[sdiy] integrator / capacitor leakage

[René Schmitz]

Hi Ian, Jürgen and all,

Ian Fritz wrote:
> Yes, with care you can measure leakage that way.  But input bias
> current of the amp can be tricky to sort out, especially if it is
> highly temperature sensitive. Rather than use the intended opamp, I
> would suggest using the *best* opamp you can get your hands on.  Also
> the test setup must be carefully insulated from leakage to the
> outside world.  I recommend building the circuit in air using teflon
> standoffs.

A good idea, if you wanted to characterise the leakage resistance of the
cap. My suggestion to use the intended OP was made since I assumed it
would need to be good enough for this anyway. Maybe a dead-bug
construction with air wireing would be even better, you don't need
standoffs then. But I guess you can already get a result on a
breadboard, if its sufficiently clean.

One could also try to decouple the cap via a switch, and let it
discharge on its own for some time, then the leakage from the OP doesn't
play such a big role.

Another idea is to use an almost ideal cap, e.g. a fully meshed air
variable with decent insulation (ceramics) to characterise the leakage
of the OP first, and then eliminate it from the other measurements.

> Still, after all this, you have only characterized leakage, not all
> the time-dependent problems caps have. :-)

Leakage per se isn't relevant here anyway, its the resulting time
constants that are, because those will add to the integrating times.
And sure there are other aspects of caps that need to be considered.

You'll find that with small caps the major contribution comes from the
OP, but with large caps this more and more gets into the background,
since the timeconstants will increase. AFAIK the product of Rleak and C
is a constant for a given type of dielectric and cap. So the value
should just be large enough that your error timeconstant due to other
leakage currents (i.e. Ibias) gets large enough to be tolerable.

But the question was if the leakage resistance is nonlinear, something
which I don't know off hand (but doubt it for the +-15V we usually
have). And this you can find out.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159