PIC ADC impedance matching

Discussion in 'Embedded Systems and Microcontrollers' started by colinDJ, Feb 3, 2010.

  1. colinDJ

    Thread Starter New Member

    Apr 29, 2009
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    0
    Hi all
    I've built a 35V PIC (16F88) LCD meter for measuring 'conductance' in a electrolysis-type setup, at 1 to 1.5mA. The resistance divider takes it to 35mV/ADC. I first measure one probe, then other, then subtract to find difference. Doing this through a 1K resistor gives me the 'amps' flowing in circuit. When not immersed, it gives a reading which is the current drawn by the adc voltage divider. How can I eliminate this to give me the correct amount of mA flowing in my circuit?
    Thank you all.
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    This is described in the data sheet. Can you post your circuit also. Your problem could be in the hardware
     
  3. colinDJ

    Thread Starter New Member

    Apr 29, 2009
    3
    0
    Hi t06afre
    Yes Seems to be a hardware thing - measuring through the 1K resistor,
    but I can't work out another way. See the attached sketch.
     
  4. colinDJ

    Thread Starter New Member

    Apr 29, 2009
    3
    0
    Actually need to monitor the electrolysis-like process going on until the current limit is reached, then to monitor the voltage dropping, and do certain things at certain voltages.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    If you want to measure current you have to "open" the current path, then insert a small resistor and measure voltage drop over this resistor.
     
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