Electronics > Metrology

ADR1399 reference

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Andreas:
Hmm,

so you have around 1 ppm/K temperature drift.
I think that your DMM contributes most of the T.C.

Did you try to thermally isolate the ADR1399?
Did you do the TC-Compensation of the LM334 from datasheet or did you adjust the 2 resistors to minimum T.C.
(according to my experiences with LM334 for a LM399 project the resistor values from datasheet are not perfectly).

with best regards

Andreas

EC8010:
Thanks for your thoughts. That's what I'm thinking; it's too close to the limits of my present measurement ability. I keep wondering about an 8 digit DMM. Or better, an array of at least four temperature controlled voltage references that can be compared using existing kit.

The 1399 is in the cardboard box with foam the evaluation board came in, but not properly sealed; I have done much better thermal control on other experiments. This is early days. I'm also wondering about that plastic hat the device is in. For instance, is the plastic triboelectric where it wobbles and scrapes on the leads? Is the plastic hat there simply because it's cheap and easy to add by the manufacturer, but not genuinely optimised? Would foam be better? The wobblyness and rattling concerns me, too. At this level, microphony is often an issue.

The LM334 is pretty much as per data sheet. I have tried tweaking the ratio of the resistors but it didn't help, suggesting that its remaining tempco was actually down to the (metal film) resistors themselves. I could perhaps do better using low-tempco wirewound resistors and trimming, but I think that's a bit of a dead end. I only put the compensated LM334 in because I had one ready-made and set for 3mA. I think the bootstrapped op-amp solution for the Zener should be better.

Alex Nikitin:
It might be simpler (and better) to use the LM317 + 330 Ohm resistor (and another 2K7 to ground as an additional load for the LM317) as a current source, the current variations should be below 10uA in a reasonable temperature range  (10C-50C) which would give a voltage change only around 1ppm (over the range) even for LM399, with less than 0.1ppm for the ADR1399.

Cheers

Alex

EC8010:
That's a thought. And I have an LM317-based variable CCS that could easily be connected. ;D If I see the same result as with the LM334, fingers will be pointed at the 34470A.

Mickle T.:
I slightly complicated the circuit diagram from the previous page, adding a few more components to the ADR1399 and the ability to reproduce AC and DC voltage with ranges of 1 and 10 V, six decades of resistance from 10 Ohm to 1 MOhm and current with five ranges from 10 μA to 100 mA. DAC - as usual AD5791A, but selected for INL<1.5 ppm.
The most difficult and time-consuming thing, as it turned out, was making the case. Although SCHROFF CompacPRO is convenient for installation and assembly, I did not like it at all due to its large and non-standard dimensions, and rather large weight (the entire assembled device weighs about 5 kg!). The extruded aluminum case is quite simple to machine, but unfortunately it does not accommodate domestic sealed PWW resistors and I have to install BMF or small-sized non-sealed PWW :(

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