[volt-nuts] Low-cost voltage reference questions

[Charles Steinmetz]

Russ wrote:

>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?

One person's opinion:

To a group like this, I'd be inclined to say that interest begins at 
a room-temperature (say, 20C +/- 3C) accuracy of 3ppm (i.e., 
guaranteed to remain within 3ppm from 18-22C for at least one year 
after purchase).  3 ppm is 0.0003%.  There is at least one 10v 
reference with specifications in this ballpark available at an asking 
price under $130 (I'm told the seller has accepted offers 
significantly lower than this).

>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%?

I, for one, do not consider 0.08% to be "much less" than 0.1%.  One 
sneeze and it's out of spec.  Indeed, I would consider a claim of 
0.1% accuracy to be bordering on fraudulent based on a calibrated 
measurement at 0.08%, unless the spec was qualified as "within 0.1% 
at [temperature within 0.1C] as is, where is -- no claim as to 
accuracy after it has been shipped to the buyer."

Speaking as someone with substantial commercial design experience, I 
would never offer a voltage reference for sale as a claimed "0.1% 
standard" that I did not have excellent justification for believing 
would stay below 0.05% for a year over a several-degree range of 
temperature and multiple trips across the country via commercial 
carriers.  I wouldn't expect to be able to charge more than $10-15 
for the product just described, and then only if the nominal output 
voltage were 10v (I think you will find that there is a very strong 
preference for 10v references over 5v, 2.5v, or other voltages).

Best regards,

Charles