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How to measure 50 pA current with high "noise" current?
Kleinstein:
I would not worry much about 0.35 pC of charge injection this would be no big problem, more like corrected mathematically if needed.
If really needed it could be some small capacitance (possibly adjustable) and a square wave signal with some 3-5 V. However the quoted charge injection depends on the input capacitance and DC voltage level. So the compensation may not work perfect in all cases.
A dummy switch maybe used on the same chip, but hardly externally, as the charge injection is different for on and off (for the off -> on transition one gets both sides, off only one. So if at all one would need a switch of halve the size.
Chances are a very low charge injection switch mal already use some compensation inside the 0.35 pC may be just the typical left over.
One may than be able to adjust the DC level for the switch to get zero charge injection at that one voltage that is needed.
The compensation idea was about the 3.7 kV times some 5 pF causing the large spikes.
Kunibert:
--- Quote from: Kleinstein on May 05, 2020, 10:10:47 am ---I would not worry much about 0.35 pC of charge injection this would be no big problem, more like corrected mathematically if needed.
Chances are a very low charge injection switch mal already use some compensation inside the 0.35 pC may be just the typical left over.
--- End quote ---
The charge injection of the analog switch is large compared to the charge introduced during the measurement period. However, the measuring range can be adjusted as required.
I will save further attempts for the V2, if the charge amplifier is promising.
--- Quote from: Kleinstein on May 05, 2020, 10:10:47 am ---One may than be able to adjust the DC level for the switch to get zero charge injection at that one voltage that is needed.
--- End quote ---
I will give it a try!
Thank you!
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