Electronics > Beginners
Logarithmic Picoammeter
David Hess:
--- Quote from: staticshock on March 22, 2018, 03:19:17 pm ---In addition, we are losing some accuracy in the low range. Other than calibration, is there anything we can do to improve it?
--- End quote ---
At the picoamp level:
1. Leakage due to surface contamination is a serious problem. Special substrates, air wiring, and guard rings on the printed circuit board are often used. DIP and through hole parts have an advantage due to greater lead separation. Air wiring sensitive nodes works well because air is an excellent insulator but Teflon insulated terminals are sometimes used. My preference is to pull the inverting input off of the board and air wire that node.
Test for surface contamination by blowing onto the circuit through a straw. The water vapor in your breath will alter the conductivity of ionic surface contaminates.
2. Accuracy falls off at high currents due to resistance and at low currents due to leakage. I did not find much discussion online about compensating for these errors in the transistor.
staticshock:
Update on the project:
We've built the circuit and the output is pretty consistent with the simulated results until 10^-10. There will be some calibration done there. However, I'd like to ask if there are ways we can increase the response time of the output especially for the very low currents. It really does take a long time for the output to stabilize after 10^-10.
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