[volt-nuts] Low-cost voltage reference questions

[acbern at gmx.de]

There are many things to factor in. Drift of reference itself over time and temperature (399 is 1ppm/K worst case alone, over say 10K, i.e. 23C+/-5K). Drift of the gain setting resistor is critical (assuming you have an amp generating 10V out of the 399 output voltage). and so on. 
Also, you need a precise reference to calibrate the units (low cal uncertainty and low drift).
Selection, statistical validation and aging is needed.
All doable but costly and more complicated than one might expect. 

Thats why I think the target price has to be reasonable, any thoughts here?




> Gesendet: Dienstag, 24. November 2015 um 10:05 Uhr
> Von: "Ian Johnston" <ian at ianjohnston.com>
> An: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
> Betreff: Re: [volt-nuts] Low-cost voltage reference questions
>
> 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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