MPXM2102AS datasheet(431/670 Pages) MOTOROLA

431 / 670 page

AN1325

3–285

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 2. Instrumentation Amplifier Interface

–

XDCR1

MPX2000 SERIES

PRESSURE SENSOR

–

B+

GND

OUTPUT

µF

MC78L08ACP

1 k

15 k

1 k

U1C

MC33274

R3 1 k

1 k

U1B

MC33274

R8 15 k

0.001

µF

0.1

µF

7.5 k

ZERO

1 k

U1D

MC33274

* NOTE: FOR MPX2010 R10 = 150 OHMS

U1A

MC33274

R10

240*

To provide the desired DC offset, a slight modification is

made in Figure 2. R3 is connected to pin 14 of U1D, which

supplies a buffered offset voltage that is derived from the wiper

of R6.

This voltage establishes a DC output for zero

differential input.

The translation is one to one. Whatever

voltage appears at the wiper of R6 will, within component

tolerances, appear as the zero pressure DC offset voltage at

the output.

With R10 at 240

Ω gain is set for a nominal value of 125,

providing a 4 V span for 32 mV of full scale sensor output.

Setting the offset voltage to 0.75 V, results in a 0.75 V to 4.75

V output that is directly compatible with microprocessor A/D

inputs.

This circuit works reasonably well, but has several notable

limitations when made with discrete components. First, it has

a relatively large number of resistors that have to be well

matched. Failure to match these resistors degrades common

mode rejection and initial tolerance on zero pressure offset

voltage. It also has two amplifiers in one gain loop, which

makes stability more of an issue than it is in the following two

alternatives. This circuit also has more of a limitation on zero

pressure offset voltage than the other two. The minimum

output voltage of U1D restricts the minimum zero pressure

offset voltage that can be accommodated, given component

tolerances. The result is a 0.75 V zero pressure offset voltage,

compared to 0.5 V for each of the following two circuits.

SENSOR SPECIFIC AMPLIFIER

The limitations associated with classic instrumentation

amplifiers suggest that alternate approaches to sensor

interface design are worth looking at. One such approach is

shown in Figure 3. 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 minus

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 zero V. For example, assume that the common

mode voltage 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 by U1C and U1D.

To see how the level

translation works, assume that the wiper of R9 is at ground.

With 4.0 V at pin 12, pin 13 is also at 4.0 V. This leaves 4.0 V

across (R3+R9), which total essentially 1 k

Ω. Since no current

flows into pin 13, the same current flows through R4,

producing approximately 4.0 V across R4, as well. Adding the

voltages (4.0 + 4.0) yields 8.0 V at pin 14. Similarly 4.0 V at

pin 10 implies 4.0 V at pin 9, and the drop across R2 is 8.0 V

– 4.0 = 4.0 V. Again 4.0 V across R2 implies an equal drop

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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