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A/D module input resistances

Discussion in 'Electronic Basics' started by Joel Moore, Sep 21, 2005.

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  1. Joel Moore

    Joel Moore Guest

    I've been shopping around lately for remote (USB/serial/ethernet) A/D input
    modules and so far the highest input resistance I've found is 100M Ohms
    (Measurement Computing's USB-1608FS). The next closest contender is around
    20M Ohms

    Out of curiosity I checked out one of their PCI analog input cards (PCI-
    DAS6013) and was surprised to see its input resistance is rated at 100G
    Ohms! That's 3 orders of magnitude larger.

    How do they achieve such a number? And why aren't there any remote modules
    that offer numbers even close to that?

    Thanks for any insight.

    Joel Moore
  2. Bob Masta

    Bob Masta Guest

    Hmm, I wonder if 100G is a typo... 100M is pretty common, 100G
    is hard to believe. Anything that really is 100G is ludicrously
    difficult to produce and to maintain. Try finding 100G resistors
    for an op-amp feedback path, for instance. They are large
    glass tubes that have to be mounted on teflon standoffs
    and even then you are lucky if you can get anywhere near
    the rated value due to humidity, etc.

    Do you really need a high value here? Even 100M is
    pretty high. Unless you are building a dedicated pH
    meter or electrometer or something, you will probably
    want some interface in front of the A/D anyway.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  3. PeteS

    PeteS Guest

    I notice on the specs they don't specify the frequency at which the
    input impedance is '100Gohm'

    If you look at the specs
    they don't specify the input capacitance (which would make a
    significant difference).

    Now, it's possible to raise the effective input impedance through the
    roof by bootstrapping the inputs - and perhaps this is what has been
    done, although 100G is awfully hard to believe *across the entire
    frequency range of operation*. I can get really good amplifiers with
    100s of fA of I(ib), which sets the input *resistance* easily in that

    At DC, it's perfectly possible. Consider an input of 3V - at 100G input
    *R*, then I(in) would be 30pA (quite believable). It's possible this is
    marketing tripe (hype) where the engineer stated the input resistance
    is 100G, but that's not the input impedance across the range.

    Even ignoring cables, there will be a minimum of a few pF input
    capacitance, so at the stated rate of 200k Samples / sec (so one can
    digitise up to 100kHz), the input would look like (for 10pF input C)
    about 10G - j160k. [100kHz, 10pF input C]. At those differences in
    range, the input impedance would be about 160k at almost zero degrees.

    Apart from that, the *effective* input frequencies involved (due to the
    sample and hold) would be higher, but at even these numbers, I don't
    see the input *impedance* being above a few hundred K. (Note that those
    numbers are quite superior in terms of general equipment).


  4. PeteS

    PeteS Guest

    That should have been not this:
    about 160k at almost zero degrees

    But this:
    about 160k at almost 90 degrees (seen from the input) [the C/R are in
    parallel and the input impedance is dominated by the input capacitance]


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