MPXM2102AS datasheet(346/670 Pages) MOTOROLA

346 / 670 page

3–200

Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1082

Simple Design for a 4-20 mA Transmitter

Interface Using a Motorola Pressure Sensor

Prepared by: Jean Claude Hamelain

Motorola Toulouse Application Lab Manager

INTRODUCTION

Pressure is a very important parameter in most industrial

applications such as air conditioning, liquid level sensing and

flow control.

In most cases, the sensor is located close to the measured

source in a very noisy environment, far away from the receiver

(recorder, computer, automatic controller, etc.)

The transmission line can be as long as a few hundred

meters and is subject to electromagnetic noise when the

signal is transmitted as voltage. If the signal is transmitted as

a current it is easier to recover at the receiving end and is less

affected by the length of the transmission line.

The purpose of this note is to describe a simple circuit which

can achieve high performance, using standard Motorola

pressure sensors, operational amplifiers and discrete

devices.

PERFORMANCES

The following performances have been achieved using an

MPXV2102DP Motorola pressure sensor and an MC33079

quad operational amplifier. The MPXV2102DP is a 100 kPa

temperature compensated differential pressure sensor. The

load is a 150 ohm resistor at the end of a 50 meter telephone

line. The 15 volt power supply is connected at the receiver

end.

Power Supply

+15 Vdc, 30 mA

Connecting Line

3 wire telephone cable

Load Resistance

150 to 400 Ohms

Temperature Range

– 40 to + 85

°C (up to +125°C

with special hardware)

Pressure Range

0 to 100 kPa

Total Maximum Error

Better than 2% full scale

Basic Circuit

The Motorola MPXV2102DP pressure sensor is a very high

performance piezoresistive pressure sensor. Manufacturing

technologies include standard bipolar processing techniques

with state of the art metallization and on–chip laser trim for

offset and temperature compensation.

This unique design, coupled with computer laser trimming,

gives this device excellent performance at competitive cost

for demanding applications such as automotive, industrial or

healthcare.

MC33078, 79 operational amplifiers are specially designed

for very low input voltage, a high output voltage swing and very

good stability versus temperature changes.

First Stage

The Motorola MPXV2102 and the operational amplifier are

directly powered by the 15 Vdc source. The first stage is a

simple true differential amplifier made with both of the

operational amplifiers in the MC33078. The potentiometer,

RG, provides adjustment for the output.

Current Generator

The voltage to current conversion is made with a unity gain

differential amplifier, one of the four operational amplifiers in

an MC33079. The two output connections from the first stage

are connected to the input of this amplifier through R3 and R5.

Good linearity is achieved by the matching between R3, R4,

R5 and R6, providing a good common mode rejection. For the

same reason, a good match between resistors R8 and R9 is

needed.

The MC33078 or MC33079 has a limited current output;

therefore, a 2N2222 general purpose transistor is connected

as the actual output current source to provide a 20 mA output.

To achieve good performance with a very long transmission

line it may be necessary to place some capacitors (C1, C2)

between the power supply and output to prevent oscillations.

Calibration

The circuit is electrically connected to the 15 Vdc power

supply and to the load resistor (receiver).

The high pressure is connected to the pressure port and the

low pressure (if using a differential pressure sensor), is

connected to the vacuum port.

It is important to perform the calibration with the actual

transmission line connected.

The circuit needs only two adjustments to achieve the

4 – 20 mA output current.

1. With no pressure (zero differential pressure), adjust Roff

to read exactly 4 mA on the receiver.

2. Under the full scale pressure, adjust RG to exactly read

20 mA on the receiver. The calibration is now complete.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 2

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS датащи(PDF) 346 Page - Motorola, Inc

MPXM2102AS датащи(HTML) 346 Page - Motorola, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL