MPXM2102AS datasheet(352/670 Pages) MOTOROLA

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AN1097

3–206

Motorola Sensor Device Data

www.motorola.com/semiconductors

The

result

twice

differential

conversion.

demonstrated below, all errors from the instrument amplifier

are cancelled. Other averaging techniques may be used to

improve the result, but the appropriated algorithm is always

determined by the maximum bandwidth of the input signal

and the required accuracy of the system.

PRESSURE

SENSOR

SYSTEM

MPX2100AP

MC74HC4053

+ 5 V

I/O

0 V

MC68HC05B6

CH1

CH2

VRH

VDD

VRL

VSS

Figure 9. Two Channel Input and One Output Port Are Used by the MCU

MC33078

–

SYSTEM CALCULATION

Sensor out 2

Sensor out 4

Vs2 = a (P) + of2

Vs4 = b (P) + of4

Amplifier out 1

Amplifier out 2

V1 = Avd (Vs2 + OF1)

V2 = Avd (Vs4 + OF2)

Inverting of the amplifier input

V1x = Avd (Vs4 + OF1)

V2x = Avd (Vs2 + OF2)

Delta = V1–V2

1st differential result

= Avd * (Vs2 of OF1) – Avd * (Vs4 + OF2)

Deltax = V2x–V1x

2nd differential result

= Avd * (Vs2 + OF2) – Vdc * (Vs4 + OF1)

Adding of the two differential results

VoutV = Delta + Deltax

= Avd*Vs2 + Avd*OF2 + Avd*OF2 – Avd*OF1

= 2 * Avd * (Vs2 –Vs4)

= 2 * Avd * [(a (P) + of2) – (b (P) + of4)]

= 2 * Avd * [V(P) + Voffset]

+ Avd*OF1 – Avd*OF2 + Avd*OF2 – Avd*OF1

There is a full cancellation of the amplifier offset OF1 and

OF2. The addition of the two differential results V1–V2 and

V2x–V1X produce a virtual output voltage VoutV which

becomes the applied input voltage to the A/D converter. The

result of the conversion is expressed in the number of counts

or bits by the ratiometric formula shown below:

count = VoutV *

255

VRH–VRL

255 is the maximum number of counts provided by the A/D

converter and VRH–VRL is the reference voltage of the

ratiometric A/D converter which is commonly tied to the 5.0 V

supply voltage of the MCU.

When the tolerance of the full scale pressure has to be in the

range of

± 2.5%, the offset of the pressure sensor may be

neglected. That means the system does not require any

calibration procedure.

The equation of the system transfer is then:

count = 2 * Avd * V(P) * 51/V where:

Avd is the differential–mode gain of the instrument amplifier

which is calculated using the equation (1). Then with Rf = 510

Ω and Rg = 9.1 kΩ Avd = 113.

The maximum counts available in the MCU register at the

Full Scale Pressure is:

count (Full Scale) = 2 * 113 * 0.02 V * 51/V = 230

knowing that the MPX2100AP pressure sensor provides

20 mV at 5.0 excitation voltage and 100 kPa full scale

pressure.

The system resolution is 100 kPa/230 that give 0.43 kPa per

count.

MC68HC05B6

VRH

CH1

CH2

VRL

VSS

VDD

+ 5 V

0 V

I/O

FINE

CAL.

Figure 10. Full Scale Output Calibration Using the

Reference Voltage VRH–VRL

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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