4-20mA Basics and2-Wire vs. 3-Wire Transmitters

Material Created by Scott Hill

Presented by Ian Williams

May 23, 2011

Background

• Prior to the 1950s, control of industrial sites was achieved with pneumatic systems

• 3 psi – 15 psi was used as the standard signal span– 3 psi 0%– 15 psi 100%– 3 psi used as a “live zero” – < 3 psi considered a fault condition or “dead zero”

• The 4-20mA current loop was developed to emulate the old 3-15 psi pneumatic system

Typical 4-20mA Applications

°T

XTRX

TR

3-WireXTR111

2-WireXTR112

RL

– Temperature, pressure, flow

– 2-wire transmitter

– Valve, actuator, heater– 3-wire transmitter

Transmit control signals from a control station out to a remote device.

Report a process variable from a remote sensor to a control station.

• 4mA represents 0% input level– Allows up to 4mA to power external input circuitry– 4mA zero level allows under-scale settings and fault detection

• 20mA represents 100% input level– Provides sufficient current to power electromechanical devices– Over-scale can also be used to detect fault conditions

SensorReceiver

250Ω

0 to 100% Signal

4 to 20mA Signal

Transmitter 1V to 5V

4-20mA Overview

• Immunity to noise– Multiple unknown noise sources can exist between

transmitter and receiver – Low impedance system prevents noise from impacting

the accurate regulation of loop current

• Long distance transmission– Signals must often travel distances > 1 mile– Impedance of long wires would severely attenuate a

voltage signal – 4-20mA current loops are lossless, even over long

distances• Kirchoff’s Current Law states that the current in a loop is equal

at any point within the loop

Why Use Current Transmitters?

T

Signal Conversion/Conditioner

Circuitry

Current Output

Current Transmitter Function

• Linearity errors on most sensors creates an unacceptable error.

Line

arity

(%

)

Temperature (°C)

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0

0.2

0 200 400

RTD Input

Ou

tpu

t

+ Full Scale

- Full Scale

Ideal Output

Bridge

Line

arity

(%)

Temperature (°C)-50 50 100

Thermistor

Why Is Signal Conditioning Required?

RL VOUTXTR+VS

IOUT4-20mA

>1000 feet

-

+

250typ 1V-5V typ

+24V Typical+7V to +44V 50 - 60 Hz

Noise

2-Wire Transmitter

3-Wire Transmitter

VLOOP

RL

+8 V to +40V+24 V typical

VOUT

XTR

+VS

IOUT4-20mA

> 1000 feet

-

+250typ 1V-5V typ

50-60 Hz Noise

Transmitter Type Diagrams

2-Wire vs. 3-Wire Transmitter

• Application-specific parameters determine which transmitter type should be used– Accessibility and location– System power requirements– Sensor input or voltage/current input

2-Wire vs. 3-Wire Transmitter

• 2-Wire Transmitter (Loop powered)– Transmitter and sensor remotely located– Local power supply not practical– Input circuitry floats with respect to loop supply ground

• 3-Wire Transmitter (Locally powered)– Transmitter located close to power supply– Input is referenced to power supply ground– Also known as a voltage-to-current converter

Common 2-Wire Transmitter Applications

• 2-Wire Transmitter– Submersible temperature sensor

– Remote location prevents local power supply

– Sends data back to control station

• 3-Wire Transmitter– Sends control signal to element

at remote location

– Local power supply is available

°T

XTR

XT

R

3-WireXTR111

2-WireXTR112

RL

Implementation Of Current Transmitters

2-Wire Transmitters

• 4-20mA span is 16mA• XTR117 has current gain of

100– Input span is 160µA

• Offset current needed if input signal reaches zero

+

-

XTR117

R1

IIN

IRET

A1

2

3

8 VREG

IO = 100 * IIN

2.475 kΩ

+5V

40uA

0 to 160μA

125k

Ω

IIN

Offset Current

2-Wire Typical Input Scaling

XTR117

IIN

IRET

2

3

8

VREG

IOUT DAC

XTR117

IIN

IRET

2

3

8

VREG

VOUT DAC

XTR117

IIN

IRET

2

3

8

VREGREF3140

Input Circuitry

2-Wire Typical Input Interface

+

-

+5V Regulator

XTR117

RLIM

R1 R2

IIN

IRET

A1

E

B

VREG

2.475 kΩ 25Ω

100Ω

1kΩ

8kΩ

V+

IO

• IRET is not GND

• Current Input• Current Output

• Current gain set by R1 and R2

• Regulator and input circuitry floats at IRET potential

Typical 2-Wire Transmitter

XTR117

RIn

VIn

RL

VLoop

IRET

IIN

IOUT

V+

2-Wire Transmitter Common Abuse

R1

R2

25 R4 100

V1 25

R7

250

A+

I LOOP

-

++

4

3

5

1

2

R3

125k

IIN 160u

XTR117

2.47

5k

40uA

20mA

5V Regulator

Grounded Loop Supply with Floating Input Source

R1

R2

25 R4 100

V1 25

R7

250

I LOOP

-

++

4

3

5

1

2

R3

125k

IIN 160u

IR1 IR2

XTR117

2.475k

40uA

19.8mA200uA20mA

5V RegulatorIOFFSET

ILOOP = IR1 + IR2

IR2 = IR1 * 99

IR1 = IIN Total

IIN Total = IIN + IOffset

Internal Current Paths

VR1 = VR2

Equal Path Voltages Created

R1

R2

25 R4 100

V1 25

RL

250

-

++

4

3

5

1

2

R3

125k

I IN 160u

IR1 IR2 V Float

ILOOP

V IRET

XTR117

2.475k

40uA

5.495V

20mA5V19.8mA200uA

5V Regulator

IRET

VFLOAT = ILOOP * RL

VR2 = R2 * IR2

V IRET = VFLOAT + VR2

V IRET = 5.495V

Calculating IRET Level

R1

R2

25 R4 100

V1 25

RL

250

-

++

4

3

5

1

2

R3

125k

I IN 0

IR1 IR2 V FloatILOOP

V IRET

XTR117

2.475k

40uA

1.099V

4mA1V3.96mA40uA

5V Regulator

IRET

VFLOAT = ILOOP * RL

VR2 = R2 * IR2

V IRET = VFLOAT + VR2

V IRET = 1.099V

V IRET

V FloatIR2IR1

I IN160u

R3

12

5k

-

++

4

3

5

1

2

R7

25

0

VLoop 25V

R4 100

R2

25

R1

AM2 AM3

AM5

40uA

2.475k

XTR117

40uA

19.96mA160uA

200uA 19.8mA 5V

20mA

5.495V

5V Regulator

ILOOP

V IRET

ILOOP

V FloatIR2IR1

I IN160u

R3

12

5k

-

++

4

3

5

1

2

R7

25

0

VLOOP 25V

R4 100

R2

25

R1

AM3

AM5

40uA

2.475k

XTR117

36.082uA

40.64mA40.82mA

196.08uA-178.25uA

4.45mV

17.82uA

0V

5V Regulator

AM2

Effects of Grounding IRET

IRET Floating IRET Grounded

3-Wire Transmitters

+

-

XTR111 OD

A1

GND

VIN

VSP

3V

+

-

Current Mirror

+

-

EF

ISET

REGF

REGS

IS

VG

RSET

• Input and Output are referenced to GND

• Voltage Input• Current Output

• Current gain set by VIN and RSET

• Regulator referenced to GND

Typical 3-Wire Transmitter

+

-

XTR111 OD

A1

GND

VSP

+

-

Current Mirror

+

-

EF

ISET

+24V

REG F

SREG

3V

REF3140

VIN

VOUT DACμC

+5V

2kΩ

3kΩ

47

0nF

2kΩRSET

15Ω

LOAD

15Ω

Q1

Q2

VG

IS

10

nF

Short-CircuitProtection

XTR111 Typical Input Interface

2-Wire General Purpose

XTR101 I_IN to I_OUT Converter, Current Excitation

XTR115 I_IN to I_OUT Converter, 5V Regulator, 2.5V Reference

XTR116 I_IN to I_OUT Converter, 5V Regulator, 4.096V Reference

XTR117 I_IN to I_OUT Converter, 5V Regulator

2-Wire RTD Conditioner

XTR105 RTD Conditioner, 800uA Current Sources

XTR108 RTD Conditioner, Digital Calibration, Input Mux

XTR112 RTD Conditioner, 250uA Current Sources

XTR114 RTD Conditioner, 100uA Current Sources

2-Wire Bridge Conditioner XTR106 Bridge Conditioner, Linearization Correction

3-Wire General Purpose

XTR110 Selectable Input/Output Ranges, Voltage to Current Converter

XTR111 Configurable Input/Output Ranges, Voltage to Current Converter

XTR300 Configurable Input/Output Ranges, Current/Voltage Output Mode

4-20mA Receiver RCV420 4-20mA input, 0-5V Output

4-20mA Product Family

Thank You!