Electronics > Projects, Designs, and Technical Stuff
Want a 5ppm/C (max) reference? Then don't buy a voltage reference...
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SilverSolder:

--- Quote from: exe on July 10, 2019, 09:48:03 am ---
--- Quote from: splin on July 10, 2019, 01:30:00 am ---I'm very dubious about those stability specs - 20ppm is very good for a plastic encapsulated part. Other references available in metal or ceramic packages as well as plastic all show much worse drift for the plastic parts - usually 40 to 50ppm or more. Ie. it is the plastic which is the culprit, no matter how good the reference itself.

--- End quote ---

I'm no expert, but I agree. May be measurements done in a controlled environment with constant humidity, pressure etc.

I wonder if it's possible to apply some coating to make plastic not absorbing moist.

--- End quote ---

Sadly, it seems the only "coating" that works is a glass envelope or metal container...   

Not sure how the ceramic versions of chips perform in this regard, perhaps the ceramic material does absorb some water vapour, but is dimensionally unaffected by it?
Gyro:
Ceramic packaged ICs are hermetic (and don't suffer humidity related internal stresses).
RandallMcRee:

--- Quote from: bdunham7 on July 09, 2019, 06:48:28 pm ---
--- Quote from: bob91343 on July 09, 2019, 05:44:44 pm ---Initial accuracy of 0.1% isn't good enough.  I need something that is closer, right out of the box, to calibrate my instruments.  Even 0.01% is marginal.

Is there anything reasonable in price that is good enough?  Back in the day you could buy a Weston standard cell and get a pretty good reference but they were expensive.

--- End quote ---

I think this is as reasonable as you can get for a quasi-professional product.  By "quasi-professional" I mean the accuracy and service provided are top-notch, while the appearance and construction are hobbyist level.

http://www.voltagestandard.com/Home_Page.html

--- End quote ---

Yes, the VoltageStandard devices are a good deal. I recently had my DMMCheck re-calibrated after one year, to verify the initial specs. It was 5.00000 volts before and 5.00000 volts after. So drift less than 10 microvolts.
Edit: Just re-read the email from Doug--it drifted pretty much exactly 10 microvolts during that year and three months.

David Hess:

--- Quote from: bob91343 on July 10, 2019, 03:04:26 am ---I do understand and appreciate your caution.  However, statistically speaking comparing 5 or 6 references over the long haul should show which if any are suspect.  I don't expect them all to be bad equally.

As I said, it's a fetish so I will probably assume they are good and blindly go on with my life.
--- End quote ---

I was not as clear as I should have been.  I meant to say that I would be cautious about directly comparing references based only on their datasheets because of previous mistakes and outright shenanigans by various companies.  Empirical testing is called for.
splin:

--- Quote from: splin on July 09, 2019, 02:16:45 am ---... buy a DAC!

--- End quote ---

Thought I'd update this as SiliconWizard did make a somewhat important point:


--- Quote from: SiliconWizard on July 09, 2019, 01:43:14 pm ---The DAC80501 is an interesting product indeed, still not available (preproduction stage).

... right now the unit cost per 1k is given at $2.86.
...
As it's in preproduction, the unit cost may not quite be definitive...

--- End quote ---

These are now available and as yet the 12 bit DAC60501 is still priced at $1.14 @ 1k price but they are now also quoting $0.99 @3k. The bad news is that the temperature coefficient spec has increased from 5ppm/K to 10ppm/K. Dissapointing but perhaps not too surprising.

Also unsurpising is that the long term drift spec has also increased from 12 uV/1900h to 20uV/1900h. Perhaps they should include a long term drift specification for the drift specification...   :-DD

But even 20uV/1900h is still hard to take seriouy - that's a mere 8ppm in 1900hs! Even the hermetic metal can version of the LM399 used in 6 1/2 digit meters including the 34401A was only specced at 20ppm/1000h! I don't know of any other (easily available) reference that has as good a specification other than the LTZ1000.

But still, they did revise the datasheet with this new figure so they must be making measurements - perhaps they really have made a ground breaking improvement in reference stability which they decided to showcase in a $0.99 DAC's reference!

The good news is that the 16 bit DAC80501 reference is still specced at 5ppm/K max; they are still $2.86 @ 1k but they are now also quoting $2.49 @ 3k which is pretty impressive.

So if you want a 5ppm/K max reference with a free - ish DAC it'll cost you $2.86 from TI - but bear in mind that the DAC specs are actually pretty good, especially the max INL at +/- 1LSB. Many low cost 16 bit DACs have max INLs of 16 bits or more - ie. 12 bit accuracy (or less).

[EDIT] typos
 
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