Analog Input 2-wire 4-20mA TransmittersTI Precision Labs – Current Loop Transmitters

Presented by Katlynne Jones

Prepared by Katlynne Jones

1

Transmitter

RIn

VIn

IRET

IIN

IOUT

V+

+

_+

_

+

_Receiver

Power

supply

Supply &

Signal Current

VLOOP

Sensor + Input

Circuitry

VOUT

GND

V+ VREG

2-wire transmitter

• 2-wire transmitter

block diagram

– 2 wires create a

loop that transmits

the signal current

and transmitter

power

– IRET cannot be

grounded to VLOOP!

– Transmitter,

sensor, and input

circuitry must

consume <4mA

2

DAC

RIN

ROS

+

R2 R1

VREG

Regulator

RLIM

Loop+

Loop-

Q1

VDAC

RLOAD

VLOOP

IRET

IDAC_IN = VDAC/RIN

IOS = VREG/ROS

IDAC

IREG

IOPA

IBJT

IOUT = IIN + IRET + IBJT

IIN = IDAC_IN + IOS

IRET = IDAC + IOPA + IREG

0A

IRET + IBJT

A1

Vcc

GND

Derivation of transfer function

V+

V-

V+ = V−

V+ = IIN ∗ R1

V− = (IRET + IBJT) ∗ R2

IBJT + IRET =IIN ∗ R1R2

IOUT =VDACRIN

+VREGROS

∗ 1 +R1R2

3

IIN =VDACRIN

+VREGROS

IIN ∗ R1 = (IRET + IBJT) ∗ R2

RIN

+

2475Ω

VREG_5V V+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

VREF_4.096V

IIN

IRET RLIM

10 nF

XTR116

Loop+

Loop-

+

-

Ref

+

INA333

143kΩ

19.6kΩ

RG2.55kΩ10kΩ

12.49kΩ

OPA333

+

-

+

VREF_4.096V

VREF_4.096V

VREG_5V

VREF_0.5V

RADJUST

50Ω

RCOMP

100nF

CHF

10µF

CLF

IRET

IRET

IRET

10nF

10nF

A1

25ΩR1 R2

Transmitter input

4

Transmitter input

5

RIN

+

2475Ω

VREG_5V V+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

25Ω

VREF_4.096V

IIN

IRETRLIM

10 nF

XTR116

Loop+

Loop-

25kΩ

ROS

A1

R1 R2

102.4kΩ

0-4V

IRET

+

2475Ω

VREG_5V V+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

VREF_4.096V

IIN

IRETRLIM

10 nF

XTR116Loop+

Loop-

ROS

A1

R1

102.4kΩ

ISOURCE

VLOOP

RLOAD

250Ω

+_

VIRET

+

_

25ΩR2

+_

+_

+VR2

VLOAD

VIRET = VR2 + VLOAD

V+ = VR2 = IIN ∗ R1

V+ = V− = VR1 = VR2

VLOAD = IOUT ∗ RLOAD

IOUT = ISOURCE +VREFROS

∗ 1 +R1R2

IIN = ISOURCE +VREFROS

6

IRET

+

2475Ω

VREG_5V V+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

25Ω

VREF_4.096V

IIN

IRETRLIM

10 nF

XTR116Loop+

Loop-

ROS

A1

R1 R2

102.4kΩ

ISOURCE

VLOOP

RLOAD

250Ω

+_

0A

4mAVIRET

+

_

1.099V

+_

+_

+0.099V

VR2

1V

VLOAD

VIRET = 0.099V + 1V = 1.099V

VIRET = VR2 + VLOAD

VR2 = 40µA ∗ 2475Ω = 0.099V

V+ = VR2 = IIN ∗ R1

V+ = V− = VR1 = VR2

VLOAD = 4mA ∗ 250Ω = 1V

VLOAD = IOUT ∗ RLOAD

IOUT = 40µA ∗ 1 +2475Ω

25Ω= 4mA

IOUT = ISOURCE +VREFROS

∗ 1 +R1R2

IIN = 0A +4.096V

102.4kΩ= 40µA

IIN = ISOURCE +VREFROS

7

IRET

+

2475Ω

VREG_5V V+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

25Ω

VREF_4.096V

IIN

RLIM

10 nF

XTR116Loop+

Loop-

ROS

A1

R1 R2

102.4kΩ

ISOURCE

VLOOP

RLOAD

250Ω

+_

160µA

+_

20mA

+_

+0.495V

VR2

5V

VLOAD

IRET

VIRET

+

_

5.495V

VIRET = 0.495V + 5V = 5.495V

VIRET = VR2 + VLOAD

VR2 = 200µA ∗ 2475Ω = 0.495V

V+ = VR2 = IIN ∗ R1

V+ = V− = VR1 = VR2

VLOAD = 20mA ∗ 250Ω = 5V

VLOAD = IOUT ∗ RLOAD

IOUT = 200µA ∗ 1 +2475Ω

25Ω= 20mA

IOUT = ISOURCE +VREFROS

∗ 1 +R1R2

IIN = 160µA +4.096V

102.4kΩ= 200µA

IIN = ISOURCE +VREFROS

8

Compliance voltage

DACA1

RIN

ROS

+

R2 R1

VREG

Regulator

RLIM

Loop+

Loop-

Q1

VDAC

RLOAD

VLOOP

+

_

VREG

+

_

VCOMP

Vcc

GND

IRET

Compliance voltage needs to be met so the regulator can supply the op amp with enough voltage.

9

Compliance voltage

+

2475Ω

VREG_5VV+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

25Ω

VREF_4.096V

IIN

IRETRLIM

10 nF

XTR116

A1

R1 R2

VLOOP

RLOAD

500Ω

24V

200ΩRWIRE

200ΩRWIRE

+_

+_

+ _

+_+

_

V+

VLOOP ≥ VCOMP + IOUT_MAX ∗ RMAX

V+ = VLOOP − IOUT_MAX ∗ R

V+ = 24V − 20mA ∗ 900Ω = 6V

RMAX =VLOOP − VCOMP

IOUT_MAX

RMAX =24V − 7.5V

20mA= 825Ω

10

4mA Current budget

RIN

+

2475Ω

VREG_5V V+

IO

V+

Q1

E

B

5V

Regulator

4.096V

Reference

25Ω

VREF_4.096V

IIN

IRET RLIM

10 nF

XTR116

Loop+

Loop-

25kΩ

ROS

A1

R1 R2

102.4kΩ

IOPA

IREG

IREF

IBJT

IOUT = IIN + IRET + IBJT

IIN

11

Input isolation

12

+ _

Receiver

+ _

Receiver

+ _

Power

supply

VLOOP

2- wire

Transmitter

(XTR116)

IRET

IIN

IOUT

V+

2- wire

Transmitter

(XTR116)

IRET

IIN

IOUT

V+

VREGVREF

250Ω

250Ω

VREGVREF

MCU

GND

ADC1+

ADC1-

ADC2+

ADC2-

Sensor2

CS2

SCLK2

SDO2

V+

CS1

SCLK1

SDO1

DAC

V+ VREF

GND

VOUT

/CS

SCLK

DIN

DAC

V+ VREF

GND

VOUT

/CS

SCLK

DIN

Sensor1

RIn

RIn

4mA

20mA

1.1V

5.5V

Input isolation

13

+ _

Receiver

+ _

Receiver

+ _

Power

supply

VLOOP

2- wire

Transmitter

(XTR116)

RIn

IRET

IIN

IOUT

V+

2- wire

Transmitter

(XTR116)

RIn

IRET

IIN

IOUT

V+

VREGVREF

250Ω

250Ω

VREGVREF

VCC1INA

INB

INC

GND1

VCC2OUTA

OUTB

OUTC

GND2

ISO7330

VCC1INA

INB

INC

GND1

VCC2OUTA

OUTB

OUTC

GND2

ISO7330

VISO

VISO

MCU

GND

ADC1+

ADC1-

ADC2+

ADC2-

CS2

SCLK2

SDO2

V+

CS1

SCLK1

SDO1

DAC

V+ VREF

GND

VOUT

/CS

SCLK

DIN

DAC

V+ VREF

GND

VOUT

/CS

SCLK

DINSensor1

Sensor2

2

1

1

Input isolation

+_

Power

supply

VISO

+ _

Receiver

+ _

Receiver

+ _

Power

supply

VLOOP

2- wire

Transmitter

(XTR116)

RIn

IRET

IIN

IOUT

V+

2- wire

Transmitter

(XTR116)

RIn

IRET

IIN

IOUT

V+

VREGVREF

250Ω

250Ω

VREGVREF

VCC1INA

INB

INC

GND1

VCC2OUTA

OUTB

OUTC

GND2

ISO7330

VCC1INA

INB

INC

GND1

VCC2OUTA

OUTB

OUTC

GND2

ISO7330

VISO

VISO

MCU

GND

ADC1+

ADC1-

ADC2+

ADC2-

CS2

SCLK2

SDO2

V+

CS1

SCLK1

SDO1

DAC

V+ VREF

GND

VOUT

/CS

SCLK

DIN

DAC

V+ VREF

GND

VOUT

/CS

SCLK

DINSensor1

Sensor2

1

2

14

XTR partsPart Description Loop

Voltage (V)

Full-Scale Input

Range

Reference Regulator Gain

XTR101 IA + Current Excitation 11.6-40 Settable Current

reference

- Settable

XTR115 IIN to IOUT Converter.

External Resistor Scales

VIN to IIN

7.5-36 40μA to 200μA 2.5V 5V 100A/A

XTR116 IIN to IOUT Converter.

External Resistor Scales

VIN to IIN

7.5-36 40μA to 200μA 4.096V 5V 100A/A

XTR117 IIN to IOUT Converter.

External Resistor Scales

VIN to IIN

7.5-40 40μA to 200μA - 5V 100A/A

15

XTR partsPart Description Sensor

Excitation

Loop

Voltage (V)

Nonlinearity

Correction

Regulator Gain

XTR105 100Ω RTD Conditioner Two 800μA 7.5-36 40:1 improvement 5.1V Settable

XTR112 1kΩ RTD Conditioner Two 250μA 7.5-36 40:1 improvement 5.1V Settable

XTR106 Bridge Conditioner 5V and

2.5V

7.5-36 20:1 improvement 5.1V Settable

XTR108 Smart Programmable RTD

or Bridge Conditioner

Settable 7.5-36 40:1 improvement 5.1V Settable

16

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