MPXM2102AS datasheet(356/670 Pages) MOTOROLA

356 / 670 page

AN1100

3–210

Motorola Sensor Device Data

www.motorola.com/semiconductors

(from

first

stage)

R11

R15

R17

R12

R13

R14

R16

Figure 4. Second Stage — Difference Amplifier and Gain

–

from D/A

Note: R14 = R12, R11 = R13

FIRST STAGE (Figure 3)

The first stage consists of the Motorola pressure sensor; in

this case the MPX2200 is used. This sensor typically gives a

full scale span output of 40 mV at 200 kPa. The sensor output

(VS) is connected to the inputs of amplifier A1 (1/4 of the

MC33079, a Quad Operational Amplifier). The gain, G1, of this

amplifier is R7/R6. The sensor has a typical zero pressure

offset voltage of 1 mV. Figure 3 shows offset compensation

circuitry if it is needed. A1 output is fed to the non–inverting

input of A2 amplifier (1/4 of a MC33079) whose gain, G2, is

1+R10/R9. G2 should be set to yield 4.5 volts out with full–rated

pressure.

THE SECOND STAGE (Figure 4)

The output from A2 (Vm = G1 x G2 x Vs) is connected to the

non–inverting input of amplifier A3 (1/4 of a MC33079) and to

the A/D where its corresponding (digital) value is stored by the

microprocessor. The output of A3 is the amplified difference

between Vm, and the digitized/calculated voltage Vc.

Amplifier A4 (1/4 of a MC33079) provides additional gain for

an amplified difference output for the desired resolution. This

difference output, D, is given by:

+ Vm – Vc

+ R14 R13 1 ) R17

R16

where G3 is the gain associated with amplifiers A3 and A4.

The theoretical resolution is limited only by the accuracy of

the

programmable

power

supply.

The

Motorola

microprocessor used has an integrated A/D. The accuracy of

this A/D is directly related to the reference voltage source

stability, which can be self–calibrated by the microprocessor.

Vexpanded is the system output that is the sum of the voltage

due to the count and the voltage due to the difference between

the count voltage and the measured voltage. This is given by

the following relation:

expanded + Vc ) D G3

therefore, PV

expanded + Vexpanded S.

Pexpanded is the value of pressure (in units of kPa) that

results from this improved–resolution system. This value can

be output to a display or used for further processing in a control

system.

CONCLUSION

This circuit provides an easy way to have high resolution

using inexpensive microprocessors and converters.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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