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increased A/D noise when isolated - Addendum

Intellectual 620 points

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Unfortunately the original thread has been closed..

I would like to add news to my topic: It seems the increased noise was caused by the input amplifier/active attenuator (formed by an inverting opamp) escpecially the input capacitance.

This highly decreased the phase margin and thus the amp tend to oscillate with high frequncies (and of coarse fundamental property of differential-like pre-emphasis).

This phenomenon is nicely described in TI Signal Book from Mr.Trump (Chapter 13: Taming the oscillating opamp).

I compensated the feedback Network "upper part"  by added capacitors and the signal became noise free.