[volt-nuts] Low-cost voltage reference questions

[Russ Ramirez]

I am comparing designs centered around the MAX6126-25 and the TL431B (0.4%
grade). I have an HP 3457 that is in cal from Agilent and a NI DAQ unit
that is more useful at seeing the noise (unshielded) on the output (which
is +/- 600 uVs or so) and logging the warm-up time to a (+/- 1 mV) stable
output. I also have a multi-channel Keithley 740 thermometer scanner so I
know the ambient at which readings are being made.

What is considered the break-over point of precision with low uncertainty
versus cost to a group like this? Is there a rule-of-thumb for the cost of
each additional digit of precision after N digits?

I mean these as simple, don't get into all of the uncertainty contributions
and general traps of metrology, questions. If I sell someone a reference
that I've ascertained is 2.50163v @70.3 F with a calculated uncertainty, is
it valuable as a 0.1% reference even though the error may be much less,
like +/- 0.08%?

Thanks for your time and feedback.
Russ