MPXM2102AS datasheet(351/670 Pages) MOTOROLA

351 / 670 page

AN1097

3–205

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 5. Instrument Amplifier Transfer Function with

Spread of the Device to Device Offset Variation

V1, V2

5 Vdc

1/2

VCC

VEE

VCC

V2X

V1X

VPS

(mV)

Figure 5 shows the transfer function of different instrument

amplifiers used in the same application. The same sort of

random errors are generated by crossing the inputs of the

instrument amplifier. The spread of the differential output

voltage (V1–V2) and (V2x–V1x) is due to the unsigned voltage

offset and its absolute value. Figures 6 and 7 show the

unit–to–unit variations of both the offset and the bias current

of the dual op amp MC33078.

Vio (mV)

(

°C)

–1

–2

–50

–25

100

125

Figure 6. Input Offset Voltage versus Temperature

UNIT 1

UNIT 2

UNIT 3

To realize such a system, the designer must provide a

calibration procedure which is very time consuming. Some

extra potentiometers must be implemented for setting both the

offset and the Full Scale Output with a complex temperature

compensation network circuit.

The new proposed solution will reduce or eliminate any

calibration procedure.

Figure 7. Input Bias Current versus Temperature

lib (nA)

(

°C)

600

450

300

150

–50

–25

100

125

UNIT 1

UNIT 2

MCU CONTRIBUTION

As shown in Figure 5, crossing the instrument amplifier

inputs generated their mutual differences which can be

computed by the MCU.

–

0 V

Figure 8. Crossing of the Instrument Amplifier

Input Using a Port of the MCU

+VS

Figure 8 shows the analog switches on the front of the

instrument amplifier and the total symmetry of the chain. The

residual resistance RDS(on)oftheswitchesdoesnotintroduce

errors due to the high input impedance of the instrument

amplifier.

With the aid of two analog switches, the MCU successively

converts the output signals V1, V2.

Four conversions are necessary to compute the final result.

First, two conversions of V1 and V2 are executed and stored

in the registers R1, R2. Then, the analog switches are

commuted in the opposite position and the two last

conversions of V2x and V1x are executed and stored in the

registers R2x and R1x. Then, the MCU computes the following

equation:

RESULT = (R1 – R2) + (R2x – R1x)

(2)

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS датащи(PDF) 351 Page - Motorola, Inc

MPXM2102AS датащи(HTML) 351 Page - Motorola, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL