Instrumentation Amp Gain
Gain of the classic three-op-amp in-amp from the gain-set resistor.
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The engineering
The three-op-amp in-amp buffers both inputs (true high-Z differential) and sets gain with one external resistor. The default 24.7 kΩ internal value makes this card match the AD620 family: G = 1 + 49.4 kΩ/Rg.
It exists to amplify millivolt bridge signals sitting on volts of common mode — gain for the difference, rejection for the rest.
Where this math comes from
The three-op-amp topology emerged in the 1960s from analog-computer differential practice, but the monolithic in-amp — laser-trimmed resistors on one die — is what made 120 dB of common-mode rejection a purchasable part rather than a trimming afternoon.
Analog Devices' AD520 (1971) was the first; the AD620 (1992) became the default front end for a generation of bridge and biomedical measurements.
- 1963Analog computing eraThree-op-amp differential topology in common use.
- 1971Analog DevicesAD520 — first monolithic instrumentation amplifier.
- 1992Analog DevicesAD620 sets the modern G = 1 + 49.4k/Rg convention.
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