[sdiy] voltage reference advice

[David G. Dixon]

> I'm building 4 VCO's in a single unit and I don't want to use the rail
> voltages for the tuning knobs, octave switches and a few other things
> (low current stuff).
> 
> I know I can use a Zener diode as a simple zener regulator
> (http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html#c2),
> but I don't know how noisy and stable that is going to be for this
> application. I was looking at the LM431 which looks good.. There's
> also the REF02 for a fixed +5V. Does anybody have any preferences for
> voltage references?

Those would be my choices.  I've used REF02 in my VCOs and it's great (but a
little bit pricey).  It's not that widely available now (at least in bulk)
but the ADR02 is a good substitute.  I now think it's probably overkill.

I'm thinking of switching to the LM431 (or it's cheaper equivalent, the
TL431) because it comes in a handy TO-92 package, and costs a tiny fraction
of the other.  The tradeoff is a tempco of 50ppm/C vs 3-10ppm/C, but this
should be good enough for even the most stringent VCO requirements.

In fact, on my PCB layouts I am now providing pads for both options.

Of course, the REF02 and ADR02 give a trimmable 5V reference, and the LM431
and TL431 give an adjustable 2.5V reference.  In fact, in VCOs it may not be
necessary to adjust these values at all, since they are generally within 1%
of their nominal values, making trimming unnecessary, and in most cases you
will be using them as current sources (through input resistors) to summing
nodes and can simply select the input resistor accordingly.  The only case
where you might need to adjust the output of the '431 is if you are using it
as the threshold reference to the comparator in a sawcore, and then two
resistors is all you need.