MPXM2102AS datasheet(334/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN935

Compensating for Nonlinearity in the MPX10

Series Pressure Transducer

Prepared by: Carl Demington

Design Engineering

INTRODUCTION

This application note describes a technique to improve the

linearity of Motorola’s MPX10 series (i.e., MPX10, MPXV10,

and MPX12 pressure sensors) pressure transducers when

they are interfaced to a microprocessor system. The

linearization technique allows the user to obtain both high

sensitivity and good linearity in a cost effective system.

The MPX10, MPXV10 and MPX12 pressure transducers

are semiconductor devices which give an electrical output

signal proportional to the applied pressure over the pressure

range of 0–10 kPa (0–75 mm Hg). These devices use a unique

transverse voltage–diffused silicon strain–gauge which is

sensitive to stress produced by pressure applied to a thin

silicon diaphragm.

One of the primary considerations when using a pressure

transducer is the linearity of the transfer function, since this

parameter has a direct effect on the total accuracy of the

system, and compensating for nonlinearities with peripheral

circuits is extremely complicated and expensive. The purpose

of this document is to outline the causes of nonlinearity, the

trade–offs that can be made for increased system accuracy,

and a relatively simple technique that can be utilized to

maintain system performance, as well as system accuracy.

ORIGINS OF NONLINEARITY

Nonlinearity in semiconductor strain–gauges is a topic that

has been the target of many experiments and much

discussion. Parameters such as resistor size and orientation,

surface impurity levels, oxide passivation thickness and

growth temperatures, diaphragm size and thickness are all

contributors to nonlinear behavior in silicon pressure

transducers. The Motorola X–ducer was designed to minimize

these effects. This goal was certainly accomplished in the

MPX2000 series which have a maximum nonlinearity of 0.1%

FS. However, to obtain the higher sensitivity of the MPX10

series, a maximum nonlinearity of

±1% FS has to be allowed.

The primary cause of the additional nonlinearity in the MPX10

series is due to the stress induced in the diaphragm by applied

pressure being no longer linear.

One of the basic assumptions in using semiconductor

strain–gauges as pressure sensors is that the deflection of the

diaphragm when pressure is applied is small compared to the

thickness of the diaphragm. With devices that are very

sensitive in the low pressure ranges, this assumption is no

longer valid. The deflection of the diaphragm is a considerable

percentage of the diaphragm thickness, especially in devices

with higher sensitivities (thinner diaphragms). The resulting

stresses do not vary linearly with applied pressure. This

behavior can be reduced somewhat by increasing the area of

the diaphragm and consequently thickening the diaphragm.

Due to the constraint, the device is required to have high

sensitivity over a fairly small pressure range, and the

nonlinearity cannot be eliminated. Much care was given in the

design of the MPX10 series to minimize the nonlinear

behavior. However, for systems which require greater

accuracy, external techniques must be used to account for this

behavior.

PERFORMANCE OF AN MPX DEVICE

The output versus pressure of a typical MPX12 along with

an end–point straight line is shown in Figure 1. All nonlinearity

errors are referenced to the end–point straight line (see data

sheet). Notice there is an appreciable deviation from the

end–point straight line at midscale pressure. This shape of

curve is consistent with MPX10 and MPXV10, as well as

MPX12 devices, with the differences between the parts being

the magnitude of the deviation from the end–point line. The

major tradeoff that can be made in the total device

performance is sensitivity versus linearity.

Figure 2 shows the relationship between full scale span and

nonlinearity error for the MPX10 series of devices. The data

shows

the

primary

contribution

nonlinearity

nonproportional stress with pressure, while assembly and

packaging stress (scatter of the data about the line) is fairly

small and well controlled. It can be seen that relatively good

accuracies (<0.5% FS) can be achieved at the expense of

reduced sensitivity, and for high sensitivity the nonlinearity

errors increase rapidly. The data shown in Figure 2 was taken

at room temperature with a constant voltage excitation of

3.0 volts.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 3

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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