Want a 5ppm/C (max) reference? Then don't buy a voltage reference... - Page 1
Forum section: electronics / projects

author: splin
author_rank: Frequent Contributor
timestamp: July 09, 2019, 02:16:45 am
content: ... buy a DAC!The cheapest TI 2.5V reference better than 5ppm/C (max) is the 3ppm/C LM4140ACM-2.5 costing $2.39 @ 1K from TI. The B-grade, LM4140BCM-2.5, 6ppm/C max is $2.0.Or, for only $1.14 you could buy aDAC80501DAC60501- a 3ppm typical, 5ppm/C max reference with a 12-bit DAC thrown in for free! Ok, it's a bit noisier with 14uVp-p (0.1 to 10Hz) v 5.4uVp-p for the LM4140 - but you get an unbelievably good drift specification of only 12uV, or 4.8ppm @ 35C over 1900 hours compared to 60ppm over 1000 hours for the LM4140!!As I said, unbelievable. Unless they have step-change technology here, it probably has similar drift to other plastic encapsulated band gap references, so nothing to see here folks.The DAC isn't too shabby either with +/- 1 LSB INL, but if you are really prepared to push the boat out and splurge another $0.86 you get a 16 bit, +/- 1LSB INL DAC for $2.86.You've got to love pricing anomolies - but it does make component selection aright pain in thegreat deal of fun. The flip side is that you are always wondering if some uber low cost camera imaging IC or the like could have provided a much cheaper ADC + data encoder solution (that would need less processing to re-encode into your required form than processing from scratch) for your latest wonder product.And lest you think this post is some sort of attempt at TI viral marketing (it isn't), the AD5310R @ $1.29 is a 10 bit DAC with a 2ppm/C typical, 5ppm/C max reference. It's a bit noisier though @ 16.5uVp-p. Typo - the DAC80501 is the 16 bit DAC, the $1.14 12 bit DAC is the DAC60501 - thanks to SiliconWizard for pointing it out.

author: krayvonk
author_rank: Regular Contributor
timestamp: July 09, 2019, 02:50:16 am
content: How much does it cost per bit of significance?

author: capt bullshot
author_rank: Super Contributor
timestamp: July 09, 2019, 06:01:42 am
content: Yes, not that surprising.If you need a simpe 12Bit ADC with 8 or more input channel multiplexer, it's cheapest to use a small uC for that.

author: exe
author_rank: Unknown
timestamp: July 09, 2019, 08:34:00 am
content: But is DAC80501 on sale yet? I don't see it in stock... anywhere.

author: tszaboo
author_rank: Super Contributor
timestamp: July 09, 2019, 08:58:37 am
content: The LM4140 is old silicon, and doesn't represent the best technology.The DAC80501 specified temperature range is 0-70C, while most of the other references that are about 5-8ppm/C from TI are specified from -40-125C. They use the box method to give you the tempco, and on the lower and higher extremes, the tempco is a lot worse than next to 0 degrees. That means, that a REF3425 with 6ppm specification will behave much better on the 0-70 C region than that DAC. Just look at Figure 3 in SBAS804B.Not to mention the specification for the DAC's reference seems to be... missing important info. Like the drift spec is for 1900 hours 35C, instead of first 1000 hour, second 1000 hours. And the value there... Let's just say I dont believe it. They claim 12uV, which is less than 5ppm drift for almost 2000 hours. It is better that this is pre-production datasheet.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2019, 08:59:18 am
content: And be aware of the other specs and factors, e.g.initial accuracy 0.1% (i.e. 1000ppm)60ppm drift after 1000 hours (10% of devices 140ppm)20ppm thermal hysteresis9uV/sqrt(Hz) noise at/below 0.1HzPCB/mounting stresses, especially for SMD deviceshumidity

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 09, 2019, 01:43:14 pm
content: The DAC80501 is an interesting product indeed, still not available (preproduction stage).http://www.ti.com/product/DAC80501Just for clarification, the DAC80501 is a 16-bit DAC (not 12-bit) and right now the unit cost per 1k is given at $2.86.The 12-bit version is theDAC60501, displayed at $1.14 per 1k indeed. As it's in preproduction, the unit cost may not quite be definitive...The initial accuracy of the internal ref is +/-0.1%, same as the LM4140. Not bad. As said above, its specs are not spectacular either and some info that is usually given for voltage references are not given here. Also compare the load regulation between the two... At 2.5V, it's almost 10x worse for the DACx0501. So it would all depend on how you're going to use it.When I need something better, I go for something like the MAX6126. About twice the cost of the LM4140. But that is what I'd call a voltage reference.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2019, 03:27:11 pm
content: Quote from: SiliconWizard on July 09, 2019, 01:43:14 pmStill 20ppm or 100ppm per 1000hours. Voltagereferencesusually stabilise over time, and the spec is ppm/sqrt(hours).Besides, "friends don't let friends use Maxim".

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 09, 2019, 03:40:54 pm
content: Quote from: tggzzz on July 09, 2019, 03:27:11 pmYeah. It's just a couple dollars IC. It's good and has served me well on occasion, but don't expect it to beat a precision, temperature-stabilized voltage reference in an oven.Quote from: tggzzz on July 09, 2019, 03:27:11 pmYeah, this is your opinion though. Maxim makes some pretty good products (I tend to favor Linear/AD for various reasons, but still). I admit they are not always the best with support and availability (especially for their MCUs as far as I'm concerned), but to knock them off altogether... sounds a bit excessive.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 09, 2019, 04:20:40 pm
content: No idea what they're like today but older engineers remember designing Maxim parts into new products, only to have the parts discontinued some months later. It's hard to forgive when that happens a couple times. Management doesn't remember the vendor as clearly as they remember the idiot who used them.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 09, 2019, 05:24:24 pm
content: Well, out of curiosity, I just took a look again at the MAX6126 page:https://www.maximintegrated.com/en/products/analog/voltage-references/MAX6126.htmlThey state "But some versions of the family are Not Recommended for New Designs", which kinda sounds, knowing Maxim, like those versions are going to be discontinued any time now. You make a point. But it has lasted for a few years (I think I selected it like 4 years ago, that's admittedly not much).Its successor doesn't look bad though:https://www.maximintegrated.com/en/products/analog/voltage-references/MAX6226.html

author: bob91343
author_rank: Super Contributor
timestamp: July 09, 2019, 05:44:44 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2019, 05:46:05 pm
content: Quote from: SiliconWizard on July 09, 2019, 03:40:54 pmMy introduction to Maxim was for a new interesting specialised device. performance was right, price was right, but there was never any availability. I had the feeling they "ran the device up the flagpole to see who saluted"; no salute => no production.Onlylaterdid I come across "friends don't let friends use Maxim". It resonated with me, i.e. it confirmed my prejudice.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 09, 2019, 05:51:46 pm
content: I understand. And now that I've just seen that the reference I used is going deprecated doesn't bode well either... this was only for a few units-project so not a huge issue here, but still. The "successor" I just saw, flagged as "new", looks pretty nice but... it costs almost twice as much!

author: tggzzz
author_rank: Super Contributor
timestamp: July 09, 2019, 05:52:22 pm
content: Quote from: bob91343 on July 09, 2019, 05:44:44 pmSaturated Weston cells are 40uV/C, i.e. ~40ppm/C. Mine is still in spec after 70 years.Newer references are based on zener diodes and are unsurprisingly expensive.One option is to get a well-aged device and get it calibrated by a calibrated HP3458.Alternatively new 6.5 digit bench multimeters are around £1000.

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2019, 06:48:28 pm
content: Quote from: bob91343 on July 09, 2019, 05:44:44 pmI 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

author: splin
author_rank: Frequent Contributor
timestamp: July 10, 2019, 12:51:27 am
content: Quote from: NANDBlog on July 09, 2019, 08:58:37 amTrue but not very relevant - the LTZ1000 is a lot older than the LM4140, but is still the best reference on the market (for TC, noise and stability - not price or power consumption or initial tolerance). This thread is not about references that 'represent the best technology' ( what even is that?) but the cheapest, or nearly so, 5ppm/C (max) reference available (from a 1st tier manufacturer).There are many characteristics other than TC which need to be considered and the DAC60501's reference will not meet the needs of many applications - but it may will be good enough for others where it's attractive price compared to the alternatives could make it a good choice, and a free 12-bit DAC, or a 16 bit DAC for only $1.74 extra may make it very attractive.Part of the reason for the OP was to point out that sometimes pricing oddities occur which mean that the 'obvious' solutions aren't always the best. Another example is that if you want an 80MSPS 12 bit ADC, by far the cheapest solution is to choose the $5.39 LPC4370 and get a free triple core, 208MHz ARM Cortex M4 micro thrown in for free. It obviously won't suit all applications, but for some it is outstanding in comparison to the free standing ADC and an FPGA and/or microcontroller solution you originally had in mind. If you were only searching for high speed ADCs you wouldn't find this option.It would be great if anyone could come up with some other hidden gems of parts that you wouldn't expect or can be repurposed to good advantage. Parts used in very high volumes, such as toys, lighting, white goods etc. can be good candidates because the pricing can be very low and because of their specialist nature aren't well known outside of their direct application area - and because datasheets may not even be generally available.QuoteIt's TC is specified as:3ppm typ, 5ppm max 0 to 70Cand5ppm typ, 8ppm max -40 to 125Cwhich is a much better way of specifying the TC than the single -45 to 125C figure given for most references - many applications aren't interested in temperatures outside the 0-70C range. Eg. a bench meter.If 8ppm max over the industrial temperature range is not good enough for you but 6ppm is, then by all means choose the more expensive REF3425.QuoteYou can't assume anything about the characteristics that aren't guaranteed limits, especially based on a graph of a single supposedly 'typical' unit in a datasheet. That graph may have been created when testing a few early pre-production samples or it could (unlikely) be an average of hundreds of production samples from many different batches - the datasheets rarely give such insights. Even then, production processes vary over time so using a typical graph of parts made years earlier when the DS was written is ill advised.Take a look the graph of the 1.8V LM4132 for example:Three of the 5 samples actually have higher TCs over 0-70C than -40 to 125C. The graphs for other voltage LM4132s have similar shapes but with greater extremes at the higher temperature end but is that because there is something special about the 1.8V version or because they came from different batches?In practice, the REF3425 may well behave better than the DAC's reference over 0-70C but you can't assume it from the DS information; if it's important then you need to characterise and qualify the parts yourself - and test every new batch you get until you are confident the batch to batch variation is small enough.QuoteYes, the drift spec is almost certainly hokum - which is why I went to the trouble to point it out in my OP:Quote

author: splin
author_rank: Frequent Contributor
timestamp: July 10, 2019, 01:30:00 am
content: Quote from: tggzzz on July 09, 2019, 03:27:11 pmI'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.For a bit more evidence however, take a look at the graphs showing the stabiity over 1000 hours for the 2.5V uMax and SO parts - the two 'typical' uMax parts show 109ppm and 133ppm drift (max) compared to the spec of 100ppm. The SO parts show 29ppm and 35ppm maximum drift compared to the 20ppm spec.A typical figure without any indication of the actual worst case spread or varience is almost useless in any case.

author: bob91343
author_rank: Super Contributor
timestamp: July 10, 2019, 03:01:07 am
content: I do have my HP 3456A that is my house standard for voltage. But some day I may want to verify and/or calibrate it. Short of an expensive deal with NIST or some local equivalent, I don't have any idea what I could do.I also have an HP 3455A but it's not working. If I can fix that I will be happy. When it worked, it agreed spot on with the 3456A but then turned belly up. If anyone has serious experience working on one of these I would love to get some advice.I also have a few 10V IC chips that agree with one another remarkably well. At one time I had them switch selectable so I could verify my voltmeters. Once they have been characterized they will probably hold calibration very well. I should reconnect that setup and take notes.Now and then I ask myself why I need accurate voltage but have no satisfactory answer other than it must be a fetish.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2019, 03:01:19 am
content: I would be cautious about directly comparing references from some companies including Analog Devices and Texas Instruments. In the past both have advertised noise and temperature coefficient specifications which were questionable or even impossible. Pease and others called them out on this but I do not know that the datasheets were ever corrected.

author: bob91343
author_rank: Super Contributor
timestamp: July 10, 2019, 03:04:26 am
content: 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.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 10, 2019, 07:20:13 am
content: Quote from: splin on July 10, 2019, 01:30:00 amIndeed. The new MAX6226, which has significantly better specs in terms of drift, only comes in a ceramic package.

author: exe
author_rank: Unknown
timestamp: July 10, 2019, 09:48:03 am
content: Quote from: splin on July 10, 2019, 01:30:00 amI'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.

author: tszaboo
author_rank: Super Contributor
timestamp: July 10, 2019, 10:24:33 am
content: Quote from: splin on July 10, 2019, 12:51:27 amThat's not how the box method works, and it is impossible to work with that graph for the box method. You have to take a bunch of samples (25-100), normalise it somewhere (for example 25 degrees) and measure the spread. Then divide it with the range. Most references will have some kind of a compensation built in, for example a zener with a regular diode in series, with different direction of the typical tempco.But anyway, look at figure 1 of the REF3425, and tell me that the typical value is not good enough indication. I know when I need a voltage reference, which one I typically use, a dedicated and characterised one or something that comes as an extra.Quote from: bob91343 on July 09, 2019, 05:44:44 pmLet's say, I can sell you a 0.001% accurate reference, that changes it's output voltage by up to 0.02%, when you solder it to a PCB. Would you buy it? If yes, than I also have a bridge to sell.

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2019, 11:01:56 am
content: Quote from: exe on July 10, 2019, 09:48:03 am"Conformal coatings" do not completely solve problems, and introduce new problems.

author: SilverSolder
author_rank: Super Contributor
timestamp: July 10, 2019, 05:18:39 pm
content: Quote from: exe on July 10, 2019, 09:48:03 amSadly, 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?

author: Gyro
author_rank: Super Contributor
timestamp: July 10, 2019, 05:56:52 pm
content: Ceramic packaged ICs are hermetic (and don't suffer humidity related internal stresses).

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: July 10, 2019, 06:37:07 pm
content: Quote from: bdunham7 on July 09, 2019, 06:48:28 pmYes, 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.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2019, 02:58:30 am
content: Quote from: bob91343 on July 10, 2019, 03:04:26 amI 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.

author: splin
author_rank: Frequent Contributor
timestamp: November 04, 2019, 03:25:51 am
content: Quote from: splin on July 09, 2019, 02:16:45 amThought I'd update this as SiliconWizard did make a somewhat important point:Quote from: SiliconWizard on July 09, 2019, 01:43:14 pmThese 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...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). typos

author: exe
author_rank: Unknown
timestamp: November 04, 2019, 09:20:38 am
content: Quote from: splin on November 04, 2019, 03:25:51 amAfaik those lm399 that go into 6.5digit dmms are aged and specially selected. A good lm399, if treated well, gives 1-2ppm long-term stability.