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Motorola Sensor Device Data
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high–side switch pulsing circuitry
signal–conditioning amplifier interface with resistors to ad-
just the sensor’s amplified, full–scale span and zero–pres-
sure offset
on–chip resources of a complete 8–bit microcontroller
(MCU)
MCU oscillator circuitry (4 MHz)
5 V
±
5% linear voltage regulator
low–voltage inhibit (LVI) supervisory voltage monitoring cir-
cuit
resistor divider connected to the sensor’s power supply bias
to sense the excitation voltage across the sensor
These subsystems are explained as follows to provide an
understanding of the system design and its intelligent features
(refer to Figure 2).
Pulsing Circuitry
As previously mentioned, the sensor’s output is ratiometric
to the excitation voltage across the sensing element; the
sensor’s sensitivity increases with increasing supply voltage.
Thus, to detect low pressures and minute changes in
pressure, it is desirable to operate the sensor at the highest
possible excitation voltage. The maximum supply voltage at
which the sensor can reliably operate is determined by one or
both of the following two limitations: (1) maximum allowable
sensor die temperature, (2) maximum supply voltage
available in the sensing application/system.
In terms of thermal/power dissipation, the maximum voltage
that can be supplied to the sensor on a continuous basis is
relatively low compared to that which can be pulsed on the
sensor at a low duty–cycle. The average power that is
dissipated in the sensor is the square of the average sensor
excitation voltage divided by the input resistance of the
sensor. When the sensor’s supply bias is operated in a pulsed
fashion, the average excitation voltage is simply the product
of the dc supply voltage used and the percent duty–cycle that
the dc voltage is “on.”
The pulsing circuitry is a high–side switch (two small–signal
switching transistors with associated bias resistors) that is
controlled via the output compare (TCMP) pin of the MCU. The
output compare timer function of the MCU provides a
logic–level pulse waveform to the switch that has a 2–ms
period
and
a
200–
μ
s
user–programmable).
on–time
(Note:
this
is
Figure 2. System Schematic
Signal Conditioning
Even with pulsing at a relatively high supply voltage, the
pressure sensing element still has a full–scale output that is
only on the order of tens of millivolts. To input this signal to the
A/D converter of the MCU, the sensing element output must be
amplified to allow adequate digital resolution. A basic
two–operational amplifier signal–conditioning circuit is used to
provide the following desired characteristics of an
instrumentation amplifier interface:
high input impedance
low output impedance
differential to single–ended conversion of the pressure sen-
sor signal
moderate gain capability
Both the nominal gain and offset reference pedestal of this
interface circuit can be adjusted to fit a given distribution of
sensor devices. Varying the gain and offset reference pedestal
F
Freescale Semiconductor, Inc.
n
.