On Fri, 4 Jan 2013 09:42:53 -0700, David DiGiacomo
<daviddigiacomo@...> wrote:
There is no fundamental connection between amplifier input impedance and noise. I've design preamplifiers with sub-300 pV/rtHz, and they could easily have GOhm input resistance. That's because the noise of the input bias resistor is shunted by the (much lower) source impedance.
In my lab (aka basement), the "noise" is mostly RFI pickup.
Did you try connecting a termination to the 7A26?
On my 7A18 that makes some difference but the noise level is still
noticeably higher at the highest sensitivity where the only difference
is removal of the last high impedance input attenuator. I usually
test this with the input set to GND anyway.
The 7A26 is more difficult to gauge since it uses a switched low
impedance gain stage at the highest sensitivity which adds a
confounding factor.
Both the 7A18 and 7A26 uses the same input arrangement and even the
same dual FET. Besides the bandwidth difference, the 7A26 uses
integrated instead of discrete gain stages and I would expect them to
be noisier anyway.
David Hess