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Limitations of Measuring Small Currents

Question asked by amf on May 7, 2017
Latest reply on Apr 12, 2018 by mark.ramos

This is a question for Marc Ramos, a followup to his answer of March 16 regarding a question about measurement limitations in amperometry.  I too have recently laid out and fabricated a custom ADuCM350 board to measure very small currents.  However, I have problems in getting accurate results (i.e. I don't measure 32768 when amperometric.c applies 0 V across the load, in my case a 4 Mohm load).



   You posted a schematic that showed using the ADuCM350 with an RTIA of 12 Mohms.  Did you really get that to work?  What value did you use for your RCAL resistor and what values for RCAL/RTIA did you type into your code?  That is, did you type in "12000000" into your code in the call in amperometric.c that asks for the value of RTIA, or did you measure the resistance after you soldered in the 12 Mohm resistor and enter the actual measured value of RTIA when it was in parallel with the ADuCM350?  


    The reason I ask, is that if I measure the value of my 3 Mohm 0.1% resistor in series with a 1 Mohm 0.1% resistor before soldering them into the pads for RTIA, I measure 3.9997 Mohms.  Fine.  However, once they are in the board, in parallel with the ADuCM350, I measure 1.22 Mohms.  So what would you type in for RTIA (i.e. does the api's error correction take care of this discrepancy)?


     A similar issue arises for RCAL.  If you remove the RCAL resistor and measure the input resistance looking into the ADuCM350 from RCAL's pads, you measure 1.54 Mohms.  I tried all sorts of values for RCAL, and all possible combinations of typing the measured in- vs. out-of-the board values into the place where amperometric.c asks for the value of RCAL, but I still never get the result of 32768 when 0V is applied across the 4 Mohm load.


    It seems then, that there are definite limitations of using the ADuCM350 for measurement of very small currents.  Would you concur?  What's the smallest increment of current you've been able to resolve?


    Or is the problem that one really needs a Keithley 2401 to apply an accurate 1.1V (for zero volts across the load)?  That is, is the problem with the ADuCM350's DAC not being accurate enough?


    What is the status of your new board for measuring small currents?  Have you run into similar problems?


Thank you.