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Motorola Sensor Device Data
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Circuit Operation
The voltage signal conditioning portion of this circuit is a
variation
on
the
classic
configuration. It is capable of providing high differential gain
and good common–mode rejection with very high input
impedance; however, it provides a more user friendly method
of performing the offset/bias point adjustment. It uses four op
amps and several resistors to amplify and level shift the
sensor’s output. Most of the amplification is done in U1A
which is configured as a differential amplifier. Unwanted
current flow through the sensor is prevented by buffer U1B. At
zero pressure the differential voltage from pin 2 to pin 4 on the
sensor has been precision trimmed to essentially zero volts.
The common–mode voltage on each of these nodes is 4 V
(one–half the sensor supply voltage). The zero pressure
output voltage at pin 1 of U1A is then 4.0 V, since any other
voltage would be coupled back to pin 2 via R5 and create a
non–zero bias across U1A’s differential inputs. This 4.0 V zero
pressure DC output voltage is then level translated to the
desired zero pressure offset voltage by U1C and U1D. The
offset voltage is produced by R4 and adjustment trimpot R12.
R7’s value is such that the total source impedance into pin 13
is approximately 1 k. The gain is approximately (R5/R6)(1 +
R11/R10), which is 125 for the values shown in Figure 2. A
gain of 125 is selected to provide a 4 V span for 32 mV of
full–scale sensor output (at a sensor supply voltage of 8 V).
The resulting .5 V to 4.5 V output from U1C is then
converted by the V/F converter to the nominal 1–10 kHz that
has been specified. The AD654 V/F converter receives the
amplified sensor output at pin 8 of op amp U1C. The full–scale
frequency is determined by R3, R13 and C3 according to the
following formula:
instrumentation
amplifier
F
out
(full-scale)
V
in
(10V)(R3
R13)C3
For best performance, R3 and R13 should be chosen to
provide 1 mA of drive current at the full–scale voltage
produced at pin 3 of the AD654 (U3). The input stage of the
AD654 is an op–amp; thus, it will work to make the voltage at
pin 3 of U3 equal to the voltage seen at pin 4 of U3 (pins 3 and
4 are the input terminals of the op amp). Since the amplified
sensor output will be 4.5 V at full–scale pressure, R3 + R13
should be approximately equal to 4.5 k
to have optimal
linearity performance. Once the total resistance from pin 3 of
U3 to ground is set, the value of C3 will determine the
full–scale frequency output of the V/F. Trimpot R13 should be
sized (relative to R3 value) to provide the desired amount of
full–scale
frequency
adjustment.
frequency is adjusted via the offset adjust provided for
calibrating the offset voltage of the signal conditioned sensor
output. For additional information on using this particular V/F
converter, see the applications information provided in the
Analog Devices Data Conversion Products Databook.
The frequency output has its edge transitions “sped” up by
a small–signal FET inverter. This final output is directly
compatible with microprocessor timer inputs, as well as any
The
zero–pressure
other high–speed CMOS logic. The amplifier portion of this
circuit has been patented by Motorola Inc. and was introduced
on evaluation board DEVB150A. Additional information
pertaining to this circuit and the evaluation board DEVB150A
is contained in Motorola Application Note AN1313.1
TEST/CALIBRATION PROCEDURE
1. Connect a +12 V supply between B+ and GND terminals
on the connector CN1.
2. Connect a frequency counter or scope probe on the Fout
terminal of CN1 or on TP1 with the test instrumentation
ground clipped to TP3 or GND.
3 . Turn the power switch, S1, to the on position. Power
LED, D1, should be illuminated. Verify that the voltage at
TP2 and TP4 (relative to GND or TP3) is 5 V and 8 V,
respectively. While monitoring the frequency output by
whichever means one has chosen, one should see a
50% duty cycle square wave signal.
4. Turn the wiper of the OFFSET
adjust trimpot, R12, to the
approximate center of the pot.
5. Apply 100 kPa to pressure port P1 of the MPX2100DP
(topside port on marked side of the package) sensor, X1.
6. Adjust the FULL–SCALE
trimpot, R13, until the output
frequency is 10 kHz. If 10 kHz is not within the trim range
of the full–scale adjustment trimpot, tweak the offset
adjust trimpot to obtain 10 kHz (remember, the offset pot
was at an arbitrary midrange setting as per step 4).
7. Apply zero pressure to the pressure port (i.e., both ports
at ambient pressure, no differential pressure applied).
Adjust OFFSET
trimpot so frequency output is 1 kHz.
8. Verify that zero pressure and full–scale pressure
(100 kPa) produce 1 and 10 kHz respectively, at Fout
and/or TP1. A second iteration of adjustment on both
full–scale and offset may be necessary to fine tune the
1 – 10 kHz range.
CONCLUSION
Transforming conventional analog voltage sensor outputs
to frequency has great utility for a variety of applications.
Sensing remotely and/or in noisy environments is particularly
challenging for low–level (mV) voltage output sensors such as
the MPX2000 Series pressure sensors. Converting the
MPX2000 sensor output to frequency is relatively easy to
accomplish, while providing the noise immunity required for
accurate pressure sensing. The evaluation board presented
is an excellent tool for either “stand–alone” evaluation of the
MPX2000 Series pressure sensors or as a building block for
system prototyping which can make use of DEVB160 as a
“drop–in” frequency output sensor solution. The output of the
DEVB160 circuit is ideally conditioned for interfacing to MCU
timer inputs that can measure the sensor frequency signal.
REFERENCES
1. Schultz, Warren (Motorola, Inc.), “Sensor Building Block
Evaluation Board,” Motorola Application Note AN1313.
F
Freescale Semiconductor, Inc.
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