MPXM2102AS datasheet(485/670 Pages) MOTOROLA

485 / 670 page

AN1551

3–339

Motorola Sensor Device Data

www.motorola.com/semiconductors

SYSTEM DESIGN

As mentioned in the introduction, the lowest pressure

devices in the Motorola portfolio are rated at a full–scale

pressure of 10 kPa (40

″ of H2O). The calibrated and

temperature compensated, 10 kPa device (MPX2010) is

specified to operate at a 10 Vdc supply voltage and produce

25 mV (nominal) at the full–scale pressure of 10 kPa. This

translates to a 0.25 mV/(V*kPa) pressure sensitivity.

Additionally, the absolute maximum supply voltage specified

is 16 Vdc. Thus, the maximum full–scale output signal that can

be achieved without exceeding the maximum supply voltage

rating is 40 mV, or 60% greater than the output at the 10 Vdc

specification. So, a 60% increase can be achieved in the

output signal of the sensor for the 0–10 kPa pressure range,

or the same signal level of 25 mV can be preserved over a

proportionally lower applied pressure range (i.e., 0–6.25 kPa).

The point here is that increasing the dc supply excitation only

produces limited improvement in the output signal level.

Much greater gains in output signal level (sensor span) can

be obtained, if it is possible to operate the sensor at

significantly higher voltages. Since the thermal/power

dissipation limitation imposed by the maximum dc supply

voltage can be avoided by using a pulsed excitation at a low

duty–cycle (on–time) and reasonable period, and second

order junction effects do not occur until much higher voltages,

the sensor output can be greatly increased by operating at a

much higher ac voltage than permitted by the dc counterpart

of this same higher voltage. As an example, industrial

applications like HVAC have 24 V commonly available, and we

want to accurately measure pressures below 10

″ H2O. To

achieve a 1–2% of full–scale accuracy (based on temperature

drift errors, system noise, device tolerance, power supply

variation/rejection, etc.), 9–12 mV is the typical minimum

full–scale span that is the desired target for the pressure range

of interest. For the MPX2010 pulsed at 24 V, we obtain 15 mV

of output for an applied pressure of 10

″ H2O (2.5 kPa). This

same sensor device will only produce 6.25 mV at its normally

specified supply of 10 V and 2.5 kPa, thus not meeting the

signal–to–noise

ratio

criteria

for

1–2%

accuracy

performance.

This smart sensing solution is intended to sense full–scale

pressures below 10

″ H2O with 1% of full–scale pressure

resolution and better than 2% of full–scale accuracy. The

following subsystems comprise the hardware portion of this

solution (see Figure 1):

Figure 1. Smart Sensing Block Diagram

PRESSURE SENSOR

SIGNAL

CONDITIONING

SWITCHING

CIRCUITRY

LOW VOLTAGE

INHIBIT

8–BIT

MICROCONTROLLER

5 V

± 5%

REGULATOR

POWER SUPPLY

REJECTION CIRCUITRY

VPP

Dout

Din

SCLK

VCC

Gnd

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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