Impedance Stuff

Discussion in 'General Electronics Chat' started by mlkcampion, Dec 12, 2006.

  1. mlkcampion

    Thread Starter Active Member

    Nov 16, 2006
    Hey Forum
    Just had a look at the discussion on Impedance matching, however
    it seemed to deal more with audio stuff, anyway my question involves a
    pH sensor working at DC levels. I know the priciples are the same but recently i got a description from the company selling the pH sensor saying that it had an initial impedance of 2-3Gigaohms, his argument was that our signal converter (-5mV -> +.5mV converted to 4mA -> 20mA) may not be able to read this. My question: So what if its impedance is high, if it outputs 4mV then why wouldn't the converter amplify it and convert it to 4-20mA???

  2. beenthere

    Retired Moderator

    Apr 20, 2004

    Your pH probe acts like a very weenie battery, and produces voltages depending on the acidity or alkanility of the solution its in. It can only source or sink currents in the range of nanoamps, so a device that is connected to it must have an input impedance of 10^12 ohms or better.

    You will need to buffer the probe's output with an op amp with a suitable input impedance. The pH probe will output +/- 414.4 millivolts full scale.
  3. Papabravo


    Feb 24, 2006
    The concept of impedance is applicable from DC to beyond blue light. It isn't limited to audio frequencies but applies to any AC source and circuit.
  4. mlkcampion

    Thread Starter Active Member

    Nov 16, 2006
    Thanks for the reply!