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Common-Mode Rejection Ratio

The instrumentation amplifier circuit shown uses three op-amps, with the rightmost op-amp and resistors forming a standard differential amplifier. The two op-amps on the left act as buffers. The single resistor between the inverting inputs of the buffer op-amps increases the differential gain while maintaining a common-mode gain of 1, improving common-mode rejection ratio. This single resistor design avoids resistor matching issues and allows easy adjustment of the overall circuit gain by varying only the value of that resistor.

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Khadar Basha
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48 views1 page

Common-Mode Rejection Ratio

The instrumentation amplifier circuit shown uses three op-amps, with the rightmost op-amp and resistors forming a standard differential amplifier. The two op-amps on the left act as buffers. The single resistor between the inverting inputs of the buffer op-amps increases the differential gain while maintaining a common-mode gain of 1, improving common-mode rejection ratio. This single resistor design avoids resistor matching issues and allows easy adjustment of the overall circuit gain by varying only the value of that resistor.

Uploaded by

Khadar Basha
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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The most commonly used instrumentation amplifier circuit is shown in the figure.

The gain of the circuit is

The rightmost amplifier, along with the resistors labelled and is just the standard differential

amplifier circuit, with gain = and differential input resistance = 2· . The two amplifiers on the

left are the buffers. With removed (open circuited), they are simple unity gain buffers; the circuit will

work in that state, with gain simply equal to and high input impedance because of the buffers. The
buffer gain could be increased by putting resistors between the buffer inverting inputs and ground to shunt

away some of the negative feedback; however, the single resistor between the two inverting inputs is
a much more elegant method: it increases the differential-mode gain of the buffer pair while leaving the
common-mode gain equal to 1. This increases the common-mode rejection ratio (CMRR) of the circuit and
also enables the buffers to handle much larger common-mode signals without clipping than would be the
case if they were separate and had the same gain. Another benefit of the method is that it boosts the gain
using a single resistor rather than a pair, thus avoiding a resistor-matching problem, and very conveniently
allowing the gain of the circuit to be changed by changing the value of a single resistor. A set of switch-

selectable resistors or even a potentiometer can be used for , providing easy changes to the gain of the
circuit, without the complexity of having to switch matched pairs of resistors.

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