[sdiy] SSM2164 Filter Cells

[David G. Dixon]

> I've started to go over the data sheet and various filter designs using
> the
> SSM2164. Haven't got into a lot of details yet but instead of re-inventing
> the wheel I thought I'd ask a couple of questions.
> 
> So far I've only seen integrator based filters for the 2164, nothing like
> the regular OTAs (cap between OTA output and output current amp). Anyone
> know why this is, are there issues with 2164 in this configuration?

Other than the fact that the 2164 has a logarithmic response to control
input, and OTAs have a linear response, I don't see much difference.  Either
one simply serves as a voltage-controlled resistor in an active filter
stage. The exact configuration of what follows (integrator, or cap-to-ground
and follower) depends on the specific filter configuration, but has nothing
to do specifically with the 2164.

> Also are there any words of wisdom that aren't in the data sheet or
> obvious when designing with the 2164?

I can't think of any, but there are a few things in the datasheet worth
heeding well.  The 7.5k impedance of the control voltage input, for
instance, and the different modes of operation possible by putting different
resistors on the mode pin.

As far as Aaron's comment about having to feed the output to virtual ground,
just remember that the 2164 is a current source, and the best way to convert
that to a voltage source is with an opamp (where the 2164 takes the place of
the input resistor in an inverting amp configuration).  A resistor-to-ground
and follower should also work just fine if you don't want to invert, but I'd
probably always use the former technique, with an inverter after if you want
to turn the signal the right way round again.