Thank-you all for your valuable insights and the book reference Jahn. I can see clearly now that doing such a project with a LM399 for example as a learning project would be interesting. Designing the board to use the MAX6126, and finding reasonably priced low tempco parts was a useful exercise.
Russ On Tue, Nov 24, 2015 at 5:55 AM, <[email protected]> wrote: > 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" <[email protected]> > > An: "Discussion of precise voltage measurement" <[email protected]> > > 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: [email protected] > > To: [email protected] > > 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" <[email protected]> > > > An: "Discussion of precise voltage measurement" <[email protected]> > > > 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 > > > > > > > > > _______________________________________________ > > > volt-nuts mailing list -- [email protected] > > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > > > and follow the instructions there. > > > > > _______________________________________________ > > volt-nuts mailing list -- [email protected] > > To unsubscribe, go to > > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > > and follow the instructions there. > > > > > > > > > > [Sent by MDaemon Mail Server at IanJohnston.com] > > _______________________________________________ > > volt-nuts mailing list -- [email protected] > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > > and follow the instructions there. > > > _______________________________________________ > volt-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
