[sdiy] low input bias current opamps

David G Dixon dixon at mail.ubc.ca
Wed Aug 15 19:24:34 CEST 2018


> A few questions:
> 
> 1. are you using the typical guard ring circuit around the 
> high impedance node on the hold cap?

Yes.

> 2. is your board clean of any impurities that might allow 
> leakage from there?

Yes.

> 3. what are you using for an analog switch?

DG409

The things to understand are:

1) I've built dozens of these things for many people, everything from bare
boards to complete modules, even a few duals and quads in all three formats,
and they work extremely well.  The design was an improvement over Ken
Stone's CGS version (with his blessing), replacing the unipolar switches
with bipolar ones and thus eliminating all the voltage shifting and dilating
circuitry.

2) The boards that didn't work with defective LF444 worked perfectly with
TL064 (the next lowest input current opamps I had on hand).

3) I was using LF444 quite happily until I bought these defective ones.  I
learned about LF444 by building my own version of the MFOS Sound Lab way
back in the day.



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