3–210
Motorola Sensor Device Data
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Vm
(from
first
stage)
R11
R15
R17
A3
A4
Vm
R12
R13
R14
R16
D
Vc
Figure 4. Second Stage — Difference Amplifier and Gain
+
–
+
–
fromD/A
Note: R14 = R12, R11 = R13
FIRST STAGE (Figure 3)
The first stage consists of the Motorola pressure sensor; in
this case the MPX2200 is used. This sensor typically gives a
full scale span output of 40 mV at 200 kPa. The sensor output
(VS) is connected to the inputs of amplifier A1 (1/4 of the
MC33079, a Quad Operational Amplifier). The gain, G1, of this
amplifier is R7/R6. The sensor has a typical zero pressure
offset voltage of 1 mV. Figure 3 shows offset compensation
circuitry if it is needed. A1 output is fed to the non–inverting
input of A2 amplifier (1/4 of a MC33079) whose gain, G2, is
1+R10/R9. G2 should be set to yield 4.5 volts out with full–rated
pressure.
THE SECOND STAGE (Figure 4)
The output from A2 (Vm = G1 x G2 x Vs) is connected to the
non–inverting input of amplifier A3 (1/4 of a MC33079) and to
the A/D where its corresponding (digital) value is stored by the
microprocessor. The output of A3 is the amplified difference
between Vm, and the digitized/calculated voltage Vc.
Amplifier A4 (1/4 of a MC33079) provides additional gain for
an amplified difference output for the desired resolution. This
difference output, D, is given by:
D
Vm – V
c
G3
G3
R14
R13 1
R17
R16
where G3 is the gain associated with amplifiers A3 and A4.
The theoretical resolution is limited only by the accuracy of
the
programmable
power
microprocessor used has an integrated A/D. The accuracy of
this A/D is directly related to the reference voltage source
stability, which can be self–calibrated by the microprocessor.
Vexpanded is the system output that is the sum of the voltage
due to the count and the voltage due to the difference between
the count voltage and the measured voltage. This is given by
the following relation:
supply.
The
Motorola
V
expanded
V
c
V
expanded
S.
D G3
therefore, PV
expanded
Pexpanded is the value of pressure (in units of kPa) that
results from this improved–resolution system. This value can
be output to a display or used for further processing in a control
system.
CONCLUSION
This circuit provides an easy way to have high resolution
using inexpensive microprocessors and converters.
F
Freescale Semiconductor, Inc.
n
.