Electronics > Projects, Designs, and Technical Stuff
How to measure 50 pA current with high "noise" current?
Kunibert:
Hello,
I am new to the forum and I am approaching you directly with a problem.
I would like to measure a slowly varying (F<<1Hz) current from -50 to 50 pA. So far I have achieved this with a simple transimpedance amplifier using a resistor + capacitor in the feedback path and a guardring.
However, in this case there is a parasitic capacitance of 3 pF between the measured object and a switched voltage source. A current of 150 mA couples in via this by voltage switching. Unfortunately, the coupling cannot be further reduced. These 3 pF are the only capacitance at the input.
Basically, it should be sufficient if the transimpedance amplifier remains in the linear range?
I have the following ideas:
1) Two low-leakage diodes anti parallel to ground. But in simulations the controller becomes unstable.
2) Input capacitor, which stores the introduced charge and limits the voltage rise. But the transimpedance amplifier can never lower the charge because of the high feedback resistance. In addition, the additional input capacitance causes instability at the controller
3) The couplings are regular and predictable. It may be possible to decouple the measuring circuit for the points in time and then there would be about 50us left for settling and measuring.
4) Making the feedback resistance variable also causes oscillations in the simulation.
Does anyone have any ideas or literature/links? How to design switches with low capacity and leakage current? Reed relays and solid state relays will probably not work?
Thanks a lot
ogden:
Usually you fight with such problems by eliminating them completely, in the source. Easiest solution: get rid of the switching supply. Depending it's one-off measurement or design of (mass market) product, just use battery + low noise LDO or mains transformer supply + low noise LDO.
Kunibert:
Unfortunately, this is not possible due to the principle. The voltage (~3750 V) generates an E-field, which is needed for the measurement. The coupling capacitance cannot be reduced because the measurement setup cannot be changed. The interference is not caused by the voltage source, but by switching the voltage. I didn't express myself very well. English is not my first language.
I am aware that far more charge is transferred by the parasitic current than by the measuring current. I hope that the DC balanced interference can be compensated somehow.
dmendesf:
What about using a sample and hold in sync with the switching power supply to sample the amplifier output when the waveform is undisturbed?
Kleinstein:
One could try adding an electronic switch to connect the signal to ground and maybe also isolate the integrator.
The usual CMOS switches have a charge injection in the 1-10 pC range, this is a lot less than some 3750 V*3 pF =11.25 nC. With something a an inductor / 100 K resistor towards the TIA one may not even need the switch to the TIA, just a connection to ground.
Edit: using low leakage diode could be a problem, as the really low leakage diode show quite some reverse recovery. So much of the charge could flow back from the diode in the following 20-50 µs. If at all one could try a transistor junction (e.g. base+collector to emitter) : these are also relatively low leakage, but faster with a reverse recovery time of more like 0.1- 1 µs compared to some 20 µs for low leakage diodes.
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