MPXM2102AS datasheet(416/670 Pages) MOTOROLA

416 / 670 page

AN1318

3–270

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 2. Instrumentation Amplifier Interface

B+

GND

µF

0.1

µF

C3 .001

µF

MC78L08ACP

U1B

MC33274

7.5 k

1 k

15 k

1 k

U1A

MC33274

R10

240*

1 k

R3 1 k

U1C

MC33274

XDCR1

MPX2000 SERIES

PRESSURE SENSOR

OUTPUT

R8 15 k

–

U1D

MC33274

* NOTE: FOR MPX2010, R10 = 150 OHMS

ZERO

For applications requiring greater precision a fully

integrated instrument amplifier such as an LTC1100CN8 gives

better results. In Figure 3 one of these amplifiers is used to

provide a gain of 100, as well as differential to single ended

conversion. Zero offset is provided by dividing down the

precision reference to 0.5 V and buffering with U2B. This

voltage is fed into the LTC1100CN8’s ground pin which is

equivalent to returning R3 to pin 14 of U1D in Figure 2. An

additional non–inverting gain stage consisting of U2A, R1 and

R2 is used to scale the sensor’s full scale span to 4 V. R2 is

also returned to the buffered .5 V to maintain the 0.5 V zero

offset that was established in the instrumentation amplifier.

Output voltage range is therefore 0.5 to 4.5 V.

Both of these instrumentation amplifier circuits do their

intended job with a relatively straightforward tradeoff between

cost and performance. The circuit of Figure 2 has the usual

cumulative tolerance problem that is associated with

instrumentation amplifiers that have discrete resistors, but it

has a relatively low cost. The integrated instrumentation

amplifier in Figure 3 solves this problem with precision

trimmed film resistors and also provides superior input offset

performance. Component cost, however, is significantly

higher.

SENSOR SPECIFIC INTERFACE AMPLIFIER

A low cost interface designed specifically for pressure

sensors improves upon the instrumentation amplifier in Figure

2. Shown in Figure 4, it uses one quad op amp and several

resistors to amplify and level shift the sensor’s output. Most of

the amplification is done in U1A which is configured as a

differential amplifier. It is isolated from the sensor’s positive

output by U1B. The purpose of U1B is to prevent feedback

current that flows through R5 and R6 from flowing into the

sensor. At zero pressure the voltage from pin 2 to pin 4 on the

sensor is 0 V. For example, let’s say that the common mode

voltage on these pins is 4.0 V. The zero pressure output

voltage at pin 1 of U1A is then 4.0 V, since any other voltage

would be coupled back to pin 2 via R6 and create a non–zero

bias across U1A’s differential inputs. This 4.0 V zero pressure

DC output voltage is then level translated to the desired zero

pressure offset voltage (VOFFSET) by U1C and U1D.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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