MPXM2102AS datasheet(530/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1660

Compound Coefficient Pressure Sensor

PSPICE Models

Prepared by: Warren Schultz

PSPICE models for Uncompensated, MPX2000 series, and

MPX5000 series pressure sensors are presented here. These

models use compound coefficients to improve modeling of

temperature dependent behavior. The discussion begins with

an overview of how the models are structured, and is followed

by an explanation of compound coefficients. The emphasis is

on how to use these models to estimate sensor performance.

They can be found electronically on a disk included in ASB200

Motorola Sensor Development Controller kits, and on the

WEB at:

http://www.mot–sps.com/home2/models/bin/sensor2.html

MODEL STRUCTURE

Models for all three sensors series share a common

structure. They are complete models set up to run as is. To

obtain output voltage versus pressure, it is only necessary to

run the model and display V(2,4) or V(1,0). V(2,4) gives the

output voltage for Uncompensated and MPX2000 series

sensors. V(1,0) applies to MPX5000 sensors. In both cases,

V(2,4) and V(1,0) correspond to the pin numbers where output

voltage would be, if probed on an actual part.

These models are divided into five sections to facilitate ease

of use. They are:

• INPUT PARAMETERS

• LINEAR TO COMPOUND CONVERSION

• MODEL COEFFICIENTS

• TRANSDUCER

• STIMULUS

Each of these sections is described in the following discussion.

INPUT PARAMETERS

This section contains input parameters that describe

measurable sensor characteristics. Inputs such as full scale

pressure (FSP), full scale span (FSS) offset voltage

(VOFFSET), and temperature coefficient of offset voltage

(TCOS) are made here. Characteristics that are specific to the

transducer,

such

bridge

impedance

(RBRIDGE),

temperature coefficient of bridge resistance (TCRB), and

temperature coefficient of span (TCSP) are also listed here.

Parameters such as VOFFSET that set an output value for

the sensor are used to calculate resistance values that

produce those outputs. For example, if you input 100 mV of

offset voltage and a 10

µV/degree temperature coefficient of

offset voltage, the model will calculate the bridge resistance

values necessary to produce 100 mV of offset voltage and a

µV/degree temperature coefficient.

In the MPX2000 and MPX5000 models, temperature

coefficient of span (TCSP) is handled differently than the other

parameters.

The

non–linear

behavior

span

over

temperature is calculated from the interaction of the

transducer’s temperature coefficient of span (TCSP), the

transducer’s temperature coefficient of resistance (TCRB),

and the effects of inserting fixed resistance, RTCSPAN, in

series with the bridge. The result is a temperature coefficient

of span that closely resembles the real thing, but is not directly

controlled by the user.

LINEAR TO COMPOUND CONVERSION

The compound coefficients used in these models are from

equations of the form:

(1)

R(Temp) = R25(1 ) TCR)(Temp – 25)

where R25 is resistance at 25 degrees Celsius , TCR is

temperature coefficient of resistance, Temp is an abbreviation

for Temperature in degrees Celsius, and R(Temp) is the

function resistance versus temperature.

The TCR (temperature coefficient of resistance) in equation

(1) is a different number than a temperature coefficient that is

stated in linear terms. The three statements in this section

convert linear coefficients to the compound values that the

models need. This conversion is based upon a 100 degree

difference between the two points at which the linear

coefficients have been measured.

MODEL COEFFICIENTS

In this section most of the calculation is performed. Values

for the transducer bridge resistors are determined from

pressure, temperature, offset, temperature coefficient of

offset, span, temperature coefficient of span, and temperature

coefficient of resistance inputs. A series of parameter

statements are used, as much as is practical, to do

calculations that will fit in an 80 character line without

wraparounds.

These

calculations

use

PSPICE’s

.PARAMETER function, making the models specific to

PSPICE. Parameters are described as follows:

KP — Pressure constant; translates pressure into a bridge

resistance multiplier

KO — Offset constant; offset component of bridge

resistance

DT — Delta temperature; Temperature

*25 degrees

Celsius

KTCO — Temperature coefficient of offset constant; trans-

lates temperature coefficient of offset into bridge resistance

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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