[volt-nuts] Low-cost voltage reference questions

[Ian Johnston]

Hi all,

I can relate to this.......and only last night I was pouring over it 
wondering what to do!

I have just designed a Handheld Precision Digital Voltage 
Source.........LM399AH & uController controlled.

About to start selling them, and so I am writing the manual & spec sheet for 
it.......but can't decide without a years worth of data & testing behind me 
what figures to use!
I've got the reference, DAC and op-amp figures, all the tempo's I need etc 
etc.

Hmmmm!

Ian.
 


-----Original Message-----
From: acbern at gmx.de
To: volt-nuts at febo.com
Date: Tue, 24 Nov 2015 09:35:24 +0100
Subject: Re: [volt-nuts] Low-cost voltage reference questions


Charles and group, 
another persons opinion:

I guess the reference to the "standards" means those sold on ebay US. If so, 
I would think it is a false expectation this would meet 3ppm acc. guaranteed 
within a year.
There is a lengthy chat in eevblog about it, in case you are not aware, and 
while the unit typically may not be too bad, it is certainly not seriously a 
3ppm guaranteed standard (even in its best version). It starts with the 
traceability, and goes on with the design and build standard. Details in the 
blog. In summary, it is not even really spec'ed, also because it can't be 
(at least not close to what it seems to raise in expectations). But at that 
price, it would be unfair to expect more than a hobbyist item with 
relatively unclear real specs. But if you mean another item, let us know, I 
guess the group would be interested.
Keep in mind, the Fluke 732B is specified/guaranteed to 2ppm per year. There 
is data available from Fluke about 732B drifts ("Predictability of Solid 
State Zener References"), and it can be seen how hard it is for them to 
guarantee 2ppm/year.
So I think your price target and spec expectation ("guaranteed to 
remain"...) just does not match. 

I would think a unit that has a traceable specification to a National 
Standard (including an error propagation analysis for the factory 
calibration how to get there), and be within say 5ppm a year, over a defined 
(limited) temperature range, with a good build standard (CU-TE spades, metal 
case, EMI filtering, PSU...), targeted at those who cannot afford/do not 
need a 732B could easily have a fair price of a couple hundred usd.
Other opinions welcome.


Cheers
Adrian


> Gesendet: Montag, 23. November 2015 um 23:26 Uhr
> Von: "Charles Steinmetz" <csteinmetz at yandex.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] Low-cost voltage reference questions
>
> 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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