MPXM2102AS datasheet(422/670 Pages) MOTOROLA

422 / 670 page

AN1318

3–276

Motorola Sensor Device Data

www.motorola.com/semiconductors

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

.96 – 4.8

24 V

PLOOP

240

RETURN

1N4002

4–20 mA OUTPUT

750

1/2 W

MPSA06

XTR101

XDCR1

MPX7000

SERIES

SENSOR

1N4565A

6.4 V @ .5 mA

100

SPAN

100 k

OFFSET

0.01

µF

–

2 mA

–

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

S 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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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