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9 mV noise on AC voltmeter

Discussion in 'Electronic Repair' started by Andre Majorel, Oct 1, 2008.

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  1. I've got a Chauvin Arnoux C.A 5220 DMM that bottoms out at 9 mV.
    Even with the inputs shorted and in "Velec" mode (270 k input
    impedance), it never goes below 9 mV.

    The problem is only for AC. In DC mode, it *does* go down to 0.

    Any idea what could be the problem ?

    It's built around a JRC NJU9210BFC.
  2. Looks like it's 50 Hz because when trying to measure a 50 Hz
    sine from a sound card, the voltage slowly oscillates by about
    +/-10 mV around a mean value (fraction of Hz beating against the
    sound card).
    No. It's long out of warranty.
  3. It keeps changing until you hold everything perfectly still. But
    even outside, it never goes below 9 mV.
    Not appreciably.

    Just tried putting everything in a tin biscuit box and still get
    the parasitic reading. Obviously, I couldn't close the box
  4. Jamie

    Jamie Guest

    Sounds normal for a cheap meter to me! :)"
  5. Guest

    Could it be that this meter needs to be calibrated? That is, nulled
    out on the AC mode? Are there any zero adjustments inside for low AC?
    Try putting a very short piece of wire across the input and ground.
    That will eliminate the "antenna" effect and pretty much prove that
    the noise is inherent in the meter amplifier. Lenny
  6. Ugh ! Fortunately for me, none of the other cheap meters I've
    used ever did something like that.
    Did that and the problem is still there. There are four presets
    inside but I'd rather not touch them without knowing what they
    do. Chauvin-Arnoux do not seem to believe in offering service
    manuals on their site (when it is not down). I've asked them,
    we'll see...
  7. Andre Majorel schrieb:

    I have an old (~1980) 4.5 digit true rms DMM; given accuracy for
    ACV: 0.25% rd. + 20D!

    300 mV AC range, input shorted -> display 000.12 (mV)
    300 mV DC range, input shorted -> display 000.00 (mV)

    Your reading could be normal for this type of DMM. What says the

    Just my 2 cents

  8. Jerry G.

    Jerry G. Guest

    It is normal that on the very low voltage AC scale and the leads are
    terminated, the meter will read something. The reading my be in the
    area of a small number of millivolts. The higher end meters tend to
    have this effect on their lowest scale.

    The lead wires can act as a small antenna and can pick up noise. The
    input circuit on the meter can also pick up its own internal noise.

    My Fluke meters all do that type of effect. I have these serviced and
    calibrated at least once per year. If I remember correctly, this
    effect is explained in the Leitch meter instruction book.

    Jerry G.
  9. I didn't make myself clear. With the leads open, you get on the
    order of 1 V, which doesn't surprise me considering the 10 M
    input impedance of the meter.

    What I don't get is how, after shorting the plugs with a 2-cm
    long piece of wire, switching to "Velec" mode (270 k input
    impedance), switching off all monitors and fluorescent lamps in
    the vicinity and going outside, you *still* get 9 mV.

    Still no service manual forthcoming from Chauvin-Arnoux. What.
    A. Surprise.
  10. This is a 4000-point meter. For AC voltages, the spec is
    "+/-1.5% reading +/-1 count".

    I'm not sure what "reading" means in this context. If it means
    "whatever number is on the display", 1.5% of 9 mV is 135 µV so
    the actual voltage could be anywhere between 7.865 mV and 10.135 mV.
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