MPXM2102AS datasheet(394/670 Pages) MOTOROLA

394 / 670 page

AN1315

3–248

Motorola Sensor Device Data

www.motorola.com/semiconductors

–

PD0

TP1

+5 V

1N914

4.7K

U1A

U1C

U1D

U1B

976

R10

340

23.7

XDCR1

121

200

6.98K

+12 V

MC33274

J8 IS INSTALLED FOR

THE MPX2010 ONLY

Figure 3.

Figure 4. Analog Interface

Referring to Figure 4, most of the amplification of the voltage

from the pressure sensor is provided by U1A which is

configured as a differential amplifier. U1B serves as a unity

gain buffer in order to keep any current that flows through R2

(and R3) from being fed back into the sensor’s negative

output. With zero pressure applied to the sensor, the

differential voltage from pin 2 to pin 4 of the sensor is zero or

very close to zero volts. The common mode, or the voltage

measured between pins 2 or 4 to ground, is equal to

approximately one half of the voltage applied to the sensor, or

4 volts. The zero pressure output voltage at pin 7 of U1A will

then be 4 volts because pin 1 of U1B is also at 4 volts, creating

a zero bias between pins 5 and 6 of U1A. The four volt zero

pressure output will then be level shifted to the desired zero

pressure offset voltage (approximately 0.7 volts) by U1C and

U1D.

To further explain the operation of the level shifting circuitry,

refer again to Figure 4. Assuming zero pressure is applied to

the sensor and the common mode voltage from the sensor is

4 volts, the voltage applied to pin 12 of U1D will be 4 volts,

implying pin 13 will be at 4 volts. The gain of amplifier U1D will

be (R10/(R8+R9)) +1 or a gain of 2. R7 will inject a Voffset (0.7

volts) into amplifier U1D, thus causing the output at U1D pin

14 to be 7.3 = (4 volts @ U1D pin 12

2) – 0.7 volts. The gain

of U1C is also set at 2 ((R5/R6)+1). With 4 volts applied to pin

10 of U1C, its output at U1C pin 8 will be 0.7 = ((4 volts @ U1C

pin 10

2) – 7.3 volts). For this scheme to work properly,

amplifiers U1C and U1D must have a gain of 2 and the output

of U1D must be shifted down by the Voffset provided by R7. In

this system, the 0.7 volts Voffset was arbitrarily picked and

could have been any voltage greater than the Vsat of the op

amp being used. The system software will take in account any

variations of Voffset as it assumes no pressure is applied to the

sensor at system power up.

The gain of the analog circuit is approximately 117. With the

values shown in Figure 4, the gain of 117 will provide a span

of approximately 4 volts on U1C pin 8 when the pressure

sensor and the 8 volt fixed voltage regulator are at their

maximum output voltage tolerance. All of the sensors listed in

Table 1 with the exception of the MPX2010DP output

approximately 33 mV when full scale pressure is applied.

When the MPX2010DP sensor is used, its full scale sensor

differential output is approximately 20 mV. J8 must be installed

to increase the gain of the analog circuit to still provide the 4

volts span out of U1C pin 8 with a 20 mV differential from the

sensor.

Diode D2 is used to protect the microprocessor’s A/D input

if the output from U1C exceeds 5.6 volts. R4 is used to provide

current limiting into D4 under failure or overvoltage conditions.

SOFTWARE

The source code, compiled listing, and S–record output for

the software used in this system are available on the Motorola

Freeware Bulletin Board Service in the MCU directory under

the filename DEVB158.ARC. To access the bulletin board,

you must have a telephone line, a 300, 1200 or 2400 baud

modem and a personal computer. The modem must be

compatible with the Bell 212A standard. Call (512) 891–3733

to access the Bulletin Board Service.

Figure 5 is a flowchart for the program that controls the

system. The software for the system consists of a number of

modules. Their functions provide the capability for system

calibration as well as displaying the pressure input to the

MPX2000 series pressure sensor.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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