[sdiy] expo accuracy? or integrator accuracy?, or both?

[Czech Martin]

>Agreed.  I especially appreciate the reminder that the discharge switch 
>leakage can easily obviate the advantages of a low leakage op amp 
>integrater.  Your use of the two-FET trick in your VCO 3 gives a good way 
>to attack that problem.  How much residual leakage you get with that 
>approach and how it depends on temperature is not obvious to me, since 
>leakage is usually speced at a very high voltage. This could be looked at 
>further.

I made some leakage observations. I used polystyrol caps
as Coulomb "counters". 

I bought a bag of them 20 years ago.
I was 16 and didn't really know what this was. But
they were cheap and shiny. 

The result was that the 30pA of the 
3140 I used outweigh any other leakage source by far.
I did not clean the circuit board, it was cheap paper/resin type,
pre drilled. Surprise to me.
Of course, this does not say anything about long term
behaviour, moisture etc.
But I have the feeling that board/soldering effects can
be neglected in the 1pA range. Certainly not in the 1fA
range which is perhaps not so interesting here.

m.c.