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How to measure 50 pA current with high "noise" current?

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dmendesf:


You can treat the large signal as the modulating   noise to me removed using a synchronous sampler...


--- Quote from: ogden on April 25, 2020, 03:58:21 pm ---You would want to consider synchronous demodulator - means of measuring low-level signals in the presence of relatively high noise levels.
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The problem here isn't uncorrelated noise (which lock-in or modulation will remove) but a large unwanted coupling from the exciting signal, its already modulated!
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Marco:
Photorelay across the feedback resistor, trigger when output voltage exceeds positive/negative threshold?

Kunibert:

--- Quote from: dmendesf on April 25, 2020, 02:07:31 pm ---What about using a sample and hold in sync with the switching power supply to sample the amplifier output when the waveform is undisturbed?

--- End quote ---
The TIA doesnt reach a steady state due to the high current pulse and the high feedback resistance


--- Quote from: Kleinstein on April 25, 2020, 02:26:09 pm ---One could try adding an electronic switch to connect the signal to ground and maybe also isolate the integrator.

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But how to design a switch with a low off-state current and fast switching capability? The input offset of the OpAmp causes a voltage across the transistor, therefore several nA current flows. The body diode could also cause problems.


--- Quote from: Jay_Diddy_B on April 25, 2020, 03:01:50 pm ---you only need a dv/dt of 16.6 V/second to get 50pA
Can you post the schematic of your TIA?

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Until now, its just a TIA like this (+Capacitor in parallel to the feedback resistor) https://upload.wikimedia.org/wikipedia/commons/thumb/f/f1/TIA_simple.svg/400px-TIA_simple.svg.png
Its a High-Voltage Source with good decoupling/ filtering. Voltage seems to be stable after switching, I will measure it again soon.


--- Quote from: David Hess on April 25, 2020, 05:38:02 pm ---What you can do instead though is clamp it from the output to the inverting input either with a low leakage FET or low leakage diodes. Transistor base-emitter junctions make good low leakage fast diodes.
If the high voltage edge is consistent, then charge compensation with another edge and capacitor could be used.

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But it needs to sink and source charge due to the positive and negative pulse. For this, the diodes would have to be connected in antiparallel. Wouldn't this lead to an undefined gain?
Charge compansation is the right Keyword. In fact, the waveform does not have to be inverted exactly. With a calibrated compensation voltage this could work!


--- Quote from: Someone on April 25, 2020, 10:42:06 pm ---The OP hasn't mentioned what they are trying to extract from the signals, if its just the DC then there are many other options.

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Its "just" the DC-current  :phew: What other options do you have in mind?


--- Quote from: Marco on April 27, 2020, 02:40:29 am ---Photorelay across the feedback resistor, trigger when output voltage exceeds positive/negative threshold?

--- End quote ---
I've been thinking about that, too. But most photocouplers are too slow (turn-on/off time up to some ms) and the off-state current is much higher than my measuring range. Could not make it work.

Many thanks for the helpful answers!

Marco:

--- Quote from: Kunibert on April 27, 2020, 04:25:32 pm ---I've been thinking about that, too. But most photocouplers are too slow (turn-on/off time up to some ms) and the off-state current is much higher than my measuring range. Could not make it work.

--- End quote ---

TLP3440S switches in 100 us and shows 1 pA or less leakage for a 5V signal at room temperature on its datasheet.

I'm sure there are others with similar spec. What is 100 uS on a <<1 Hz signal? Make the trans impedance slightly higher bandwidth and do the rest of the filtering digitally BTW. It's much easier to do non-linear filtering to clean up the remaining glitch digitally.

Kleinstein:
Campling to ground can work, if there is enough resistance between the switch and the TIA. With some 100 µV offset and 100 K ohms the current would be in the 1 nA range. This is not good but could be still manageable for the TIA without going to saturation.

I think the clamping to the output should be the better solution. With the regular clamping to the output one could use a charge amplifier and see the phase with the high current pulse as integrator reset. So one would measure the current from the rate the charge accumulates after the reset.
The input of the charge amplifier is reasonable close to ground, so that switching should not be so difficult and with a well defined charge injection. This could be a JFET switch with some compensation of the gate charge. A modern CMOS switch chip is probably easier to use. Suitable switches can be below 1 pC charge injection at a suitable voltage. Switching speed can be below 100 ns, which should be fast enough. It more like needs several µs for the integrator reset.
One may have to do a separate zero measurement to compensate for the leakage currents from the CMOS switch and the OP. Leakage current can be below 10 pA, at least for a typical value.

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