MPXM2102AS datasheet(483/670 Pages) MOTOROLA

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Motorola Sensor Device Data

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AN1551

Low-Pressure Sensing with the

MPX2010 Pressure Sensor

Prepared by: Jeffery Baum

Systems Engineering Group Leader

Sensor Products Division

Motorola Semiconductor Products Sector

Phoenix, AZ

INTRODUCTION

Until recently, low–cost semiconductor pressure sensors

were designed to measure typical full–scale pressures only as

low as 10 kPa (1.5 psi). Of course, “measure” is a relative term.

“Measure” is used here to imply that an output of reasonable

magnitude, signal–to–noise ratio, and accuracy is produced

by the sensing device. Such sensor products are available in

various levels of integration and package types. Depending

on the level of application customization required and the

budget available, a sensor user may choose from a range of

low–pressure

sensor

products

such

kPa

“bare–element” (uncompensated) device, a 10 kPa calibrated

and

temperature

compensated

device,

fully

signal–conditioned

(high–level

output),

calibrated,

and

temperature compensated integrated 10 kPa device. These

options are typically available as well for higher pressures

ranging up to 1000 kPa.

What if the sensor user must measure full–scale pressures

that are two, four, or even ten times lower than what

conventional sensor technology is capable of measuring? “Do

such applications and customers exist?’’ The answer is “yes”

and “yes.” There are many potential customers that require

such low–pressure sensing ability, the two application

examples discussed here are: (1) heating ventilation and

air–conditioning (HVAC) in the context of building controls and

(2) water–level sensing in appliance applications such as

clothes washing machines.

For the purposes of measuring low pressures, the units of

inches of water (

″ H2O) or millimeters of water (mm H2O) will

be used. Typical HVAC applications have a full–scale

pressure of 40 mm H2O and washing machines have either

300 or 600 mm H2O, depending on the region of the world

(

Note: just for reference purposes, 10 kPa

[ 40″ H2O [ 1000

mm H2O [ 1.5 psi).

Of course, a sensor intended for a higher pressure range

than the one of interest can be used. However, the effect is that

only a small portion on the device’s dynamic output range is

used for the actual operating range. This low–level output may

then be paired up with a larger than ideal amplifier gain. Thus,

a poor signal–to–noise ratio is usually the result. Some sensor

manufacturers have recently introduced pressure sensors

designed for 4

″ and 5″ H2O full–scale ranges (approx.

100–125 mm H2O). These devices typically employ silicon

with very thinly micromachined diaphragms or other sensing

technologies that are significantly larger in form factor without

any additional functionality. Thin diaphragm devices tend to be

extremely fragile and unstable. Even in cases where the

device is sufficiently robust for the intended operating

pressure range, the sensor has very poor overpressure

capability.

Now that the pressure range of interest has been

established, the stage has been set to consider the system

solution that is the enabling technology for achieving such

low–pressure sensing capability. Also important in presenting

this low–pressure system solution are some of the other

application characteristics besides the pressure range. For

example, the desired pressure resolution, accuracy, available

power

supply

voltage,

and

end–equipment

system

architecture

play

major

role

determining

the

implementation of this system solution.

DEVELOPMENT HISTORY

For simplicity’s sake, let’s refer to this low–pressure sensing

system solution as the “smart sensing” or “smart sensor

system.” One of the key performance advantages of the smart

sensor system is that the output of the actual sensing element

is ratiometric (linearly proportional) to the excitation voltage

applied to the sensing element. Since most semiconductor

pressure sensors are characterized with a constant voltage

power supply, current excitation will not be discussed.

Although a sensor’s operation is specified at a given power

supply voltage, there is some maximum supply that can be

applied, beyond which power dissipation and self–heating

produce significant output errors or exceed the package’s

thermal handling capability. This means that the strategy of

increasing the sensor’s excitation to improve the sensor’s

sensitivity (increase signal output for a given applied pressure)

can be done in a dc fashion only up to some maximum supply

voltage. For Motorola pressure sensors, this limit allows only

about a 50% to 60% increase in sensitivity, depending on the

specific device family.

About five years ago, some of my colleagues were working

on pulsing the sensor supply voltage with a conventional

voltage and very low duty–cycle, sampling–and–holding the

resulting output, and then filtering the output to produce a dc

sensor output with very low–power consumption. This was the

impetus to consider pulsing a sensor at a much higher than

recommended voltage and a low duty–cycle (10% or less) for

the purpose of increased sensitivity. It is true that some of the

sensor’s parasitic drawbacks, like its zero–pressure offset

voltage and temperature coefficient of offset, are increased as

well, but some of the sensor’s negative characteristics are

lessened. In addition, other sources of error and noise in the

system are not subjected to the higher amplifier gain that

would be required if operating the sensor at a conventional

supply voltage.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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