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Motorola Sensor Device Data
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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
= 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 s the reference pressure. Note that (VFSO
VOFF/PMAX PREF) can be hought of as he slope of he ine
and VOFF as they y–intercept. The slope is also referred to as
the sensitivity, S, of the sensor.
PREF)]*P
Figure 1. Definition of Span, Full–Scale Output,
Offset and Sensitivity
SENSOR OUTPUT
VFSO
VOFF
PREF
PMAX
PRESSURE
SPAN
S
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
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 withoutaffecting the
sensitivity. We can therefore correct this error by sampling the
pressure at zero reference pressure (atmosphere) and
subtracting this from the sensor output.
VOFF)/S.
Figure 2. Effect of Offset Errors
SENSOR OUTPUT VOUT
VFSO
VOFF
PREF(atm
PMAX
PRESSURE
VOFF
VP
P
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.
SEMICONDUCTOR APPLICATION NOTE
REV 1
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Freescale Semiconductor, Inc.
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