3–276
Motorola Sensor Device Data
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A relatively straightforward circuit for converting pressure to
frequency is shown in Figure 8. It consists of three basic parts.
The interface amplifier is the same circuit that was described
in Figure 4. Its 0.5 to 4.5 V output is fed directly into an AD654
voltage–to–frequency converter. On the AD654, C3 sets
nominal output frequency. Zero pressure output is calibrated
to 1 kHz by adjusting the zero pressure input voltage with R3.
Full scale adjustments are made with R12 which sets the full
scale frequency to 10 kHz. The output of the AD654 is then fed
into a buffer consisting of Q1 and R10. The buffer is used to
clean up the edges and level translate the output to 5 V.
Advantages of this approach are that the frequency output is
easily read by a microcomputer’s timer and transmission over
a twisted pair line is relatively easy. Where very long distances
are involved, the primary disadvantage is that 3 wires (VCC,
ground and an output line) are routed to the sensor.
A 4–20 mA loop reduces the number of wires to two. Its
output is embedded in the VCC and ground lines as an active
current source. A straightforward way to apply this technique
to pressure sensing is shown in Figure 9. In this figure an
MPX7000 series high impedance pressure sensor is mated to
an XTR101 4–20 mA two–wire transmitter. It is set up to pull
4 mA from its power line at zero pressure and 20 mA at full
scale. At the receiving end a 240 ohm resistor referenced to
signal ground will provide a 0.96 to 4.8 V signal that is suitable
for microcomputer A/D inputs.
Figure 9. 4–20 mA Pressure Transducer
2
3
4
1
9
1
3
7
1
4
2
1
.96 – 4.8
V
24 V
PLOOP
240
RETURN
D1
1N4002
4–20 mA OUTPUT
+
R1
750
1/2 W
Q1
MPSA06
U1
XTR101
–
XDCR1
MPX7000
SERIES
SENSOR
D2
1N4565A
6.4 V @ .5 mA
R3
30
R5
100
SPAN
R6
100 k
OFFSET
R4
1M
12
1
1
1
0
8
4
5
6
3
C1
0.01
μ
F
–
2 mA
+
R2
1 k
Bias for the sensor is provided by two 1 mA current sources
(pins 10 and 11) that are tied in parallel and run into a 1N4565A
6.4 V temperature compensated zener reference. The
sensor’s differential output is fed directly into XTR101’s
inverting and non–inverting inputs. Zero pressure offset is
calibrated to 4 mA with R6. Biased with 6.4 V, the sensor’s full
scale output is 24.8 mV. Given this input R3
+
R5 nominally
total 64 ohms to produce the 16 mA span required for 20 mA
full scale. Calibration is set with R5.
The XTR101 requires that the differential input voltage at pins
3 and 4 has a common mode voltage between 4 and 6 V. The
sensor’s common mode voltage is one half its supply voltage
or 3.2 V. R2 boosts this common mode voltage by
1 k
2 mA or 2 V, establishing a common mode voltage for the
transmitter’s input of 5.2 V. To allow operation over a 12 to 40
V range, dissipation is off–loaded from the IC by boosting the
output with Q1 and R1. D1 is also included for protection. It
prohibits reverse polarity from causing damage. Advantages of
this topology include simplicity and, of course, the two wire
interface.
F
Freescale Semiconductor, Inc.
n
.