Electronics > Projects, Designs, and Technical Stuff
Want a 5ppm/C (max) reference? Then don't buy a voltage reference...
bdunham7:
--- 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
splin:
--- Quote from: NANDBlog on July 09, 2019, 08:58:37 am ---The LM4140 is old silicon, and doesn't represent the best technology.
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True 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.
--- Quote ---The DAC80501 specified temperature range is 0-70C
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It's TC is specified as:
3ppm typ, 5ppm max 0 to 70C and
5ppm typ, 8ppm max -40 to 125C
which 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.
--- Quote --- 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.
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You 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.
--- Quote ---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.
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Yes, the drift spec is almost certainly hokum - which is why I went to the trouble to point it out in my OP:
--- Quote ---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.
--- End quote ---
splin:
--- Quote from: tggzzz on July 09, 2019, 03:27:11 pm ---
--- Quote from: SiliconWizard on July 09, 2019, 01:43:14 pm ---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.
--- End quote ---
Still 20ppm or 100ppm per 1000hours. Voltage references usually stabilise over time, and the spec is ppm/sqrt(hours).
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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.
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.
bob91343:
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.
David Hess:
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.
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