Low Current AC Ammeter with good filtering AND linearity possible?

Discussion in 'The Projects Forum' started by Michael_V, Oct 9, 2014.

  1. Michael_V

    Thread Starter New Member

    Oct 9, 2014
    I have been given a work assignment to build the perfect AC Ammeter. The signal source is a current transformer secondary that supplies 0 to 5mA AC. The metermovement is DC from 0 to 1mA.

    I need to add filtering so the meter will display 0 to 5mA AC RMS at 60 Hz, but any harmonics 120Hz and higher would have to be attenuated enough so the meter needle is barely influenced.

    On top of this, I still need good linearity, so at 4mA AC RMS input the needle deflects close to 80%, at 3mA it deflects close to 60%, and so on.

    So far I can make a meter with awesome filtering but lousy linearity, or awesome linearity but crap filtering, but not be very good at both.

    The attached .pdf is the best I've come up with so far. Don't get hung up on the resistor values, I change them constantly.

    Is there any different strategy that would work that I haven't thought of, or is what I'm trying to achieve just not possible? I can't use any externally powered filtering method here either.
  2. DickCappels


    Aug 21, 2008
    I think the diodes might cause problems unless your load is a true current source.

    Have you considered using an RMS to DC converter such as the Analog Devices AD636 and its cousins? You could follow it with an opamp to drive your moving needle meter.

    As for rejecting harmonics, many poles of analog filtering is probably your best bet.