[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[Dave Leith]

might be off this topic but has anyone used the ADR03ARZ - precision
2.5V 3-10 ppm voltage reference chip to solve any temperature depency
problems. Looks interesting

http://www.analog.com/UploadedFiles/Data_Sheets/ADR01_02_03_06.pdf

On 9/5/09, Jerry Gray-Eskue <jerryge at cableone.net> wrote:
>
>  <<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?>>
>
>
> <<<I also know that the effectiveness of the tempco relies on
>  coming within about +/- 250 microvolts of the optimum tempco voltage. >>>
>
>
> Don't forget that common Zeners have a Temperature Coefficient too. You can
>  however get the composite (Zener/Silicon set) type (like the LM329 used in
>  the Ian Fritz Dial-a-Temp co converter) that has temperature compensation
>  built in, or use a true voltage reference part.
>
>  If this temp co voltage is this twitchy you want as stable and temperature
>  independent reference voltage as possible.
>
>
>  - Jerry
>
>
>  -----Original Message-----
>  From: synth-diy-bounces at dropmix.xs4all.nl
>  [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of David G. Dixon
>
> Sent: Saturday, September 05, 2009 1:17 PM
>  To: 'cheater cheater'; 'synth-diy'
>  Subject: RE: [sdiy] Temperature Compensated Exponential
>  ConverterUsingSSM2164
>
>
>
> 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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