[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[cheater cheater]

Most interesting. Thanks Neil.

Got a long night behind me, but am I wrong to think that with the
SSM2164 you can have a monolithic pair of expo converters?
Even if not.. they all depend on one voltage reference.
This means that you could possibly dedicate one EC to a
microcontroller which constantly retunes the circuit by adding
corrective voltage. I think they do that in the Andromeda, but I might
be wrong.

D.

On Sun, Sep 6, 2009 at 8:54 PM, Neil Johnson<neil.johnson97 at ntlworld.com> wrote:
> Hi,
>
> David G. Dixon wrote:
>>
>> 1.  The "obvious" option:  Take 15V straight from the supply rail and
>> divide
>> it down according to the scheme as outlined before.  I'm presuming that
>> the
>> supply rail won't bounce all over the place!
>
> WRONG - supply rails bounce all over the place.  Why do you think op-amp
> manufacturers specify PSRR?  UNLESS you have a circuit which is ratiometric
> so doesn't care about the exact value (and even then I still think its dumb
> - why let supply rail noise into your circuit?) never rely on the supply
> rails being 'constant' - they are anything but constant.
>
>> 2.  The "clever" option:  Drop the rail voltage with a zener diode rated
>> between 4.7 and 5.6 volts...
>
> Ok, not so dumb as using the supply rails, and fine if you're not concerned
> about temperature stability too much. 0.01%/K = 100ppm/K.  Not bad.
>
> Better yet try something designed for this, like a bandgap or buried zener.
>  For example, the REF02C gives you +5V at 20ppm/C typical, max. 65ppm/C.
>  Bandgap ref. diodes give you similar stability, e.g. the ZRB500 will give
> you typ. 15ppm/C.
>
> Neil
> --
> http://www.njohnson.co.uk
>
>
>
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