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Motorola Sensor Device Data
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SYSTEM DESIGN
As mentioned in the introduction, the lowest pressure
devices in the Motorola portfolio are rated at a full–scale
pressure of 10 kPa (40
″
of H2O). The calibrated and
temperature compensated, 10 kPa device (MPX2010) is
specified to operate at a 10 Vdc supply voltage and produce
25 mV (nominal) at the full–scale pressure of 10 kPa. This
translates to a 0.25 mV/(V*kPa) pressure sensitivity.
Additionally, the absolute maximum supply voltage specified
is 16 Vdc. Thus, the maximum full–scale output signal that can
be achieved without exceeding the maximum supply voltage
rating is 40 mV, or 60% greater than the output at the 10 Vdc
specification. So, a 60% increase can be achieved in the
output signal of the sensor for the 0–10 kPa pressure range,
or the same signal level of 25 mV can be preserved over a
proportionally lower applied pressure range (i.e., 0–6.25 kPa).
The point here is that increasing the dc supply excitation only
produces limited improvement in the output signal level.
Much greater gains in output signal level (sensor span) can
be obtained, if it is possible to operate the sensor at
significantly higher voltages. Since the thermal/power
dissipation limitation imposed by the maximum dc supply
voltage can be avoided by using a pulsed excitation at a low
duty–cycle (on–time) and reasonable period, and second
order junction effects do not occur until much higher voltages,
the sensor output can be greatly increased by operating at a
much higher ac voltage than permitted by the dc counterpart
of this same higher voltage. As an example, industrial
applications like HVAC have 24 V commonly available, and we
want to accurately measure pressures below 10
″
H2O. To
achieve a 1–2% of full–scale accuracy (based on temperature
drift errors, system noise, device tolerance, power supply
variation/rejection, etc.), 9–12 mV is the typical minimum
full–scale span that is the desired target for the pressure range
of interest. For the MPX2010 pulsed at 24 V, we obtain 15 mV
of output for an applied pressure of 10
″
H2O (2.5 kPa). This
same sensor device will only produce 6.25 mV at its normally
specified supply of 10 V and 2.5 kPa, thus not meeting the
signal–to–noise ratio criteria for a 1–2% accuracy
performance.
This smart sensing solution is intended to sense full–scale
pressures below 10
″
H2O with 1% of full–scale pressure
resolution and better than 2% of full–scale accuracy. The
following subsystems comprise the hardware portion of this
solution (see Figure 1):
Figure 1. Smart Sensing Block Diagram
PRESSURE SENSOR
SIGNAL
CONDITIONING
SWITCHING
CIRCUITRY
LOW VOLTAGE
INHIBIT
8–BIT
MICROCONTROLLER
5 V
±
5%
REGULATOR
POWER SUPPLY
REJECTION CIRCUITRY
VPP
Dout
Din
SCLK
CS
VCC
Gnd
F
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n
.