[sdiy] offset compensation for variable slew rate circuits: idiot ?

[Czech Martin]

Sorry guys, forget about my first proposal for offset compensation
for variable, voltage controlled slew rate circuits. I'm an idiot.
(not that this fact wasn't abvious to you all for some time).
It won't work, since the averaging will disable the ability to
compensate fast control feedthrough.

But the second (chopper) could basically work. 
Chopper means that the output voltage is hold (s&h),
while the input is clamped to 0 and the large timing cap
is switched to something considerable faster.
Then the error is sampled (another s&h) and everything is switched
back to normal operation.

On first hearing this sounds crazy, but some of the worlds
lowest offset op amps (and lowest drift) are choppers,
there exist a few chopper add on chips by LT and NS.

This could be translated as follows:
every ms the signal path is switched, the output is on hold.
Then the measurement phase beginns (few us).
Then back to sample.
Risks: AC feedthrough (spikes) into output, strange
behaviour during CV change.

If you have a nice, low offset ota, this is of course
the best solution.

Some of the continuos schemes measure the input
offset voltage directly (surprise), that would mean
right at the ota. Now, the differences should be small
here and also basically constant (CV:= const).
So, maybe I'm rescued and not such a bloddy idiot.

This could work, without chopper, because the integration
time for error can be short. The integrator and output op
amp must have low offset, low offset drift and low bias
current, because it is not in-loop. 411?

Who want's to try that?

m.c.