True RMS vs. Analytic(?) voltmeter

Discussion in 'General Electronics Chat' started by MartinV05, Aug 19, 2012.

  1. MartinV05

    Thread Starter New Member

    Aug 19, 2012
    I'm reading about RMS voltmeter and there are 2 types, one that measures by the analytic definition and the other that measures the true RMS value (AD637 in this particular case). As I can see from the circuit map of the AD637 it pretty much pushes the input signal through the analytic formula for RMS (the same is done by those 1st ones, as I can see from this block scheme I have here). How is AD637 different from the first one?
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    Hello, Matin and welcome to All About Circuits.

    Perhaps a little more detail of what you mean might help.

    There are several types of RMS voltmeter, but there is only one type that is always valid and this is the original calorimetric type, which you didn't mention.
    Last edited: Aug 19, 2012
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Correct, the only accurate TRMS is the thermal type. Also the most expensive.
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    There are scopemeters that do FFTs. RMS could be calculated based on that. I don't know how accurate that would be. I also don't know if any multimeters sample fast enough to do this.
  5. crutschow


    Mar 14, 2008
    Thermal type TRMS voltmeters are often used for microwave measurements, but the analytic type are typically used for lower frequency AC measurements since they are cheaper and accurate enough for the typical maximum crest-factor (ratio of peak to RMS voltage) of common AC waveforms.
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    You might also try looking at AD's extensive app notes on this subject ....