[sdiy] Temperature Compensated Exponential ConverterUsingSSM2164

[Jerry Gray-Eskue]

<<For example, if we divide 10V
down to, say, 300mV, then the temperature coefficient is divided from
3300ppm/K to 100ppm/K.>>

Careful - you have to "divide both sides of the equation".
By dividing the 10V reference down you reduce the magnitude of the error but
not the percentage, since your "new" reference is 300mV you now have a 300mV
reference with a relative temperature coefficient of 3300ppm/K.


- 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: Sunday, September 06, 2009 12:16 AM
To: 'cheater cheater'; 'synth-diy'
Subject: RE: [sdiy] Temperature Compensated Exponential
ConverterUsingSSM2164


I think that the temperature dependence of the voltage source to the tempco
voltage divider is basically a non-issue.  Even if it is 3300ppm/K, by the
time the voltage is divided down (particular if it comes from a 15V rail),
this temperature effect will be very small.  For example, if we divide 10V
down to, say, 300mV, then the temperature coefficient is divided from
3300ppm/K to 100ppm/K.  This is on par with the temperature coefficient of
the metal film resistors.

> Maybe you could use a voltage reference that has a negative temp
> dependence ? And then adjust the tempco in the expo circuit itself to
> account for that?
> I.E. the tempco in the expo circuit goes half of the way, the voltage
> reference's 'temp compensation' does the other half ... not sure how
> that would work though :) but it seems it's easier to balance out the
> imperfections of a circuit, than it is to find parts that are
> absolutely perfect.
>
> D.
>
> On Sat, Sep 5, 2009 at 7:31 PM, 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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