MPXM2102AS datasheet(430/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1325

Amplifiers for Semiconductor Pressure Sensors

Prepared by: Warren Schultz

Discrete Applications Engineering

INTRODUCTION

Amplifiers for interfacing Semiconductor Pressure Sensors

to electronic systems have historically been based upon

classic instrumentation amplifier designs.

Instrumentation

amplifiers have been widely used because they are well

understood

standard

building

blocks

that

also

work

reasonably well.

For the specific job of interfacing

Semiconductor Pressure Sensors to today’s mostly digital

systems, other circuits can do a better job. This application

note presents an evolution of amplifier design that begins with

a classic instrumentation amplifier and ends with a simpler

circuit that is better suited to sensor interface.

INTERFACE AMPLIFIER REQUIREMENTS

Design

requirements

for

interface

amplifiers

are

determined by the sensor’s output characteristics, and the

zero to 5 V input range that is acceptable to microcomputer

A/D converters. Since the sensor’s full scale output is typically

tens of millivolts, the most obvious requirement is gain. Gains

from 100 to 250 are generally needed, depending upon bias

voltage applied to the sensor and maximum pressure to be

measured. A differential to single–ended conversion is also

required in order to translate the sensor’s differential output

into a single ended analog signal. In addition, level shifting is

necessary to convert the sensor’s 1/2 B+ common mode

voltage to an appropriate DC level. For microcomputer A/D

inputs, generally that level is from 0.3 – 1.0 V. Typical design

targets are 0.5 V at zero pressure and enough gain to produce

4.5 V at full scale. The 0.5 V zero pressure offset allows for

output saturation voltage in op amps operated with a single

supply (VEE = 0). At the other end, 4.5 V full scale keeps the

output within an A/D converter’s 5 V range with a comfortable

margin for component tolerances. The resulting 0.5 to 4.5 V

single–ended analog signal is also quite suitable for a variety

of other applications such as bar graph pressure gauges and

process monitors.

CLASSIC INSTRUMENTATION AMPLIFIER

A classic instrumentation amplifier is shown in Figure 1.

This circuit provides the gain, level shifting and differential to

single–ended conversion that are required for sensor

interface. It does not, however, provide for single supply

operation with a zero pressure offset voltage in the desired

range.

–

1 k

* NOTE: FOR MPX2020 R10 = 150 OHMS

R 3 1 k

1 k

R9 15 k

R8 15 k

–

U1A

MC33274

U1B

MC33274

VEE

R10

240*

VCC

OUTPUT

R4 1k

U1C

MC33274

C3 0.001

µF

Figure 1. Classic Instrumentation Amplifier

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 2

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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