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Protecting ADC Inputs

Discussion in 'Electronic Basics' started by Richard, Dec 28, 2004.

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  1. Richard

    Richard Guest

    I'm built a circuit board for my Analog card. My normal inputs will be 0-5
    vdc but the max allowed on the ADC is 10 vdc.
    I was thinking about putting some zeners across the input circuit so the
    voltage will never go above 10 volts. I left a space from each input to
    ground for a surface mount 1/2 watt resistor. These were there so we could
    do a 4-20ma conversion.

    So, First, will zeners work without affecting my normal input voltage?
    Second, are zeners available close to the 10 volt range?

    Any better simplier solutions?

    Thanks,
    Richard
     
  2. John Larkin

    John Larkin Guest

    Zeners are good. 6.8 volts, 1 watt maybe, should absorb some pretty
    good zaps. A 6-volt or better zener will conduct tiny currents at 5
    volts. A series polyfuse might be nice, too, in case somebody decides
    to connect +48 volts to your input or something.

    If your ADC input is high impedance, a series RC ahead of it is
    prudent too. That gives additional zap/latchup protection and filters
    noise, too. It's a generally good policy to avoid connecting any IC
    pin directly to the outside world.

    John
     
  3. Bob Masta

    Bob Masta Guest


    My choice would be to use a series resistor with
    reverse-biased clamp diodes to each rail. (This
    is a popular method used for example on the
    inputs to CMOS logic gates.) This assumes that
    the input impedance of the ADC is high, and the
    small RC with the effective diode capacitance is
    compatible with your signals.

    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
     
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