Hello,
the below mentioned 3ppm/year are not easy to get (even with expensive
references).
Your selected parts are both band gap references. Which are poor
regarding long term stability + noise.
The best references (stability + noise) are buried zener references.
Plastic packages suffer from humidity effects. (even good references can
have 10-15 ppm shift over 30% rH change).
So with high demands you would need a hermetically sealed reference (at
best in metal can case)
Further effects are temperature hysteresis and mechanical stress from
the PCB (or soldering).
Practically you have to pre-age the references (6-12 months) and to sort
out the
less performing parts if you really want to go below 10 ppm stability.
Book recommendation:
Current Sources and Voltage References: A Design Reference for
Electronics Engineers (Linden T. Harrison)
With best regards
Andreas
Am 23.11.2015 um 23:41 schrieb Russ Ramirez:
Excellent points, especially the consideration of shipment effects
year-round and across major temperature variations. Thank-you Charles.
Russ
On Mon, Nov 23, 2015 at 4:26 PM, Charles Steinmetz <[email protected]>
wrote:
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
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