[sdiy] 2164 Expo correction

[David G. Dixon]

> This is most likely the error when the output current should 
> be zero, if there is no built in way to zero this offset 
> (check you data sheet) you probably can supply the opposite 
> fixed current to the output to sink or source the error 
> current. This sort of offset zeroing mechanism has to be 
> calibrated for each specific 2164, the +- 50nA is the range 
> of the correction required. Calibrate the correction to a net 
> 0nA when the output is Theoretically = 0nA. Unlike a Span 
> error offset is a constant value, it may however drift some 
> with temperature.  

If that truly is what this offset is all about, then it's probably the
source of my LF tracking problems.  It should also make the triangles
seriously lopsided at low frequencies (presuming it doesn't change polarity
with the input current).  Compensating for it should be fairly easy.  Since
it is terminating to the integrator's virtual ground, summing a small
current from a divider off the rails into the integrator would fix it.

> I hope this helps and I did not misread your question and 
> state the obvious...

Nothing's obvious to me, Jerry!  :)