[sdiy] 1v/oct expo converter front end for SSM2164 quadpolevcf..

[David G Dixon]

> I read in the SSM2164 specs:
> Gain constant Temperature coefficient: -3300ppm/degC Could I 
> put a 2 resistors voltage divider (with the ground resistor 
> being a 1k +3300ppm TEMPCO)  rignt between the summing opamp 
> out and the 4 linked CV's inputs to compensate for the SSM 
> temp drift ?
> (The TEMPCO glued to the SSM2164 top)
> This way that would only cost me a TEMPCO and I'd keep the 
> SSM's 4 available stages...?

Yes, however you'll have to buffer the divider, as the 2164 VC pin has an
input impedance of only 5k (so 4 in parallel would be 1.25k).

As I said before, I can send you a model for predicting the correct gains
depending on the tempco you use.  NOTE: A 3300 PPM TEMPCO WILL NOT WORK
HERE!!!  You'll need a larger tempco coefficient if you are going to use the
divider method.  We used a 470-ohm, 3900-ppm Panasonic ERAS 0805 tempco chip
resistor in the Dr. Octature.  For this, the upper leg of the divider must
be 2.5k (2k + 1k multiturn trimmer).  In order to avoid saturating the CV
summer, I used a 1V/oct gain of 50% (100k input resistor, 49.9k feedback
resistor), which means that the non-inverting stage must have a gain factor
of roughly 40% to get the overall -20% gain required by the 2164 VC input.
For this, I used a 16k feedback resistor and a 10k resistor to ground from
the non-inverting input.

Here is the whole circuit:

1V/oct --> 100k --> CV summer w (50k feedback) --> 2k --> 1k trim --> (+) of
non-inverting amp (gain = 2.6) --> 470R, 3900ppm tempco --> GND

and the output of the non-inverting amp --> 2164 VC pins.