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DMM Frequency response

Discussion in 'Electronic Basics' started by shan, Nov 13, 2005.

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  1. shan

    shan Guest

    Hi, everyone! I'm new to here.

    I have some questions about measuring AC voltages.

    I know that low-cost DMMs cannot measure accurately the AC (rms)
    voltage for too high frequency normally below several hundred Hz even
    for sinusoidal signal. But why? Is it due to the electronics inside or
    is it a loading effect?

    What above analog multimeters? Are they better/worse?

    Besides the internal resistance ~ 10Mohm, what are the typical parallel
    capacitance / series inductance values across a DMM inputs? Is it
    normally neccessary to take these into account?

    Thanks!
     
  2. If a DMM has an input impedance of 10 M Ohms and 20 pF, the 3 dB frequency
    is 1/(2piRC) = 800 Hz. The capacitance in this case is a shunt effect to
    ground after the 10 Megs of resistance ... so it forms a low-pass filter.

    A 1 volt signal at 800 Hz will display as 0.7 volts.

    Scopes are better for higher frequencies.
     
  3. shan

    shan Guest

    So, the equivalent circuit is something like this?

    -----\/\/\/\/----- Vdmm
    Vin R |
    C =
    |
    -------------------

    If we put DC voltage across the inputs why not the DC voltage shows up
    (in AC measurement mode)?

    How scopes and other more sophisticated devices do? How to represent
    the these as equivalent circuit?

    Thanks very much!
     
  4. Tim Williams

    Tim Williams Guest

    Because it's made to ignore DC potential...

    My RadioShank DMM (before it died...sigh) started dropping off at 3kHz,
    based on my observations.

    Tim
     
  5. There is a series blocking capacitor.
    It's selectable on a scope ... dc coupling (no blocking capacitor) ... or ac
    coupling (capacitor). dc coupling is also called direct coupling.
     
  6. Jasen Betts

    Jasen Betts Guest

    couldn't they bypass the 10M with a suitable cap to reduce that ?

    wether the DMM can sample the input fast enoug could be another problem..


    Bye.
    Jasen
     
  7. shan

    shan Guest

    If we add a series cap (denoted it as C1) at Vin, there should still be
    DC voltage across C. Maybe C1>>C so that most voltage is on C1? By the
    way what is the typical capacitance needed for DC blocking? What are the
    main considerations?
    Even we can select dc/ac coupling, it only deals with the DC component,
    how can it avoid low-pass phenomenen? It should not be simple otherwise
    all cheap DMMs can do this, I suppose?

    Thanks.
     
  8. shan

    shan Guest

    If they bypass 10M, I think the impedance may be too low.
     
  9. You are thinking about the distribution of a dc voltage across two
    capacitors in series ... does not apply here. The capacitor is selected to
    have a low reactance at the lowest frequency of interest. At dc, 0 Hz, its
    reactance is infinite; hence the dc blocking function.
     
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