Electronics > Metrology

Lowest drift, lowest noise voltage reference (ADR1000AHZ)

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branadic:
Hi folks,

after I played with the simplified version of W/F7000 schematic with LTZ and achived -0.0106 ppm/K after t.c. trimming for the bare ovenized zener, I now played with a very new, lowest drift, lowest noise voltage reference I had on the desk. Actually I already had it for some months already, but it always takes some time to get all the components and board design ready.

I treated it the same way as mentioned in the linked thread before, thus first setting the z.t.c. temperature to a specific temperature with the oven turned off. Next step was to turn on the oven and adjusting the oven to that specific z.t.c. temperature.

I then captured some low frequency noise data with a 0.1 ... 10 Hz LNA (80 dB, 100 nVpp or 16 nVrms) and an Advantest R9211E used as a DSO.

Afterwards I've measured the remaining t.c. of the board and trimmed the t.c. in multiple steps. The last trimming was slightly overcompensating things., turning the t.c. into opposite direction. The measurement is already limited by the lab temperature stability and the t.c. of the meter. Thus, it needs another run of adjustment and another run in the thermal chamber plus comparison to F7000-2 in the next run, but results are looking good so far.
Compensating the ambient temperature influence, I can find the t.c. already being at -0.03713 ppm/K, but I'm sure we can do better than that.

The board is currently missing the gain in the boost stage, but uses the amplifier as a buffer only. This is part of the next steps that will be performed soon.

-branadic-

branadic:
After some tweaking I'm now down to about -10 ppb/K. With some more iterations I'm sure I could get it even smaller, but hey the result is not too bad, or is it?

-branadic-

Kleinstein:
The temperature effect looks good. However this is the easy part - expecially for a rather limited tempearture range.

The noise performance also looks really good, though the time window is relatively short to really tell. Sometimes popcorn noise happens on a long time scale and some 100 seconds without a jump are well possible even for a LM399. The standard to LF noise charactrization is 0.1 to 10 Hz, because this is a window that can still be measured with AC coupling. However the LF noise that really matters is often the even lower frequency part, like 1-100 mHz. This is especially the case for a precision DC voltage reference. This is really hard to measure - so it is very understandable to measure the sharter time scale. It could still help to have a longer window.

branadic:
At least in the observed time frame, a t.c. measurement takes about 10h 40min, no popcorn noise was present.
The only real measurement setup for very low frequency noise I know of is propably a JVS and a nV-meter. As the LNA already limits the bandwidth to 0.1 ... 10 Hz, longer time frames with this setup don't make any sense at all.

-branadic-

Kleinstein:
I know the problem with the very low frequency noise. One could do a measurement with 2 identical reference, but it is quite some effort.

The TC measurements take quite some time, but the vertical resolution is not as good and the reference in the meter is involved too. So hard to tell which ref. is causing the change.

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