[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[Neil Johnson]

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