MPXM2102AS datasheet(486/670 Pages) MOTOROLA

486 / 670 page

AN1551

3–340

Motorola Sensor Device Data

www.motorola.com/semiconductors

• high–side switch pulsing circuitry

• signal–conditioning amplifier interface with resistors to ad-

just the sensor’s amplified, full–scale span and zero–pres-

sure offset

• on–chip resources of a complete 8–bit microcontroller

(MCU)

• MCU oscillator circuitry (4 MHz)

• 5 V ±5% linear voltage regulator

• low–voltage inhibit (LVI) supervisory voltage monitoring cir-

cuit

• resistor divider connected to the sensor’s power supply bias

to sense the excitation voltage across the sensor

These subsystems are explained as follows to provide an

understanding of the system design and its intelligent features

(refer to Figure 2).

Pulsing Circuitry

As previously mentioned, the sensor’s output is ratiometric

to the excitation voltage across the sensing element; the

sensor’s sensitivity increases with increasing supply voltage.

Thus, to detect low pressures and minute changes in

pressure, it is desirable to operate the sensor at the highest

possible excitation voltage. The maximum supply voltage at

which the sensor can reliably operate is determined by one or

both of the following two limitations: (1) maximum allowable

sensor die temperature, (2) maximum supply voltage

available in the sensing application/system.

In terms of thermal/power dissipation, the maximum voltage

that can be supplied to the sensor on a continuous basis is

relatively low compared to that which can be pulsed on the

sensor at a low duty–cycle. The average power that is

dissipated in the sensor is the square of the average sensor

excitation voltage divided by the input resistance of the

sensor. When the sensor’s supply bias is operated in a pulsed

fashion, the average excitation voltage is simply the product

of the dc supply voltage used and the percent duty–cycle that

the dc voltage is “on.”

The pulsing circuitry is a high–side switch (two small–signal

switching transistors with associated bias resistors) that is

controlled via the output compare (TCMP) pin of the MCU. The

output compare timer function of the MCU provides a

logic–level pulse waveform to the switch that has a 2–ms

period

and

200–

µs

on–time

(

Note:

this

user–programmable).

Figure 2. System Schematic

Signal Conditioning

Even with pulsing at a relatively high supply voltage, the

pressure sensing element still has a full–scale output that is

only on the order of tens of millivolts. To input this signal to the

A/D converter of the MCU, the sensing element output must be

amplified to allow adequate digital resolution. A basic

two–operational amplifier signal–conditioning circuit is used to

provide

the

following

desired

characteristics

instrumentation amplifier interface:

• high input impedance

• low output impedance

• differential to single–ended conversion of the pressure sen-

sor signal

• moderate gain capability

Both the nominal gain and offset reference pedestal of this

interface circuit can be adjusted to fit a given distribution of

sensor devices. Varying the gain and offset reference pedestal

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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