had to limit the voltage to 3.3V. An

Simple circuit serves as milliohmmeterAM Hunt, Lancaster Hunt Systems Ltd, Shepperton, UK

(continued from pg 74)

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LM317, configured as aconstant-current source,delivers an output voltage

equal to the in-put if the outputresistance is too high. Be-cause I wanted to use abench supply or a 9V bat-tery, the voltage would fryany 3.3V logic on theboard. Ideally, I wantedvoltage to be limited to1.5V. So,I came up with theconfiguration in Figure 1.

IC1

controls the base ofthe npn Darlington transis-

tor,Q1. The IC regulates thevoltage across the selected resistor to formthe constant-current source. The currentsource delivers either 10 or 100 mA, de-pending on which emitter resistor is in thecircuit.The purpose of S

1is to give longer

battery life. You can calibrate the currentsource by strapping a resistive load be-

tween test points A and B and measuringthe voltage across the resistor using aDVM (digital voltmeter). I used 5 and10 and set one S

2position for 10 mA

and the other for 100 mA. To measure asmall resistance, you attach test points Aand B across the resistance. You set the

DVM on a millivolt range.The DVM reads a voltagethat is proportional to the

resistance under test.If youcalibrate the circuit as sug-gested, then the reading is10/V on the 100-mArange and 100/V on the10-mA range.

To track down pc-boardshort circuits, attach theunit with test points A andB across the suspectedshorted signals.Attach oneDVM probe to test point A

and use the other to

probe the circuit. Con-stant voltage along a trace indicates thatno current is flowing and that the trace isnot the source of the short circuit. Lookfor high readings on the trace with thelow reading and low readings on the tracewith the high reading, to locate thesource of the short circuit.

LM317

IC1

S1A

S1B

VIN

VOUT

P1

100

Q1

BD636

P2

10

R1

68

R2

6.8

VADJ

+

+

3V

BATTERY

9V

BATTERY

OR BENCH

SUPPLY

A

B

F igure 1

Make your own milliohmmeter, using a voltage-regulator IC and some resistors.

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