[sdiy] Temperature Compensated Exponential Converter Using SSM2164

[Neil Johnson]

Hi David,

David G. Dixon wrote:
> Point taken.

Apologies if I came across a little terse.  I've been fighting a  
power supply stability issue that's killing a PLL at the moment, and  
its not looking pretty.  So its a sore point at the moment.

>>> 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.

True, but if you can achieve better then only they (the resistors)  
become the limitation.

>> 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!

There are plenty of devices like that.  Usually the more you pay the  
less (temperature instability) you get :)  They'll be binned on the  
production line, and the marketing people say you should pay more for  
a better device, hence the prices.

What frustrates/intrigues/annoys me is that good stable voltage  
references are really quite cheap these days.  So why-o-why don't  
more people use them in their synth module designs instead of  
assuming that a supply rail (a) if it says "+15V" it really is +15V,  
and (b) it will be stable and noise free???

Ok in my modules I just use 2V7 zeners instead of bandgap diodes, but  
they're as cheap as resistors, smaller, and means you won't get  
(much) audible modulation by any rubbish on the supply rails.

Cheers,
Neil
--
http://www.njohnson.co.uk