3–290
Motorola Sensor Device Data
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Table 3. MPX2100AP Electrical Characteristics
Characteristic
Symbol
Minimum
Typical
Max
Unit
Pressure Range
POP
VS
VFSS
Voff
S
0
100
kPa
Supply Voltage
10
16
Vdc
Full Scale Span
38.5
40
41.5
mV
Zero Pressure Offset
±
1.0
mV
Sensitivity
0.4
mv/kPa
Linearity
0.05
%FSS
Temperature Effect on Span
0.5
%FSS
Temperature Effect on Offset
0.2
%FSS
Pressure Sensor
The first and most important subsystem is the pressure
transducer. This device converts the applied pressure into a
proportional, differential voltage signal. This output signal will
vary linearly with pressure. Since the applied pressure in this
application will approach a maximum level of 30.5 in–Hg. (100
kPa) at sea level, the sensor output must have a linear output
response over this pressure range. Also, the applied pressure
must be measured with respect to a known reference pressure,
preferably absolute zero pressure (vacuum). The device should
also produce a stable output over the entire operating
temperature range.
The desired sensor for this application is a temperature
compensated and calibrated, semiconductor pressure
transducer, such as the Motorola MPXM2102A series sensor
family. The MPX2000 series sensors are available in
full–scale pressure ranges from 10 kPa (1.5 psi) to 200 kPa
(30 psi). Furthermore, they are available in a variety of
pressure configurations (gauge, differential, and absolute)
and porting options. Because of the pressure ranges involved
with barometric pressure measurement, this system will
employ an MPXM2102AS (absolute with single port). This
device will produce a linear voltage output in the pressure
range of 0 to 100 kPa. The ambient pressure applied to the
single port will be measured with respect to an evacuated
cavity (vacuum reference). The electrical characteristics for
this device are summarized in Table 3.
As indicated in Table 3, the sensor can be operated at
different supply voltages. The full–scale output of the sensor,
which is specified at 40 mV nominally for a supply voltage of
10 Vdc, changes linearly with supply voltage. All non–digital
circuitry is operated at a regulated supply voltage of 8 Vdc.
Therefore, the full–scale sensor output (also the output of the
sensor at sea level) will be approximately 32 mV.
8
10
40 mV
The sensor output voltage at the systems minimum range
(15 in–Hg.) is approximately 16.2 mV. Thus, the sensor output
over the intended range of operations is expected to vary from
32 to 16.2 mV. These values can vary slightly for each sensor
as the offset voltage and full–scale span tolerances indicate.
Signal Conditioning Circuitry
In order to convert the small–signal differential output signal
of the sensor to MCU compatible levels, the next subsystem
includes signal conditioning circuitry. The operational
amplifier circuit is designed to amplify, level–shift, and ground
reference the output signal. The signal is converted to a
single–ended, 0.5 – 4.5 Vdc range. The schematic for this
amplifier is shown in Figure 3.
This particular circuit is based on classic instrumentation
amplifier design criteria. The differential output signal of the
sensor is inverted, amplified, and then level–shifted by an
adjustable offset voltage (through Roffset1). The offset voltage
is adjusted to produce 0.5 volts at the maximum barometric
pressure (30.5 in–Hg.). The output voltage will increase for
decreasing pressure. If the output exceeds 5.1 V, a zener
protection diode will clamp the output. This feature is included
to protect the A/D channel input of the MCU. Using the transfer
function for this circuit, the offset voltage and gain can be
determined to provide 0.1 in–Hg of system resolution and the
desired output voltage level. The calculation of these
parameters is illustrated below.
In determining the amplifier gain and range of the trimmable
offset voltage, it is necessary to calculate the number of steps
used in the A/D conversion process to resolve 0.1 in–Hg.
(30.5
15.0)in-Hg * 10sHg
155 steps
The span voltage can now be determined. The resolution
provided by an 8–bit A/D converter with low and high voltage
references of zero and five volts, respectively, will detect 19.5
mV of change per step.
VRH
5 V, VRL
0 V
Sensor Output at 30.5 in–Hg = 32.44 mV
Sensor Output at 15.0 in–Hg = 16.26 mV
Sensor Output =
SO = 16.18 mV
Gain
3.04 V
SO
187
Note:
30.5 in–Hg and 15.0 in–Hg are the assumed
maximum and minimum absolute pressures, respectively.
F
Freescale Semiconductor, Inc.
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