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LMP2231

www.ti.com

SNOSB01E – JANUARY 2008 – REVISED MARCH 2013

Figure 49. Noise Reduction Circuit

PRECISION INSTRUMENTATION AMPLIFIER

Measurement of very small signals with an amplifier requires close attention to the input impedance of the

amplifier, gain of the overall signal on the inputs, and the gain on each input of the amplifier. This is because the

difference of the input signal on the two inputs is of the interest and the common signal is considered noise. A

classic circuit implementation is an instrumentation amplifier. Instrumentation amplifiers have a finite, accurate,

and stable gain. They also have extremely high input impedances and very low output impedances. Finally they

have an extremely high CMRR so that the amplifier can only respond to the differential signal. A typical

instrumentation amplifier is shown in Figure 50.

Figure 50. Instrumentation Amplifier

There are two stages in this amplifier. The last stage, output stage, is a differential amplifier. In an ideal case the

two amplifiers of the first stage, input stage, would be set up as buffers to isolate the inputs. However they

cannot be connected as followers because of mismatch of amplifiers. That is why there is a balancing resistor

between the two. The product of the two stages of gain will give the gain of the instrumentation amplifier. Ideally,

the CMRR should be infinite. However the output stage has a small non-zero common mode gain which results

from resistor mismatch.

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