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Protecting the OPA129

Gerhard Kreuzer
Expert 2630 points


 

Dear Team,

I need to buffer a very high impedanz voltage divider, Ri = 48Meg. So I want to use a OPA129, 100fA max for Ib  and the needed +-15V supply made it an excellent choice.

Problem is, how to protect the circuit against high voltages, which could accidently appear at the input terminals? I have to say, that I need to switch off this divider, if input voltages drop.

The normal technique, two revered diodes between the input terminals and the supply rails wouldnt work, because I only found diodes with 500pA leakage current, which gives about 1V!!!

Thanks for helping.

With best regads

DI Gerhard Kreuzer

  • 0
    Intellectual 405 points

    The DPAD1, a dual low-leakage pico-amp diode originally from Vishay Siliconix but now I believe available from Linear Systems, specifies a maximum reverse leakage current of 1pA at 25dgC for Vr=20V. Of course it has a logarithmic increase with temperature.

    regards

    Steve