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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
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 as 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
μ
V/degree temperature coefficient of
offset voltage, the model will calculate the bridge resistance
values necessary to produce 100 mV of offset voltage and a
10
μ
V/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 of 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
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.
TCR)(Temp – 25)
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
.PARAMETER function, making the models specific to
PSPICE. Parameters are described as follows:
use
PSPICE’s
KP — Pressure constant; translates pressure into a bridge
resistance multiplier
KO — Offset constant; offset component of bridge
resistance
DT — Delta temperature; Temperature
Celsius
25 degrees
KTCO — Temperature coefficient of offset constant; trans-
lates temperature coefficient of offset into bridge resistance
SEMICONDUCTOR APPLICATION NOTE
REV 1
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Freescale Semiconductor, Inc.
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