MPXM2102AS datasheet(432/670 Pages) MOTOROLA

432 / 670 page

AN1325

3–286

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 3. Sensor Specific Amplifier

* NOTE: FOR MPX2010 R5 = 75 OHMS

µF

B+

GND

OUT

R9 200

ZERO

CAL.

1.5 k

XDCR1

MPX2000 SERIES

PRESSURE SENSOR

–

U1A

MC33274

TP2 +8 V

0.1

µF

R6 7.5 k

120*

–

U1B

MC33274

820

–

U1C

MC33274

R1 2 k

R2 2 k

–

U1D

MC33274

R4 1 k

MC78L08ACP

across R1, and the voltage at pin 8 is 4.0 V – 4.0 V = 0 V. In

practice, the output of U1C will not go all the way to ground,

and the voltage injected by R8 at the wiper of R9 is

approximately translated into a DC offset.

Gain is approximately equal to R6/R5(R1/R2+1), which

predicts 125 for the values shown in Figure 3. A more exact

calculation can be performed by doing a nodal analysis, which

yields 127.

Cascading the gains of U1A and U1C using

standard op amp gain equations does not give an exact result,

because the sensor’s negative going differential signal at pin

4 subtracts from the DC level that is amplified by U1C. Setting

offset to 0.5 V results in an analog zero to full scale range of

0.5 to 4.5 V. For this DC output voltage to be independent of

the sensor’s common mode voltage it is necessary to satisfy

the condition that R1/R2 = (R3+R9)/R4.

This approach to interface amplifier design is an

improvement over the classic instrument amplifier in that it

uses fewer resistors, is inherently more stable, and provides

a zero pressure output voltage that can be targeted at .5 V. It

has the same tolerance problem from matching discrete

resistors that is associated with classic instrument amplifiers.

SENSOR MINI AMP

Further improvements can be made with the circuit that is

shown in Figure 4. It uses one dual op amp and several

resistors to amplify and level shift the sensor’s output. To see

how this amplifier works, let’s simplify it by grounding the

output of voltage divider R3, R5 and assuming that the divider

impedance is added to R6, such that R6 = 12.4 k. If the

common mode voltage at pins 2 and 4 of the sensor is 4.0 V,

then pin 2 of U2A and pin 6 of U2B are also at 4.0 V. This puts

4.0 V across R6, producing 323

µA. Assuming that the current

in R4 is equal to the current in R6, 323

µA • 100 Ω produces

a 32 mV drop across R4 which adds to the 4.0 V at pin 2. The

output voltage at pin 1 of U2A is, therefore, 4.032 V. This puts

4.032 – 4.0 V across R2, producing 43

µA. The same current

flowing through R1 again produces a voltage drop of 4.0 V,

which sets the output at zero. Substituting a divider output

greater than zero into this calculation reveals that the zero

pressure output voltage is equal to the output voltage of

divider R3, R5. For this DC output voltage to be independent

of the sensor’s common mode voltage it is necessary to satisfy

the condition that R1/R2 = R6/R4, where R6 includes the

divider impedance.

Gain can be determined by assuming a differential output at

the sensor and going through the same calculation. To do this

assume 100 mV of differential output, which puts pin 2 of U2A

at 3.95 V, and pin 6 of U2B at 4.05 V. Therefore, 3.95 V is

applied to R6, generating 319 uA. This current flowing through

R4 produces 31.9 mV, placing pin 1 of U2A at 3950 mV + 31.9

mV = 3982 mV. The voltage across R2 is then 4050 mV –

3982 mV = 68 mV, which produces a current of 91

µA that

flows into R1. The output voltage is then 4.05 V + (91

µA •

93.1 k) = 12.5 V. Dividing 12.5 V by the 100 mV input yields

a gain of 125, which provides a 4 V span for 32 mV of full

scale sensor output. Setting divider R3, R5 at 0.5 V results

in a 0.5 V to 4.5 V output that is comparable to the other two

circuits.

This circuit performs the same function as the other two with

significantly fewer components and lower cost. In most cases

it is the optimum choice for a low cost interface amplifier.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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