life speed wrote:
Message: 6
From: Pete Rawson<[email protected]>
Subject: Re: [time-nuts] Advice on 10 MHz isolation/distribution
Clay,
Analog devices AD4899-1 voltage noise = 2nV/rtHz @ 10Hz; GBW = 300MHz.
Pete Rawson
That is an interesting part.
So, how does one think about reverse isolation in a feedback amp? Is the
open-loop gain of the amp applied to the spurious signal to cancel it? The
spurious, reduced by the ratio of the 50 ohm source impedance, to the output
impedance of 0.7 ohms of the amp (-18 dB) appears at the inverting input . . .
And what would appear at the non-inverting input?
Off to the simulator.
Clay
Reverse isolation is highest if one uses a non inverting amplifier
rather than an inverting one.
The signal at the inverting input is coupled to the non inverting input
via the differential input impedance (predominately capacitive) of the
opamp.
ie finite input stage current gain and emitter base (along with package
strays) capacitances are the principal contributors at least for
frequencies where the input stage output voltage swing is small.
Bruce
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