[volt-nuts] Low-cost voltage reference questions

Tue Nov 24 11:52:00 EST 2015

Wow, there's a ton to know about NTC thermistors by themselves! Lars, is
the part for the Geller style of unit similar to this one, (
http://www.digikey.com/product-detail/en/NXRT15XH103FA1B030/490-8601-ND/3788625)
or very different?

Russ

On Tue, Nov 24, 2015 at 8:51 AM, Lars Walenius <lars.walenius at hotmail.com>
wrote:

> Hello,
>
>
> My name is Lars Walenius and I live in Sweden. I am new to this forum but
> a volt nut for decades. I must admit I have to many DIY voltage reference
> boxes. Some years ago I was fortunate to work with Joe Geller on the SVR-T.
> For me the SVR-T still is the best 10V hobby reference sold. So it is a
> pity the AD587LQ isn´t available any more as my opinion is that the
> reference IC is the most important in a DIY reference.
>
>
> I can just second what Andreas say. Maybe the comment about metal cans
> need to be commented. Metal cans are good for humidity but I have seen high
> long term drifts on LT1031 that I have several powered on for years. Also
> if power is off for a or two month they go back many ppms. On REF102CM in
> metal cans (obsolete) I have seen about 5ppm if turned off a longer time.
> In the same test AD587LQ (obsolete) changed less than a ppm. AD587JQ and
> AD587UQ are still available but I would not recommend the UQ as my and
> others observations are in the 10ppm/C class ( probably due to adaption to
> a very wide temp range). I have several JQ with reasonable (less than
> 5ppm/C that can be compensated with NTC) but you should check noise (LF). I
> have seen several ppm p-p on some. So by selection I think the AD587JQ is
> the best 10V ref available today (from eg Digikey).
>
>
> Charles, I have to say thank for all your good comments both here and in
> time nuts but if the 130USD ref is the D105 on Ebay have a look on EEVBlog
> and my humidity tests. The 2ppm ref has 20ppm drift for a change 40 to
> 90%RH in just a week!!
>
>
>
> http://www.eevblog.com/forum/testgear/calibratory-d-105-dc-precision-voltage-reference-standard/630/
>
>
>
>
>
>
> Lars
>
>
>
>
>
>
> Från: Russ Ramirez
> Skickat: ‎tisdag‎ den ‎24‎ ‎november‎ ‎2015 ‎15‎:‎29
> Till: Discussion of precise voltage measurement
>
>
>
>
>
> 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, <acbern at gmx.de> 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" <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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> > >
> > >
> > >
> > > [Sent by MDaemon Mail Server at IanJohnston.com]
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