An instrumentation amplifier's inputs are buffered and many times even amplified and buffered to to its inputs. Why is that so and how do they know how much the signal should be amplified? Couldn't find an "easy" explanation.
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Take a look at the basic schematic of a diff amp created from a single opamp. You will notice that the two inputs aren't high impedance, and that one of them is also coupled to the other. By adding buffers in front of each of these inputs, the inputs of the combined diff amp become high impedance and not coupled to each other.
The designer of this 3-opamp diff amp (often called a instrumentation amp) can trade off where gain is applied. The main diff amp can be configured to provide the gain, or each of the buffer amps can also have gain. Usually the buffer amps have unity gain because anything else can't be done as accurately. Any mismatch in the gain of the buffer amps reduces common mode rejection.
answered Dec 2, 2013 at 23:01
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\$\begingroup\$ yes two inputs aren't high impedance, and that one of them is also coupled to the other. but aren't they coupled via an infinitive input resistance of the differential amplifier where no current can flow? u wrote: "the inputs of the combined diff amp become high impedance and not coupled to each other." what is the benefit of this? preventing the voltage drop? \$\endgroup\$ Commented Dec 2, 2013 at 23:11
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\$\begingroup\$ @user: With a single amp, the inputs are not high impedance because of the circuitry around the amp, even if the amp is ideal with infinite input impedance. High input impedance is useful for voltage amplifiers since that disturbs the signal less. \$\endgroup\$ Commented Dec 2, 2013 at 23:16
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1\$\begingroup\$ Adding gain to the input stage correctly does not impact CMRR (other than adding differential gain w/o adding common mode gain improves CMRR by definition!). The input stage of a 3-input op amp provides a differential gain by virtue of the resistor BETWEEN the two buffers. You change the gain of the input stage by changing this one resistor (and leaving the other two precision trimmed resistors, which live in the chip, alone), and this has no impact on the CMRR. This one resistor is often the "Rext" that you see in the data sheets of instrumentation amps. \$\endgroup\$ Commented Dec 2, 2013 at 23:58