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Q: Construction of a 8.5 digit multimeter

Discussion in 'Electronic Design' started by Harald Noack, Feb 13, 2005.

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  1. Harald Noack

    Harald Noack Guest


    Has someone of you experience how a high precision multimeter is constructed


    How is the voltage reference stabilized? (by using an oven?)

    Are the input voltage divider temperature stabilized ? (temp. coeff.)

    How is the self calibration done?

    What type of ADC is used (maybe time to digital converter) ?

    How is the integral and differential non-linearity measured and compensated

    THANKS a lot for your help

    Harald Noack
  2. Ken Smith

    Ken Smith Guest

    It could well be an ovened reference, but over a fairly narrow temperature
    band, references can be made to be nearly flat. Without reading the
    spec's carefully, you won't know.


    These sort of things usually use a "multiple slope" converter. Many
    simple ones use a dual slope converter. A capacitor is charged up by the
    input for a fixed time and then discharged by a reference current. The
    time taken for the capacitor to return to zero gives the reading. This
    assumes that the capacitor is ideal.

    When you get into higher end DVMs, things are trickier. The circuit
    doesn't assume that the capacitor is ideal. Instead, it is run up and
    down and extra time using reference currents. The soakage and resistance
    effects of the capacitor are canceled.
    Multi-slope converters generally have very good differential linearity.
    With careful design capacitors can be made very linear. The design of the
    integrator is very likely to be the magic of the design.
  3. Harald Noack wrote...
    These are questions you have to answer and thoroughly understand to
    evaluate a 5.5 or 6.5 digit voltmeter. You could start by studying
    the service manual of Agilent's popular 34401A 6.5-digit multimeter.
    For example, study its input-protection circuitry. A 7.5 or 8.5-
    digit instrument raises these and many other issues, such as guards,
    dc thermoelectric voltages, normal-mode ac-line-signal rejection,
    etc., to dramatically higher levels.
  4. Joerg

    Joerg Guest

    Hello Ken,
    Sometimes companies go as far as having their own capacitors custom made.

    Regards, Joerg
  5. John Larkin

    John Larkin Guest

    I have one of these. It's really a 4.5 digit box, except on AC where
    it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
    on dual-slope with HC4051's as the switches. Its biggest problem is
    the huge amount of noise the VF display kicks into the front end. An
    old Fluke 8842 has less digits but is a far better instrument.

  6. Remember that an 8.5 digit DMM will typically not have anything like 8.5 digits absolute accuracy.
  7. John Larkin wrote...
    Sheesh, there must be something wrong with your meter! We have six
    of them, of various vintages (including some from eBay) and haven't
    experienced the degradation you describe. Furthermore, used in the
    SLOW high-resolution 6.5-digit mode (integrates over 100 power-line
    cycles), they show an amazing capability for a $1k instrument.

    John, have you checked out how to get the 34401A in this mode (use
    the MEASurement menu), and tried it? BTW, this instrument clearly
    wasn't meant to be a micro-volt meter. Note, the Fluke 8842 has a
    20mV scale, compared to the 34401A's 200mV lowest scale. Perhaps
    that's what you like about it. We use Keithley meters for most of
    our low-voltage measurements.

    We have several high-performance digital voltmeters in my lab, but
    they're large 19" relay-rack beasts and real space hogs on the bench.
  8. John Larkin

    John Larkin Guest

    Well, mine kicks out so much spikes it disturbs active circuits. Check
    yours; mine's an early unit, so maybe they fixed it without changing
    the "A" in the model number.

    On AC, as you reduce the input voltage, at some point it just drops
    off to zero indication, clearly a software kluge to hide the spikes. A
    careful reading of the spec shows that they fudged the specs and the
    firmware, rather than cleaning up the noise problem. Tacky.
    Yes, but don't get me started on the nightmare menu structure!
    Yes; at low level, it's much better than the Agilent, even though
    superficially it has less resolution.
    After mixed experience, we only buy Fluke handhelds and Keithley
    benchtops now. Too bad Fluke gave up the precision end of the

  9. Chris Carlen

    Chris Carlen Guest

    Well Fluke still has the 8050A 8.5 digit multimeter. How about that one?

    I bought a Fluke 45 for home instead of the Agilent. I liked the dual
    display, and a little bit cheaper. 0.025% DC is pretty good for testing
    batteries ;-)
  10. John Larkin

    John Larkin Guest

    Can't find that one. 8508A maybe? Looks expensive... price is $call,
    not a good sign. Specs look pretty good.

  11. Chris Carlen

    Chris Carlen Guest

    Oops, sorry, that's the one I meant.

    price is $call,
    Well I'd think anyone considering to buy an 8.5 digit meter would pass
    the old test of "if you have to ask the price..."

    Good day!

    Christopher R. Carlen
    Principal Laser/Optical Technologist
    Sandia National Laboratories CA USA
    -- NOTE: Remove "BOGUS" from email address to reply.
  12. I have used many of theese at work and liked it so much I bought one for
    my home lab. I have never seen what you describe. On the contrary - our
    Q-dept has increased the calibration intervals to two years because they
    never find out-of-spec 34401A's


  13. John Larkin

    John Larkin Guest

    Have you checked it for the kickout spikes and low-level AC dropout?
    It would be interesting to compare.
    I never said that mine was out of spec. I said that the VF display
    made a lot of noise, and that the specs and firmware were fudged to
    compensate. I measure a lot of low-level stuff, AC and DC, and the
    Agilent is seriously inferior to a Kiethley 2000 or an old Fluke 8842.

  14. Treeline

    Treeline Guest

    Thanks for the clarifications. I once looked into those when I guess
    Agilent was still Hewlett-Packard. The digit numbers were impressive
    and its ability to hold a calibration was impressive; but, when I studied
    the specs closely and compared them to the Fluke 8842 - that's when
    I began to suspect that you get what you pay for. I was hoping for
    a great bargain, more numbers at lower cost. Neither device
    could do what I needed so I forgot about them.

    Hewlett Packard HP 34401A Digital Multimeter, 6.5 Digits, sells for
    $1136.00 or so?

    A dozen years later, and it's still selling for the same price?
    You would think in a dozen years, for the same price, better
    specifications for the money, or not? I guess here someone will chime
    in with the value of the dollar but still...

    Now I remember. I wanted to match resistors to a far higher degree
    than I could order easily, even from Vishnay?. The reason was to reduce
    the CMRR in the front-end of a circuit dealing with lots of noise
    at millionths of a volt and picoamperes.

    I was also trying to match capacitors, but that was even more of a problem.

    Pease, a great analog engineer and writer, suggested an elementary op-amp
    circuit instead to achieve specs on the compared parts.
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