[sdiy] simple 2164 linear to expo CV?

Neil Harper metadata at gmx.com
Thu Nov 14 21:25:49 CET 2019

On 11/14/19 4:35 AM, Neil Johnson wrote:

> Ok, sounds like you have a linear CV in the range 0V to, say, 5V, and
> you want to convert that to an exponential voltage in the range 0V to
> +5V?
> For this you'll need a 2164 gain cell, a -5V reference voltage (at a
> push you could use a 5V1 zener), an op-amp to convert the 2164's
> output current into the output voltage, and an op-amp to invert and
> shift the incoming CV to match the range of the control pin of the
> 2164.  You want to convert the 0-to-5V range to 3.3-downto-0V.  The
> 3.3V end sets the range of the exponential CV, so you may find you
> need to reduce this down to, say, 2V.  You can use the same -5V
> reference as the offset input, suitably scaled.
> Cheers,
> Neil

hey neil, i saw your name in the SSI datasheet and looking forward to
breadboarding that 3-pole vcf, thanks for your reply.

yes exactly, 0-5v linear cv to 0-5v exponential cv. not too fussed over
exponential accuracy or temperature stability (unless it's something
crazy like .1V/C). my main concern would be the 0V point not moving with
regard to temperature since this cv is destined to control existing
vca's - i don't think a slight change in output volume would be
noticeable, but an 'off' vca starting to pass signal would definitely be

should i just leave the MODE pin n.c. for this? will breadboard it today.

the circuit you describe is basically what i was hoping could be done,
exponentially attenuating a DC voltage with the 2164, thank you. i never
thought of using a -5v zener reference. is that for when you can't trust
the actual rails? i noticed that most designs by Thomas Henry for
example that I've built usually will just reference to the rails...
maybe this isn't a problem for me, cause in my diy modular i can adjust
my lm317/337 power supply to ensure it's at +/- 15.0V?

/// Neil Harper
/// Every Wave is New Until it Breaks

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