MPXM2102AS datasheet(487/670 Pages) MOTOROLA

487 / 670 page

AN1551

3–341

Motorola Sensor Device Data

www.motorola.com/semiconductors

is desirable since pressure sensors’ full–scale span and

zero–pressure offset voltages will vary somewhat from lot to

lot and unit to unit. During software calibration, each sensor

device’s specific offset and full–scale output characteristics

will be stored. Nonetheless, a variable gain amplifier circuit is

desirable to coarsely tune the sensor’s full–scale span, and a

positive or negative dc level shift (offset pedestal adjustment)

of the pressure sensor signal is needed to translate the

pressure sensor’s signal–conditioned output span to a

specific level (e.g., within the high and low reference voltages

of the A/D converter).

Microcontroller

The microcontroller performs all of the necessary tasks to

give the smart sensor system the specified performance and

intelligent

features.

The

following

describes

its

responsibilities:

• Creates the control signal to pulse the sensor.

• Samples the pressure sensor’s output.

• Signal averages a programmable number of samples for

noise reduction.

• Samples a scaled–down version of the pressure sensor

supply voltage. Monitoring the power supply voltage allows

the microcontroller to reject sensor output changes result-

ing from power supply variations.

• Uses serial communications interface (SPI) to receive com-

mands from and to send sensor information to a master

MCU.

Resistor Divider for Rejection of Supply Voltage

Variation

Since the pressure sensor’s output voltage is ratiometric to

its supply voltage, any variation in supply voltage will result in

variation of the pressure sensor’s output voltage. By

attenuating the supply voltage (since the supply voltage may

exceed the 5 V range of the A/D) with a resistor divider, this

scaled voltage can be sampled by the microcontroller’s A/D

converter. By sampling the scaled supply voltage, the

microcontroller can compensate for any variances in the

pressure sensor’s output voltage that are due to supply

variations.

This

technique

allows

correct

pressure

determination even when the pressure sensor is powered with

an unregulated supply.

5 V Regulator

A 5 V

±5% voltage regulator is required for the following

functions:

• To provide a stable 5 V for the high voltage reference (VRH)

of the microcontroller’s A/D converter. A stable voltage ref-

erence is crucial for sampling any analog voltage signals.

• To provide a stable 5 V for the resistor divider that is used

to level shift the amplified zero–pressure offset voltage.

Low Voltage Inhibit (LVI) Circuitry

Low voltage inhibit circuitry is required to ensure proper

power–on–reset (POR) of the microcontroller and to put the

MCU in a known state when the supply voltage is decreased

below the MCU supply voltage threshold.

SOFTWARE DESCRIPTION

The smart sensor system’s EPROM resident code provides

the control pulse for the sensor’s excitation voltage and

performs calibration with respect to a wide range of excitation

voltages (20 ~ 28 V typically for HVAC). Pressure

measurement averaging is also incorporated to reduce both

signal error and noise. In addition, the availability of a serial

communications interface allows a variety of software

commands to be sent to the smart sensor system.

The following brief outline provides a more detailed

description about the software features included in the smart

sensor system.

Software Calibration and Power Supply Rejection

Only six 8–bit words of information are stored both to

calibrate the smart sensor system for a given sensor device

and to store the relationship between sensor output and power

supply voltage. This information is used to reduce errors due

to device–to–device variations and to reject variations in

power supply voltage that can introduce error into the pressure

measurement.

The

sensor’s

amplified

output

the

zero–pressure offset and full–scale pressure are stored at

each of two different supply voltages. In addition, the scaled

and digitized representation of the applied supply voltages is

stored. Compensating for power supply variation in software

allows higher performance with lower tolerance, or even

unregulated, supply voltages. For HVAC applications, where

a 24–Vac line voltage will be simply rectified and filtered to

provide a crude 24–Vdc supply, this approach has major

performance benefits. The impact on applications where a

regulated supply is available is that a lower–cost regulator or

dc–to–dc converter can be used without compromising

system accuracy significantly.

A/D Sample Averaging

Noise inherent to the 8–bit A/D successive approximation

conversion method used by the smart sensor accounts for

±1–bit resolution. Signal noise, which exhibits a measured

peak–to–peak range larger in magnitude than 1 bit of A/D

resolution, can be minimized by a sample averaging

technique.

The current technique uses 16 A/D converted pressure

samples, sums the result, and divides by 16 (the number of

samples) to get the average:

AVG =

; where n = 16

(1)

(an)

Assuming a gaussian distribution of noise, this averaging

technique improves the signal–to–noise ratio (SNR).

Smart Sensor Unit ID and Software Revision

Level

This solution may be implemented as a single sensing

system using a nondedicated MCU to provide the sensing

function and smart features or as a slaved smart sensor (with

dedicated sensing MCU) that communicates over a serial bus

to a master controller or microprocessor (Host). Part

identification and software revision level can also be read on

request from the master MCU. This information is utilized by

the master MCU to determine what the full–scale pressure

range of a given smart sensor unit is. This allows for multiple

sensor units with different pressure ranges to be controlled

and sensed from a single master MCU.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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