Datasheet

EXCITATION

SOURCE

R + 'R

R - 'R

R - 'R R + 'R

OUT

(b)

OUT

x V

SOURCE

EXCITATION

SOURCE

OUT

(a)

OUT

1 +

x V

SOURCE

LMP2234

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SNOSAW4D –SEPTEMBER 2007–REVISED MARCH 2013

Often in sensor applications at lease one of the resistors is a variable resistor, or a sensor. The deviation of this

active element from its initial value is measured as an indication of change in the measured quantity. A change in

output voltage represents the sensor value change. Since the sensor value change is often very small, the

resulting output voltage is very small in magnitude as well. This requires an extensive and very precise

amplification circuitry so that signal fidelity does not change after amplification.

Sensitivity of a bridge is the ratio of its maximum expected output change to the excitation voltage change.

Figure 50(a) shows a typical bridge sensor and Figure 50(b) shows the bridge with four sensors. R in

Figure 50(b) is the nominal value of the sense resistor and the deviations from R are proportional to the quantity

being measured.

Figure 50. Bridge Sensor

Instrumentation amplifiers are great for interfacing with bridge sensors. Bridge sensors often sense a very small

differential signal in the presence of a larger common mode voltage. Instrumentation amplifiers reject this

common mode signal.

Figure 51 shows a strain gauge bridge amplifier. In this application one of the LMP2234 amplifiers is used to

buffer the LM4140A's precision output voltage. The LM4140A is a precision voltage reference. The other three

amplifiers in the LMP2234 are used to form an instrumentation amplifier. This instrumentation amplifier uses the

LMP2234's high CMRR and low V

and TCV

to accurately amplify the small differential signal generated by

the output of the bridge sensor. This amplified signal is then fed into the ADC121S021 which is a 12-bit analog to

digital converter. This circuit works on a single supply voltage of 5V.

Product Folder Links: LMP2234