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Motorola Sensor Device Data
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Prepared by: Eric Jacobsen and Jeff Baum
Sensor Design and Applications Group, Motorola Phoenix, AZ
INTRODUCTION
The Pressure Switch concept is simple, as are the additions
to conventional signal conditioning circuitry required to
provide a pressure threshold (or thresholds) at which the
output switches logic state. This logic–level output may be
input to a microcontroller, drive an LED, control an electronic
switch, etc. The user–programmed threshold (or reference
voltage) determines the pressure at which the output state will
switch. An additional feature of this minimal component
design is an optional user–defined hysteresis setting that will
eliminate multiple output transitions when the pressure sensor
voltage is comparable to the threshold voltage.
This paper presents the characteristics and design criteria
for each of the major subsystems of the pressure switch
design: the pressure sensor, the signal conditioning (gain)
stage, and the comparator output stage. Additionally, an entire
section will be devoted to comparator circuit topologies which
employ comparator ICs and/or operational amplifiers. A
window comparator design (high and low thresholds) is also
included. This section will discuss the characteristics and
design criteria for each comparator circuit, while evaluating
them in overall performance (i.e., switching speed, logic–level
voltages, etc.).
BASIC SENSOR OPERATION
Motorola’s MPX2000 Series sensors are temperature
compensated and calibrated (i.e., offset and full–scale span
are precision trimmed) pressure transducers. These sensors
are available in full–scale pressure ranges from 10 kPa
(1.5 psi) to 200 kPa (30 psi). Although the specifications (see
Table 1) in the data sheets apply only to a 10 V supply voltage,
the output of these devices is ratiometric with the supply
voltage. For example, at the absolute maximum supply
voltage rating, 16 V, the sensor will produce a differential
output voltage of 64 mV at the rated full–scale pressure of the
given sensor. One exception to this is that the full–scale span
of the MPX2010 (10 kPa sensor) will be only 40 mV due to the
device’s slightly lower sensitivity. Since the maximum supply
voltage produces the most output voltage, it is evident that
even the best case scenario will require some signal
conditioning to obtain a usable voltage level. For this specific
design, an MPX2100 and 5.0 V supply is used to provide a
maximum sensor output of 20 mV. The sensor output is then
signal conditioned to obtain a four volt signal swing (span).
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Table 1. MPX2100 Electrical Characteristics for VS = 10 V, TA = 25
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SEMICONDUCTOR APPLICATION NOTE
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