| Electronics > Projects, Designs, and Technical Stuff |
| Want a 5ppm/C (max) reference? Then don't buy a voltage reference... |
| (1/7) > >> |
| splin:
... 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 a DAC80501 DAC60501- 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 a right pain in the great 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. [EDIT] Typo - the DAC80501 is the 16 bit DAC, the $1.14 12 bit DAC is the DAC60501 - thanks to SiliconWizard for pointing it out. |
| krayvonk:
How much does it cost per bit of significance? |
| capt bullshot:
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. |
| exe:
But is DAC80501 on sale yet? I don't see it in stock... anywhere. |
| tszaboo:
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. |
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