[sdiy] integrator / capacitor leakage

[Ian Fritz]

At 03:31 AM 12/5/05, jhaible at debitel.net wrote:

>I haven't used OTAs. I've built it with THAT2180 VCAs, to get rid of the
>offset issues of OTAs.

I'm not familiar with that chip.  Isn't it still a transconductance 
amp?  Any idea how good the offset vs Iabc is?

>Unfortunately, these VCAs only have one input
>(inverting, to get a noninverting VCA in combination with an inverting
>opamp I->V converter), so I had to add an extra opamp inverter stage.
>Unlike your OTA solution, that extra stage will break the "symmetry"
>between the two integrator stages, which isn't good for a constant phase
>difference.

You might still think about adding some phase compensation in the opamp 
inverter.

>CA3280, diskrete FET buffers, voltage swing 1Vpp, boosted (outside the loop!)
>to 20Vpp, AD633 multipliers, 10nF capacitors.
>Works great (except some offset issues) at 7mHz (2.5sec period), and up
>to 14kHz. The latter is with Iabc of several mA, CA3280 getting warm, no
>good tracking expected. But these 7mHz aren't the lower limit - it's just the
>frequency I've counted with a stopwatch

Over 21 octaves?  Not bad.  :-)

   Ian