MPXM2102AS datasheet(350/670 Pages) MOTOROLA

350 / 670 page

AN1097

3–204

Motorola Sensor Device Data

www.motorola.com/semiconductors

PRESSURE SENSOR CHARACTERISTICS

Figure 2 shows the differential output voltage of the

MPX2100 series at +25

°C. The dispersion of the output

voltage determines the best tolerance that the system may

achieve without undertaking a calibration procedure, if any

other elements or parameters in the chain do not introduce

additional errors.

Vout (mV)

VS = 5 Vdc

TA = 25°C

(kPa)

FULL–SCALE

–5

100

OFFSET

Figure 2. Spread of the Output Voltage versus the

Applied Pressure at 25

°C

The effects of temperature on the full scale output and offset

are shown in Figure 3. It is interesting to notice that the offset

variation is greater than the full scale output and both have a

positive

temperature

coefficient

respectively

+8.0

µV/degree and +5.0 µV excitation voltage. That means that

the full scale variation may be compensated by modifying the

gain somewhere in the chain amplifier by components

arranged to produce a negative TC of 250 PPM/°C. The dark

area of Figure 3 shows the trend of the compensation which

improves the full scale value over the temperature range. In

the area of 40 kPa, the compensation acts in the ratio of

40/100 of the value of the offset temperature coefficient.

Figure 3. Output Voltage versus Temperature. The

Dark Area Shows the Trend of the Compensation

Vout (f) ∆T

POSITIVE

FULL SCALE

VARIATION

(kPa)

100

OFFSET VARIATION

–15

°C

+85

°C

OP AMP CHARACTERISTICS

For systems with only one power supply, the instrument

amplifier configuration shown in Figure 4 is a good solution to

monitor the output of a resistive transducer bridge.

The instrument amplifier does provide an excellent CMRR

and a symmetrical buffered high input impedance at both

non–inverting and inverting terminals. It minimizes the

number of the external passive components used to set the

gain of the amplifier. Also, it is easy to compensate the

temperature variation of the Full Scale Output of the Pressure

Sensor by implementing resistors “Rf” having a negative

coefficient temperature of –250 PPM/

°C.

The differential–mode voltage gain of the instrument

amplifier is:

Avd =

V1–V2

Vs2–Vs4

= 1 +

2 Rf

(1)

+Vs

0 V

–

Figure 4. One Power Supply to Excite the Bridge

and to Develop a Differential Output Voltage

–

The major source of errors introduced by the op amp is

offset voltages which may be positive or negative, and the

input bias current which develops a drop voltage

∆V through

the feedback resistance Rf. When the op amp input is

composed of PNP transistors, the whole characteristic of the

transfer function is shifted below the DC component voltage

value set by the Pressure Sensor as shown in Figure 5.

The gain of the instrument amplifier is calculated carefully

to avoid a saturation of the output voltage, and to provide the

maximum of differential output voltage available for the A/D

Converter. The maximum output swing voltage of the

amplifiers is also dependent on the bias current which creates

∆V voltage on the feedback resistance Rf and on the Full

Scale output voltage of the pressure sensor.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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