MPXM2102AS datasheet(414/670 Pages) MOTOROLA

414 / 670 page

AN1316

3–268

Motorola Sensor Device Data

www.motorola.com/semiconductors

Circuit Operation

The voltage signal conditioning portion of this circuit is a

variation

the

classic

instrumentation

amplifier

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:

Fout (full-scale) +

(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

Ω 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.

The

zero–pressure

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

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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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