[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[cheater cheater]

>> It bears noting that the temperature compensation is not perfect, and
>> deteriorates rapidly as control voltages go above 0V (i.e., the sum of
>> square errors gets geometrically larger with increasing control voltages
>> above ground).
>
> I made the classic mistake of failing to non-dimensionalize my errors.  When
> I divide the square error by the square of the desired value, I find that
> the tempco gets progressively worse the further out the control voltage is
> from 0V, on either side, as expected.  High-frequency compensation may still
> be necessary in this circuit because of this fact.

How do you know which calculation is correct and what the correct way
of understanding the results is?

Also, I see no point running my synths at 5 degrees Celsius.

D.

On Sun, Aug 30, 2009 at 9:31 AM, David G. Dixon<dixon at interchange.ubc.ca> wrote:
> Oops!  Please disregard the following statement from my last message:
>
>> It bears noting that the temperature compensation is not perfect, and
>> deteriorates rapidly as control voltages go above 0V (i.e., the sum of
>> square errors gets geometrically larger with increasing control voltages
>> above ground).
>
> I made the classic mistake of failing to non-dimensionalize my errors.  When
> I divide the square error by the square of the desired value, I find that
> the tempco gets progressively worse the further out the control voltage is
> from 0V, on either side, as expected.  High-frequency compensation may still
> be necessary in this circuit because of this fact.
>
> Also, it bears noting that an algebraic solution of the equations gives the
> following values:
>
> Tempco voltage: 0.301V
>
> Feedback resistance: 56.8k
>
> These values give perfect conformance at 25 degrees C, but the conformance
> deteriorates significantly at lower voltages, and much less so at higher
> voltages.  The best fit values of 0.289V and 54.4k give better conformance
> overall, and the deterioration is symmetrical about 0V, but there is a local
> maximum conformance error at 25 degrees C!
>
> In other words, this design is still a compromise.  However, it is probably
> superior to using a tempco resistor, but I need to do more analysis to
> determine this for sure.
>
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