MPXM2102AS datasheet(470/670 Pages) MOTOROLA

470 / 670 page

AN1525

3–324

Motorola Sensor Device Data

www.motorola.com/semiconductors

• Since the third op–amp contributes a gain of two to the

overall system, design the gain that the two op–amp circuit

contributes to the system to be one–half the desired sys-

tem gain. The gain term for the two op–amp circuit is:

G =

2R4

+ 1

which is the same as presented in Equation 16.

• Similarly, since the third op–amp also amplifies VREF by

two (refer to Equation 22), the resistor divider that creates

VREF should be designed to provide one–half the desired

positive dc voltage level shift needed for the final output.

When designing the voltage divider for VREF, use the same

design constraints as were given in the section on Applica-

tion to Pressure Sensor Circuits.

With the above simplification of R5 = R6, the negative dc

level shift, V–shift, which is also created by a voltage divider, is

now amplified by a factor of unity. When designing the voltage

divider, use the same design constraints as were presented in

the section on Application to Pressure Sensor Circuits.

CONCLUSION

The amplifier circuits discussed in this paper apply to

pressure sensor applications, but the amplifier circuits can be

interfaced to low–level, differential–voltage output sensors, in

general. All of the circuits exhibit the desired instrumentation

amplifier characteristics of high input impedance, low output

impedance,

high

gain

capability,

and

differential

single–ended conversion of the sensor signal. Each

amplifier circuit provides positive dc level shift capability, while

the last two circuit topologies presented are also able to

provide a negative dc voltage level shift. This enables the user

to position the sensor’s dynamic output within a specified

range (e.g., within the high and low references of an A/D

converter). Also detailed is a method of using an additional

feedback resistor to adjust easily the differential voltage gain,

while not sacrificing common mode rejection. Combining the

appropriate sensor device and amplifier interface circuit

provides sensor users with a versatile system solution for

applications in which the ideal fully single–conditioned sensor

does not exist or in which such signal flexibility is warranted.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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