MPXM2102AS datasheet(357/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1303

A Simple 4-20 mA Pressure Transducer

Evaluation Board

Prepared by: Denise Williams

Discrete Applications Engineering

INTRODUCTION

The two wire 4 – 20 mA current loop is one of the most widely

utilized transmission signals for use with transducers in

industrial applications. A two wire transmitter allows signal

and power to be supplied on a single wire–pair. Because the

information is transmitted as current, the signal is relatively

immune to voltage drops from long runs and noise from

motors, relays, switches and industrial equipment. The use of

additional power sources is not desirable because the

usefulness of this system is greatest when a signal has to be

transmitted over a long distance with the sensor at a remote

location. Therefore, the 4 mA minimum current in the loop is

the maximum usable current to power the entire control

circuitry.

Figure 1 is a block diagram of a typical 4 – 20 mA current

loop system which illustrates a simple two chip solution to

converting pressure to a 4 – 20 mA signal. This system is

designed to be powered with a 24 Vdc supply. Pressure is

converted to a differential voltage by the Motorola MPX5100

pressure sensor.

The voltage signal proportional to the

monitored pressure is then converted to the 4 – 20 mA current

signal with the Burr–Brown XTR101 Precision Two–Wire

Transmitter. The current signal can be monitored by a meter

in series with the supply or by measuring the voltage drop

across RL. A key advantage to this system is that circuit

performance is not affected by a long transmission line.

SENSOR

PRESSURE

PORT

PRESSURE

SOURCE

PRESSURE

SENSOR

TRANSMITTER

CIRCUITRY

4 – 20 mA PRESSURE TRANSDUCER

TRANSMISSION

LINE

CURRENT

METER

24 VDC

Figure 1. System Block Diagram

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 4

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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