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protecting/isolating ADC inputs

Discussion in 'Electronic Design' started by Dave Boland, Nov 1, 2004.

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  1. Dave Boland

    Dave Boland Guest

    In the past I have used a couple of Schottky diodes to protect the ADC
    inputs. Now I see that most ADC's have this built-in. So my first
    question is how effective are they for over-voltage protection, and
    should I plan on adding external diodes or a Transorb diode pair (or a
    MOV, but I believe they are slow and leaky)?

    Another need that my come up is isolation from devices that may be part
    of another system (with own power supply, ground reference, etc). How
    is this done with commercial DVM's that run off line power and can
    safely measure the voltages (single ended and differential) from another
    system? I have seen references in this NG to galvanic isolation and
    optical isolation, but have no experience with either. Any insight will
    be appreciated.

    Thanks,
    Dave
     
  2. Joerg

    Joerg Guest

    Hi Dave,
    The data sheet should contain two abs max ratings: One for max input
    voltage and one for max current into those internal diodes. If it is a
    fast ADC you may need a low impedance driver that could exceed the
    internal diode capabilities, else a resistor in front might be the ticket.
    Line powered DVM have a power supply transformer which can be designed
    almost to whatever isolation requirement is needed.

    When I have to design highly isolated interfaces I usually go for a
    toroid transformer method with pulse transfer and clamping. The main
    reason is low production cost because it is usually for something that
    needs to be produced for many years. For a simple prototype you could
    look at isolation amplifiers in a can but I have never used one of those.

    Regards, Joerg
     
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