[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[David G. Dixon]

That is an excellent point!

Incidentally, I have looked at the temperature sensitivity specs of metal
film resistors (~1.5 ppm) and 18-turn trimmers (~100 ppm max), and as long
as the bulk of the voltage drop is taken across the metal film resistor, the
effect should be minimal.

Also, we mustn't forget that the control pin of 2164 has a 5k input
impedance, so the tempco voltage must be buffered, particularly if multiple
expos are to be hooked up to it, as in a polyphonic application.

So, is it best to divide the tempco voltage input to the buffer directly
from the rail, or to drop it across a zener first?


> But what you get in exchange is the possibility to have multiple
> expo's which drift in the same way. Which makes it perfect for poly
> synths.
> 
> D.
> 
> On Wed, Sep 2, 2009 at 1:26 AM, David G. Dixon<dixon at interchange.ubc.ca>
> wrote:
> >> I think you need to trim it since values varies and repeatability of a
> >> design needs to be achieved. Also, how do you repeatably achieve
> >> 0,2678345615807 V or whatever we crank out as the optimal voltage?
> >
> > Yes.  What I was arguing was that the adjustment could be confined to
> one
> > trimmer, specifically, the one which gives the tempco voltage.  The
> > appropriate feedback resistance could simply be calculated and applied
> to
> > the nearest 0.1k.  I believe that I have demonstrated successfully that
> the
> > trimmed values are more or less insensitive to the VCA gain temperature
> > coefficient, and that the optimal feedback resistance at the expected
> value
> > of the VCA gain coefficient will still serve admirably for tempco even
> if
> > that gain coefficient is several percent off spec, which is highly
> unlikely
> > for 2164s.
> >
> > Having said that, I also know that the effectiveness of the tempco
> relies on
> > coming within about +/- 250 microvolts of the optimum tempco voltage.
>  Even
> > being 1 mV away from the optimum puts the first octave about 0.25% out
> of
> > tune even at the reference temperature.  This extreme sensitivity is
> > probably the "Achilles' heel" of the design.  Drift in the supply
> voltage,
> > and even temperature sensitivity of the resistor and trimmer, could
> become
> > serious issues.
> >
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