Low current readings!

Discussion in 'The Projects Forum' started by rigers, Mar 13, 2012.

  1. rigers

    Thread Starter New Member

    Jun 14, 2011
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    Hello,
    I want to be able to measure currents all the way down to the nA. I wanted to do it with a current shunt amplifier but I see that because of the offset voltages and stuff. They're not that practical for that range. I searched online and I found that the best way to measure low currents is to use a feedback ammeter.
    Anybody know any good ones. Also I want to be able to implement this directly on a PCB, so it will be part of a system.
     
  2. mcasale

    Member

    Jul 18, 2011
    210
    12
    Back "in the old days" we designed discrete electrometers that measured well below this -- like better than picoamps. This is not an easy thing to do on a PCB. Leakage currents can kill you. You will probably need guard rings around all your high impedance points.

    You did not describe your application. What is the maximum current you will measure? What is the frequency range of interest (DC to 1Hz or what)? How much noise can you tolerate?

    An op amp is essentially a current amplifier.
     
  3. #12

    Expert

    Nov 30, 2010
    16,255
    6,754
    If you have a 10 megohm input impedance voltmeter, 1 nanoamp will show 10 millivolts on a low voltage scale.

    I set my Fluke 27 on DC, millivolts, and run the current through it. With the resolution of .1 millivolt, I can see 10 picoamps.
     
  4. rigers

    Thread Starter New Member

    Jun 14, 2011
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    Well I need to go all the way to 2 amps, but not from nA to mA to A directly. In order to measure higher currents (mA) I can switch to a different method or change the gain of the amplifier.

    But after doing some more research I realized maybe I can use a logarithmic amplifier?

    I am creating something that will charge and discharge a battery at different currents. I need to be able to set the current and keep it there. Also I want to be able to set the potential to something that I want and need to measure minute changes in current. So essentially it's all DC.
     
    Last edited: Mar 13, 2012
  5. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    With the same high-impedance voltmeter, whether purchased or built, you can measure current over that whole scale. It's just a matter of choosing the right shunt resistors. The meter only measures voltage.

    There are op-amps with extremely high impedance. I think last time I looked I was looking at the LMC6035 or LMC660. I never got around to using either of them. Designing to avoid interference is a concern, so I'd plan on lots of shielding.
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  7. mcasale

    Member

    Jul 18, 2011
    210
    12
    Here's another trick you can use with an op amp.

    It's essentially a current divider in the feedback path so you don't have to use super-large resistors. By switching in different resistors you can change the gain. Divide the current by 10 and you get 10 times more gain, for example.

    This is an inverting amplifier with dual power supplies. This is more like an electrometer.

    I don't know what current you are measuring, or how.

    If you are measuring battery load current across a shunt resistor, this will not be your front end.
     
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