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Is it possible to cascade two instrumentation amplifiers (for instance INA128)? let's say each has a gain (G) of 100 up to 100 kHz. Would this result in a G=10k up to the same frequency?

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    \$\begingroup\$ Maybe, but... The input offset voltage of this difference amp is +/- 100µV for 0V common mode, and +/-500µV max for 12V common mode. At 100µV, if you cascade two amps for a gain of 10000, then 100µV input offset results in 1V output offset voltage. \$\endgroup\$
    – bobflux
    Commented Nov 30, 2019 at 9:36
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    \$\begingroup\$ Yes, in principle. But there are a lot a problems that come with such a high gain, though. Besides the offset amplification problem that @peufeu mentioned, keep in mind that the frequency bandwidth is defined for small signals. With high gain, expect higher signal amplitude at the second stage, in which case your signal most likely will be limited by the maximum slew rate of the amplifier before it hits the maximum small signal frequency limit. \$\endgroup\$
    – joribama
    Commented Nov 30, 2019 at 9:50
  • \$\begingroup\$ Yes, also it is a current sense amp, not optimized for low noise... Datasheet says 40nV/rtHz input referred so over 100k bandwidth and with 10k gain you got 0.13V RMS noise on the output, that's a bit meh. \$\endgroup\$
    – bobflux
    Commented Nov 30, 2019 at 12:11
  • \$\begingroup\$ Thanks a lot for your answers. I am interested in sensing a very narrow bandwidth which is also know beforehand. @peufeu About your answer about the input offset, is it not possible to compensate that with some feedback integration into the reference? \$\endgroup\$ Commented Dec 4, 2019 at 13:56
  • \$\begingroup\$ If this is AC, then DC offset does not matter, you can arrange the amp to have unity gain at DC and high gain at AC. Note INA181 is not an instrumentation amp, it is a difference amplifier. Instrumentation amp doesn't care about source impedance imbalance but difference amp needs equal source impedance on both inputs. \$\endgroup\$
    – bobflux
    Commented Dec 4, 2019 at 19:30

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Of course it's possible, but there is no reason to do it. By the time you've gone through one Instrumentation amplifier, you've already converted your differential signal to single ended, and you've already dealt with Common Mode Noise. Subsequent stages should just be op amps.

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