MPXM2102AS datasheet(457/670 Pages) MOTOROLA

457 / 670 page

AN1517

3–311

Motorola Sensor Device Data

www.motorola.com/semiconductors

TEST/CALIBRATION PROCEDURE

1. Before testing the circuit, the user–defined values for

RTH, RH and Roff should be calculated for the desired

application.

The sensor offset voltage is set by

off +

off

) R

off

Then, the amplified sensor voltage corresponding to a given

pressure is calculated by

Vsensor = 201 x 0.0002 x APPLIED PRESSURE + Voff,

where 201 is the gain, 0.0002 is in units of V/kPa and

APPLIED PRESSURE is in kPa.

The threshold voltage, VTH, at which the output changes

state is calculated by determining Vsensor at the pressure that

causes this change of state:

) R

VTH = Vsensor (@ pressure threshold) =

If hysteresis is desired, refer to the LM311 Used in a

Comparator section to determine RH.

2. To test this design, connect a +5 volt supply between pins

3 and 4 of the connector CN1.

3. Connect a volt meter to pins 1 and 4 of CN1 to measure

the output voltage and amplified sensor voltage,

respectively.

4. Connect an additional volt meter to the VTH probe point

to verify the threshold voltage.

5. Turn on the supply voltage.

6. With no pressure applied, check to see that Voff is correct

by measuring the voltage at the output of the gain stage

(the volt meter connected to Pin 4 of CN1). If desired, Voff

can be fine tuned by using a potentiometer for Roff.

7. Check to see that the volt meter monitoring VTH displays

the desired voltage for the output to change states. Use a

potentiometer for RTH to fine tune VTH, if desired.

8. Apply pressure to the sensor. Monitor the sensor’s output

via the volt meter connected to pin 4 of CN1. The output

will switch from low to high when this pressure sensor

voltage reaches or exceeds the threshold voltage.

9. If hysteresis is used, with the output high (pressure

sensor voltage greater than the threshold voltage), check

to see if VTH has dropped by the amount of hysteresis

desired.

A potentiometer can be used for RH to fine tune the

amount of hysteresis.

CONCLUSION

The pressure switch design uses a comparator to create a

logic level output by comparing the pressure sensor output

voltage and a user–defined reference voltage. The flexibility

of this minimal component, high performance design makes

it compatible with many different applications. The design

presented here uses an op amp with a transistor output stage,

yielding excellent logic–level outputs and output transition

speeds

for

many

applications.

Finally,

several

other

comparison stage designs, including a window comparator,

are evaluated and compared for overall performance.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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