How to correctly measure a mixed low&high current

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  • edugimeno
    Badcaps Veteran
    • May 2017
    • 568
    • Spain

    #1

    How to correctly measure a mixed low&high current

    Hi there!
    Im trying to determine the consumtion rates of a module Im testing. Particularly it's a Decawave DWM1001 module, which is expected to use around nanoamps during normal state so a LiFePo4 battery should last months

    This module has peak spikes of around 100mA once a second, while the TX module is active, and then goes to sleep for the rest of the time (this is when the uA rates are expected)

    But I'm having issues to measure this current.

    1.-Using a multimeter. Obviously all I see is values jumping all the time, as this is not expected to measure such a unstable current

    2.-Using my Rigol DS1052E oscilloscope. For this I use a 0R22 shunt in series with the battery.

    When I disable the Low Power mode, I measure 4-5mV (avg, with higher spikes) which is actually the expected 15-20mA current (shunt=0.22 so current is voltage * 4.5)

    But when I enable Low Power mode, which is what I actually need to measure in detail, voltage gets down to ~1mv, which means 4-5mA, not reasonable. Then I disconnect the battery from the module and I keep reading the same average voltage!!! ~1mv.

    What is worse...Then I plug my probe tip to the Rigol ground test lead, and it keeps reading 800-1200nV...WHY??

    I tried an auto-recalibration in the Rigol Tools menu, which ended correctly, but I can't correctly measure voltages under 1mV.

    Is this normal? Is this expected from this scope.

    Anyway, this is what I have. What other options do I have to fairly-decently measure a current in the range of nA?

    I know I could take a bigger shunt, like 5 Ohm so the read voltage would be bigger and more noticeable by my scope, but in this case, the spikes (100-200mA) would drop the voltage thru the shunt resistor below the acceptable supply and it would reset or erratically fail, during this burst...


    Anyone...?? Thanks!!
  • eccerr0r
    Solder Sloth
    • Nov 2012
    • 8687
    • USA

    #2
    Re: How to correctly measure a mixed low&high current

    Yes this is normal due to the limitations of the scope, in what you're trying to do.

    You'll need to design a jig for extending the dynamic range of your current measurement.

    Comment

    • edugimeno
      Badcaps Veteran
      • May 2017
      • 568
      • Spain

      #3
      Re: How to correctly measure a mixed low&high current

      Ok, and what sort of jig works fine for this. As I have no idea on what to start with

      Thank you

      Comment

      • eccerr0r
        Solder Sloth
        • Nov 2012
        • 8687
        • USA

        #4
        Re: How to correctly measure a mixed low&high current

        I think the easiest to understand jig is using a LNA with the 0.22 ohm resistor to bring the range into something you can accurately measure with the scope and you simply ignore the regions that go out of bounds when it hits 100mA - you measure those separately - hence improving dynamic range by measuring twice. However I think many op amps that have very low noise figures aren't cheap, cheap ones like LM741 and LM358 will not do.)

        Comment

        • edugimeno
          Badcaps Veteran
          • May 2017
          • 568
          • Spain

          #5
          Re: How to correctly measure a mixed low&high current

          Ok, I understand. I think that instead of starting from scratch designing / building / soldering... a board for this I'm groing to see if there's some pre-made module using a LNA IC that I can use for this

          Thanks a lot!

          Comment

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