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measurement of differential analog signal with high impedance

Discussion in 'Electronic Design' started by [email protected], Aug 21, 2007.

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  1. Guest

    Hello!

    I asked something similar before but I think it is best to open
    another topic as it is bit different.

    To measure the high impedance differetial analog input signal, I am
    thinking of using 2 FET or BiFET voltage followers for each input (V+
    and V-), and then connecting voltage followers outputs to ADC's Vin+
    and Vin-.

    Is the above circuit right? Any recommended parts for the voltage
    follower (or so called ADC buffer) and the ADC ?
    The analog signal is between +/-5V and the ADC has to be a 16-bit
    resolution one (requirement); It is best to have single power supply
    for buffer and ADC; and I need at least 16 differential channels to
    measure.

    Can someone help?

    Thanks
     
  2. whit3rd

    whit3rd Guest

    Sounds good, but it seems you're talking about IC followers
    (integrated
    amplifier chips); discretes are better for preamp applications,
    usually.

    'High impedance' means that there is some acceptable approximation
    that one impedance is much higher than another. Is your source to be
    the 'high impedance'? Or the load? What bandwidth do you require?
    Is DC important? Are there long wires involved between the measuring
    instrument and the source?

    Highest impedance amplifiers are vacuum tubes. Noise performance
    is poor if you ask for TOO high an impedance, or insist on large
    bandwidth.
     
  3. Winfield

    Winfield Guest

    It's good to consider taking the input difference and
    removing the common-mode voltage before presenting the
    signal to the ADC, even if it is a differential ADC.
    Look up instrumentation amplifier ICs. Many of these
    contain low-offset JFET input follower opamps plus a
    high-performance output difference amplifier. And
    they let you amplify the output voltage if you wish.
    Analog Devices and Burr-Brown (now TI) are two good
    sources for instrumentation amplifiers.
     
  4. Guest

    Thanks for your reply.
    The source is high impedance to the ADC input. No bandwidth
    requirement, just DC signal measurement.
    I believe the wires between the source and the instument shouldn't be
    very long, probably several meters.
     
  5. whit3rd

    whit3rd Guest

    The least expensive option would be multiplexing the inputs to a
    single
    amplifier; this will add some high-frequency requirement to the
    situation,
    due to the multiplexer switching, and multiplexer feedthrough/leakage
    currents can
    be troublesome. Better but pricier would be to have an
    instrumentation amplifier
    on each channel, and multiplex the amplifier output; you say there are
    lots of inputs, so IC instrument amps are the easy option. They're
    mass-produced and pre-trimmed. Something like INA2126 might be
    ideal.

    The 'few meters' of wire is a potential problem. Shielded twisted-
    pair wires
    are excellent. Unshielded twisted pair is next best. Some input
    filtering
    will be important, too, because even radio pickup can affect the low
    bits
    of a 16-bit converter.
     
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