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Protect Instrumentation Amp Inputs

Discussion in 'Electronic Design' started by Ken Petrac, Dec 22, 2010.

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  1. Ken Petrac

    Ken Petrac Guest

    I just blew a $35 instrumentation amp (differnetial) by accidientally
    over-voltaging the inputs. Without degrading performance, how can I
    modify the inputs to ensure this will not happen again?

    It was an AMP01 and I thought it had a degree of internal protection.
    Obviously needs something more.

    Ken Petrac
     
  2. Ken Petrac

    Ken Petrac Guest

    Something like this then?

    www.analog.com/static/imported-files/tutorials/MT-069.pdf

    It specifies low leakage diodes. What values would you recommend for
    the zeners with a +/-6VDC supply?

    Ken Petrac
     
  3. It would be nice if they could put 8 of them in an SO-8.
     
  4. Nico Coesel

    Nico Coesel Guest

    I agree. I rather use some resistors and capacitors for filtering and
    a TVS or varistor to clamp the inputs.
     
  5. It's sometimes better to use separate devices for conducting the
    current to the clamp and the clamp itself. A TVS might have 1,000 x
    the capacitance of a switching diode. The clamp might be leakier too-
    with separate devices you can bias the clamp beyond the operating
    range.
     
  6. Grant

    Grant Guest

    Yes, I posted a circuit here a while back that used low-leakage diode from signal
    to a 4.3V voltage at low source impedance, by running several mA through the zener.

    This let me reduce the signal series resistor to 1k (I think) and thus have
    little effect on the signal integrity (eventually goes to a dual slope converter,
    so more noise not an issue here). In my case I'm protecting the input of a
    +/- 5V powered CMOS device (4052) from an opamp powered from +12V/-5V's output,
    in order to get clean, accurate signal performance from -3V to 5V. No clamp
    for -ve signal as opamp and cmos use same -ve supply.

    Point is to make the clamp voltage low impedance so it absorbs the over-voltage,
    not let CMOS diodes pump up the supply rails, which is a Bad Thing.

    Grant.
     
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