MPXM2102AS datasheet(521/670 Pages) MOTOROLA

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AN1636

Implementing Auto Zero for Integrated

Pressure Sensors

Prepared by Ador Reodique

Motorola Sensor Systems and Applications Engineering

INTRODUCTION

This application note describes how to implement an auto–

zero function when using a Motorola integrated pressure sen-

sor with a microcontroller and an analog to digital converter

(MCU and an A/D). Auto–zero is a compensation technique

based on sampling the offset of the sensor at reference pres-

sure (atmospheric pressure is a zero reference for a gauge

measurement) in order to correct the sensor output for long–

term offset drift or variation.

Sources of offset errors are due to device to device offset vari-

ation (trim errors), mechanical stresses (mounting stresses),

shifts due to temperature and aging. Performing auto–zero will

greatly reduce these errors. The amount of error correction is

limited by the resolution of the A/D.

In pressure sensing applications where a zero–pressure

reference condition can exist, auto–zero can be implemented

easily when an integrated pressure sensor is interfaced to

an MCU.

EFFECTS OF OFFSET ERRORS

Figure 1 illustrates the transfer function of an integrated

pressure sensor. It is expressed by the linear function:

VOUT =VOFF + [(VFSO *VOFF)/(PMAX *PREF)]*P

=VOFF + S*P.

Here, VOUT is the voltage output of the sensor, VFSO is the

full–scale output, VOFF is the offset, PMAX is the maximum

pressure and PREF is the reference pressure. Note that (VFSO

*VOFF/PMAX*PREF)canbethoughtofastheslopeoftheline

and VOFF as they y–intercept. The slope is also referred to as

the sensitivity, S, of the sensor.

Figure 1. Definition of Span, Full–Scale Output,

Offset and Sensitivity

SENSOR OUTPUT

VFSO

VOFF

PREF

PMAX

PRESSURE

SPAN

A two–point pressure calibration can be performed to

accurately determine the sensitivity and get rid of the offset

calibration errors altogether. However, this can be very

expensive in a high volume production due to extra time and

labor involved. The system designer therefore designs a

pressure sensor system by relying on the sensitivity and offset

data given in the data sheet and using a linear equation to

determine the pressure. Using the later, the sensed pressure

is easily determined by:

P = (VOUT *VOFF)/S.

If an offset error is introduced due to device to device varia-

tion, mechanical stresses, or offset shift due to temperature

(the offset has a temperature coefficient or TCO), those errors

will show up as an error,

∆P, in the pressure reading:

P +

∆P = [VOUT *(VOFF + ∆VOFF)]/S.

As evident in Figure 2, offset errors,

∆VOFF, have the effect

of moving the intercept up and down

without affecting the

sensitivity.

We can therefore correct this error by sampling the

pressure at zero reference pressure (atmosphere) and

subtracting this from the sensor output.

Figure 2. Effect of Offset Errors

SENSOR OUTPUT VOUT

VFSO

VOFF

PREF(atm)

PMAX

PRESSURE

∆VOFF

∆P

AUTO–ZERO CONSIDERATIONS

IN APPLICATIONS

There is an important consideration when implementing

auto–zero. In order to use this technique,

a zero pressure

reference condition must be known to exist in the system.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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