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Impedance Calculation

Discussion in 'Electronic Basics' started by Rob, Apr 2, 2007.

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  1. Rob

    Rob Guest

    I have a quick question.
    I have an RC low pass filter and I need to calculate the impedance so
    I can match it to a SAR ADC.

    What would the steps be to calcuate the impedance?

  2. Pete D

    Pete D Guest

  3. Bob Masta

    Bob Masta Guest

    You probably shouldn't be doing any such thing as
    "impedane matching". The SAR should have a
    high input impedance. That will not only prevent it
    loading down the LPF and changing its response,
    but also prevent the LPF from messing up the SAR.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
  4. Noway2

    Noway2 Guest

    You need to be careful about the impedance you drive the ADC with. The
    sample and hold needs to be able to quickly charge / discharge to the
    next sample value. In this sense quickly is relative to the sample rate
    and the operational parameters of your converter. Some converters
    have a setting for the track time before they will isolate the sample
    cap from the driving circuit. For best results, you really need to
    understand the characteristics of your converter.

    In the absence of other advice, I would suggest driving the circuit with
    a low impedance buffer or amplifier circuit rather than attempting to
    match its impedance. You will want at an impedance at least 10X greater
    at the input to the converter than the driving circuit to account for
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