MPXM2102AS datasheet(415/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1318

Interfacing Semiconductor Pressure Sensors

to Microcomputers

Prepared by: Warren Schultz

Discrete Applications Engineering

INTRODUCTION

The

most

popular

silicon

pressure

sensors

are

piezoresistive bridges that produce a differential output

voltage in response to pressure applied to a thin silicon

diaphragm. Output voltage for these sensors is generally 25

to 50 mV full scale. Interface to microcomputers, therefore,

generally involves gaining up the relatively small output

voltage, performing a differential to single ended conversion,

and scaling the analog signal into a range appropriate for

analog to digital conversion. Alternately, the analog pressure

signal can be converted to a frequency modulated 5 V

waveform or 4–20 mA current loop, either of which is relatively

immune to noise on long interconnect lines.

A variety of circuit techniques that address interface design

are

presented.

Sensing

amplifiers,

analog

digital

conversion, frequency modulation and 4–20 mA current loops

are considered.

PRESSURE SENSOR BASICS

The essence of piezoresistive pressure sensors is the

Wheatstone bridge shown in Figure 1. Bridge resistors RP1,

RP2, RV1 and RV2 are arranged on a thin silicon diaphragm

such that when pressure is applied RP1 and RP2 increase in

value while RV1 and RV2 decrease a similar amount.

Pressure on the diaphragm, therefore, unbalances the bridge

and produces a differential output signal. One of the

fundamental properties of this structure is that the differential

output voltage is directly proportional to bias voltage B+. This

characteristic implies that the accuracy of the pressure

measurement depends directly on the tolerance of the bias

supply. It also provides a convenient means for temperature

compensation. The bridge resistors are silicon resistors that

have positive temperature coefficients. Therefore, when they

are placed in series with zero TC temperature compensation

resistors RC1 and RC2 the amount of voltage applied to the

bridge increases with temperature. This increase in voltage

produces an increase in electrical sensitivity which offsets and

compensates for the negative temperature coefficient

associated with piezoresistance.

Since RC1 and RC2 are approximately equal, the output

voltage common mode is very nearly fixed at 1/2 B

+. In a

typical MPX2100 sensor, the bridge resistors are nominally

425 ohms; RC1 and RC2 are nominally 680 ohms. With these

values and 10 V applied to B

+, a delta R of 1.8 ohms at full

scale pressure produces 40 mV of differential output voltage.

Figure 1. Sensor Equivalent Circuit

RETURN

B+

RC1

RV1

S+

RP1

RP2

RV2

S–

RC2

PRESSURE

INSTRUMENTATION AMPLIFIER INTERFACES

Instrumentation amplifiers are by far the most common

interface circuits that are used with pressure sensors. An

example of an inexpensive instrumentation amplifier based

interface circuit is shown in Figure 2. It uses an MC33274 quad

operational amplifier and several resistors that are configured

as a classic instrumentation amplifier with one important

exception. In an instrumentation amplifier resistor R3 is

normally returned to ground. Returning R3 to ground sets the

output voltage for zero differential input to 0 V DC. For

microcomputer interface a positive offset voltage on the order

of 0.3 to 0.8 V is generally desired. Therefore, R3 is connected

to pin 14 of U1D which supplies a buffered offset voltage that

is derived from the wiper of R6. This voltage establishes a DC

output for zero differential input. The translation is one to one.

Within the tolerances of the circuit, whatever voltage appears

at the wiper of R6 will also appear as the zero pressure DC

offset voltage at the output.

With R10 at 240 ohms, gain is set for a nominal value of 125.

This provides a 4 V span for 32 mV of full scale sensor output.

Setting the offset voltage to .75 V results in a 0.75 V to 4.75 V

output that is directly compatible with microprocessor A/D

inputs. Over a zero to 50

° C temperature range, combined

accuracy for an MPX2000 series sensor and this interface is

on the order of

± 10%.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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