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| Want a 5ppm/C (max) reference? Then don't buy a voltage reference... |
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| bob91343:
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. |
| SiliconWizard:
--- 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 --- Indeed. The new MAX6226, which has significantly better specs in terms of drift, only comes in a ceramic package. |
| exe:
--- 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. |
| tszaboo:
--- Quote from: splin on July 10, 2019, 12:51:27 am --- 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? --- End quote --- That'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 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 --- Let'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. |
| tggzzz:
--- 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 --- "Conformal coatings" do not completely solve problems, and introduce new problems. |
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