MPXM2102AS datasheet(355/670 Pages) MOTOROLA

355 / 670 page

AN1100

3–209

Motorola Sensor Device Data

www.motorola.com/semiconductors

A/D

MPU

OUTPUT

CIRCUITRY

D/A

Figure 2. Expanded Block Diagram

Control

ANALOG CIRCUITRY

–

Figure 2 shows the block diagram of a system that can be

used to reduce the inaccuracies caused by the limited A/D

resolution. The microprocessor would use the stored value M,

as described above, to cause a D/A to output the

corresponding voltage, Vc. Vc is subtracted from the

measured voltage, Vm, using a differential amplifier, and the

resulting voltage is amplified. Assuming a gain, G, of 10 for the

amplifier, the output would be:

+ (Vm–Vc)

(9)

+ (2790 mV–2783 mV)

+ 70 mV

The microprocessor will receive the following count from the

A/D:

+ 70mV 19.60 mV count

(10)

+ 3.6

+ 3 full counts

The microprocessor then computes the actual pressure

with the following equations:

Expanded Voltage

+ Vc ) C

R) G)(11)

+ 2783 ) 3

19.60) 10)

+ 2789 mV,

NOTE: R is resolution of 8-bit d/A

Corresponding Pressure

+ 2789 mV

(12)

+ 22.5 mV kPa

+ 123.9 kPa

Thus the error is:

Pressure Error

+ Actual – Measured

(13)

+ 124 kPa – 123.9 kPa

+ 0.1 kPa

Figures 3 and 4 together provide a more detailed

description of the analog portion of this system.

R10

Vm (to Second Stage)

Figure 3. First Stage – Differential Amplifier, Offset Adjust and Gain Adjust

–

Note: R7 = R2, R1 = R6

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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