Am 2022-07-10 9:11, schrieb Erik Kaashoek via time-nuts:
@Bob,
You mentioned "dual supplies with high voltage" for the first gain
opamp. How much impact would dual voltage bring as its a pain to
implement.
I think is was Rubiola who wrote that he exploded a costly microwave
mixer
with a LT1028 that had lost one of its power rails.
If you go AC coupling, don't forget that the input capacitor must not be
selected for f-3dB but that it must be much bigger to short the thermal
noise
of the bias network to pV levels through the low impedance source.
Otherwise you see a noise rise towards 0 Hz that is MUCH steeper than
1/f.
Scott Wurzer (designer of ad797) saw that immediately on my 20 * ada4898
220pV/rtHz amplifier. Wish he was more explicit. It took me some time to
get it. :-)
I had 10K/100u foil, ended up with 10k/4700uF wet tantalum, which opens
another can of worms.
I have converted to FETs now. A few pVrtHz more, but much less noise
current.
In cross correlation setups, the noise current of both amplifiers
produces
a common voltage drop in the (common) source resistance, and that does
not
average away. (May apply only to voltage measurements from a single
source.)
Gerhard
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