amplifier AD8421 becouse I need to compensate the effect of this one in the
next stage of my design.
We did not measure the output impedance at high frequencies, but we can
approximate this very closely based on the AD8421 frequency response curves and
the output stage biasing.
The approximate value will be about 0.1Ω near dc, increasing to about 100Ω at
8MHz. It will approach the 100Ω value at +20dB/decade as the loop gain of the
output amplifier is decreasing with higher frequencies. This behavior will be
gain-independent, because the output amplifier noise-gain is always 2
regardless of the pre-amplifier gain.