[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[Oren Leavitt]

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.
> 
> 
> Point taken.
> 
> 
>>>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.
> 
> 
> ...and about all one could expect from the metal-film resistors in the
> circuit, in any case.
> 
> 
>>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, this is the first I've ever heard of these parts!
> 
> I see that Digikey carries REF02 5V references from Texas Instruments for
> $3.11 for 15ppm/K, and $5.04 for 10ppm/K.  This seems a fairly small price
> to pay for rock-solid tempco.
> 
> Thanks for the tip!
> 

Referece ICs are definately good friends!
I do keep a small stock REF0x ICs handy.

Include an onboard +/-Vref source (consult Vref IC app notes for +/- 
ideas). Point *everything* that needs a *fixed* voltage reference to the 
onboard Vref rails - coarse/fine/offset panel controls, comparator trip 
points, bias current resistors, offset trims, etc... You'll have an 
oscillator (or whatever) that is well shielded from power supply 
variance and can even run on +/- 12 or +/- 15 volt supplies without any 
readjustment.

- Oren