4mA 至20mA电路

Contributors:

Richard Markell - Linear T echnology

4-20 mA Circuit

Note: APP-105

Revised: August 1999

INTRODUCTION

A signal conditioning circuit for a two-wire 4 - 20 mA transmitter is shown in Figure 1. T wo-wire transmitters are used when the pressure sensor is far away from its associated display or meter, since transmitters are unaffected by voltage drops along the supply and signal lines.

CIRCUIT DESCRIPTION

The two-wire operation is achieved by referencing all signals to the emitter of transistor Q2. Feedback resistors R 11 - R 12 control the voltage across resistor R 14, drawing a constant current from the input terminals.

CALIBRATION

Zero adjustment is achieved by applying the voltage at the slider of potentiometer P 1 to the second differential input of amplifier A 4, created by resistors R 7 - R 10. Span calibration is realized by bridge current change using potentiometer P 2.

FREQUENCY RESPONSE

Frequency response may be shaped by capacitors C 1, C 2 with a 3dB frequency , f=1/(2πR 11C 2), where C 1=C 2, R 11=R 12, and f is measured in Hz.

VOLTAGE REGULATOR

An optional voltage regulator provides reverse polarity protection (CR1), extends the maximum supply voltage limit and eliminates the dependency of the output current on the excitation voltage.

ADDITIONAL INFORMATION

A detailed discussion of the temperature compensation circuit can be found in Application Note TN-001, “Signal Conditioning For IC Pressure Sensors.”

Figure 1. Transducer Circuit - T

wo Wire Current Transmitter

Application Notes

4-20 mA Circuit