>
> > al from Technical University Berl> >>
> >> how are very low currents (fA range) measured?
> >
> > See:formatting link> Read the "Teflon" and "Femtoampere" articles.
> >
> > [edited] The best "Teflon Capacitor" we tried was a few
> > centimeters of RG-188 Teflon coax (outer braid to output
> > and center to the summing point) "trim" by snipping off
> > small pieces. 10-15pF is a convenient value for measuring
> > in the fA range. use the second channel of the 662 dual
> > for the integrator...
> > Paul Grohe (The "Paul" mentioned in the Teflon article).
> Yes, Paul, I remember your name in that fascinating account.
> One painful issue, as Pease discusses, is a suitable circuit
> to reset the integrator. Bernie Gottschalk, an engineer at
> the Harvard cyclotron, used NSC cmos opamps and had a clever
> solution: reverse the supply voltages. He used a resistor in
> series (perhaps 3.3k, etc.) with both opamp supply leads to
> limit the reversal current. The resistors with the opamp's
> protection diodes act as a bridge to zero the capacitor (the
> opamp's + input is grounded, although I'm not sure that's
> important). Significantly, when restoring the supply-voltage
> polarity the NSC opamp's protection-diode charge-transfer is
> balanced, leaving the integrating-capacitor voltage properly
> reset. Of course, in integrator applications like this, the
> exact capacitor voltage at reset isn't very critical.
> Very impressive, cute and elegant!
> A single HC4053 chip could be used to make a +/-5V supply-
> reversal circuit.
> The scheme is especially attractive with many channels, where
> one supply-reversal circuit resets all the channels at once,
> without any extra parts!
> I haven't tried this trick myself, but Bernie didn't mention
> any problems or special considerations (e.g., no additional
> bypass caps for the opamps after the series resistors, IIRC),
> so try it and let us know how it works out.
> Thanks,
> - Win
Just exhuming this very interesting thread from 20 years back. Recently we were talking about the trick of inverting the power supplies to reset an op amp integrator, and I think this is the first mention with attribution.
Cheers
Phil Hobbs