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Motorola Sensor Device Data
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is desirable since pressure sensors’ full–scale span and
zero–pressure offset voltages will vary somewhat from lot to
lot and unit to unit. During software calibration, each sensor
device’s specific offset and full–scale output characteristics
will be stored. Nonetheless, a variable gain amplifier circuit is
desirable to coarsely tune the sensor’s full–scale span, and a
positive or negative dc level shift (offset pedestal adjustment)
of the pressure sensor signal is needed to translate the
pressure sensor’s signal–conditioned output span to a
specific level (e.g., within the high and low reference voltages
of the A/D converter).
Microcontroller
The microcontroller performs all of the necessary tasks to
give the smart sensor system the specified performance and
intelligent
features.
The
responsibilities:
Creates the control signal to pulse the sensor.
Samples the pressure sensor’s output.
Signal averages a programmable number of samples for
noise reduction.
Samples a scaled–down version of the pressure sensor
supply voltage. Monitoring the power supply voltage allows
the microcontroller to reject sensor output changes result-
ing from power supply variations.
Uses serial communications interface (SPI) to receive com-
mands from and to send sensor information to a master
MCU.
following
describes
its
Resistor Divider for Rejection of Supply Voltage
Variation
Since the pressure sensor’s output voltage is ratiometric to
its supply voltage, any variation in supply voltage will result in
variation of the pressure sensor’s output voltage. By
attenuating the supply voltage (since the supply voltage may
exceed the 5 V range of the A/D) with a resistor divider, this
scaled voltage can be sampled by the microcontroller’s A/D
converter. By sampling the scaled supply voltage, the
microcontroller can compensate for any variances in the
pressure sensor’s output voltage that are due to supply
variations.
This
technique
determination even when the pressure sensor is powered with
an unregulated supply.
allows
correct
pressure
5 V Regulator
A 5 V
±
5% voltage regulator is required for the following
functions:
To provide a stable 5 V for the high voltage reference (VRH)
of the microcontroller’s A/D converter. A stable voltage ref-
erence is crucial for sampling any analog voltage signals.
To provide a stable 5 V for the resistor divider that is used
to level shift the amplified zero–pressure offset voltage.
Low Voltage Inhibit (LVI) Circuitry
Low voltage inhibit circuitry is required to ensure proper
power–on–reset (POR) of the microcontroller and to put the
MCU in a known state when the supply voltage is decreased
below the MCU supply voltage threshold.
SOFTWARE DESCRIPTION
The smart sensor system’s EPROM resident code provides
the control pulse for the sensor’s excitation voltage and
performs calibration with respect to a wide range of excitation
voltages (20 ~ 28 V typically for HVAC). Pressure
measurement averaging is also incorporated to reduce both
signal error and noise. In addition, the availability of a serial
communications interface allows a variety of software
commands to be sent to the smart sensor system.
The following brief outline provides a more detailed
description about the software features included in the smart
sensor system.
Software Calibration and Power Supply Rejection
Only six 8–bit words of information are stored both to
calibrate the smart sensor system for a given sensor device
and to store the relationship between sensor output and power
supply voltage. This information is used to reduce errors due
to device–to–device variations and to reject variations in
power supply voltage that can introduce error into the pressure
measurement. The sensor’s amplified output at the
zero–pressure offset and full–scale pressure are stored at
each of two different supply voltages. In addition, the scaled
and digitized representation of the applied supply voltages is
stored. Compensating for power supply variation in software
allows higher performance with lower tolerance, or even
unregulated, supply voltages. For HVAC applications, where
a 24–Vac line voltage will be simply rectified and filtered to
provide a crude 24–Vdc supply, this approach has major
performance benefits. The impact on applications where a
regulated supply is available is that a lower–cost regulator or
dc–to–dc converter can be used without compromising
system accuracy significantly.
A/D Sample Averaging
Noise inherent to the 8–bit A/D successive approximation
conversion method used by the smart sensor accounts for
±
1–bit resolution. Signal noise, which exhibits a measured
peak–to–peak range larger in magnitude than 1 bit of A/D
resolution, can be minimized by a sample averaging
technique.
The current technique uses 16 A/D converted pressure
samples, sums the result, and divides by 16 (the number of
samples) to get the average:
AVG =
; where n = 16
(1)
(
a
n)
n
n
1
Assuming a gaussian distribution of noise, this averaging
technique improves the signal–to–noise ratio (SNR).
Smart Sensor Unit ID and Software Revision
Level
This solution may be implemented as a single sensing
system using a nondedicated MCU to provide the sensing
function and smart features or as a slaved smart sensor (with
dedicated sensing MCU) that communicates over a serial bus
to a master controller or microprocessor (Host). Part
identification and software revision level can also be read on
request from the master MCU. This information is utilized by
the master MCU to determine what the full–scale pressure
range of a given smart sensor unit is. This allows for multiple
sensor units with different pressure ranges to be controlled
and sensed from a single master MCU.
F
Freescale Semiconductor, Inc.
n
.