Very low ref voltage

Il 26/06/2012 15:16, George Herold ha scritto:

So you're saying I can't get my low refs... What about the ideas proposed by other friends?

Marco

A 1-volt reference is pretty useless for calibrating thermocouple instrumentation.

The most accurate and stable voltage reference that I know of is only about 500-700uV, which is inconvenient in some ways, but worth working with in some cases.

Hey, I can now generate ratios of AC voltages on my bench with a digitally selected resolution of 0.01ppm. Happy happy, joy joy.

Hi Marco, Sorry I should have kept my mouth shut. I see no problem with dividing down the 1 volt reference.

George H.

Do tell. I guess I was just thinking about the old Weston cells. I really don't know what is used for voltage references today. =A0(I tend just to believe my bench DMM.)

George H.

The best reference that I know of that is commercially available is thousands of Josephson junctions in series that are pumped (?) by ~70GHz microwaves. The guts needs to be cold (4K) to work.

But you can't find one on ebay. :)

Il 26/06/2012 16:51, George Herold ha scritto:

Don't worry, thank you anyway.

Marco

ext -

OK, I did that as a lab back in grad school. PITA to get a sample to work. I remember staying up all night trying sample after sample. (samples were just pieces of niobium wire twisted together.) I fianly made a probe that would hold 4 samples at a time and got some OK data. Say are there any high Tc super Josephsom junction? I know there are HTc squids.

George H.

From what I've read, it sounds like the JJs nowadays are a lot better.

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Yeah a flood of research.

Starcryo is selling Mr. Squid.

A single JJ could be a failed squid, (I think?)

George H.

Starcryo is selling Mr. Squid.

A single JJ could be a failed squid, (I think?)

George H.

Art

A current shunt bearing the dissipation of being used like a little 'crystal oven', will constantly drop the same voltage and that can be a good, low reference for your probe calibration session.

Likely not for his needs.

The noise floor simply becomes too 'everpresent'. It's like the cosmic background. You keep trying to fix your circuit, when your reference choice was the problem the whole time.

Gotta chill it all and put it in a cage, and make the room a cage as well. Maybe then.

So, about how much noise is there in a 10 ohm Thévenin-equivalent divider, assuming, say, a 10Hz bandwidth and lab environment, relative to the ~10uV resolution he wants?

Best regards, Spehro Pefhany

PARDon me, but ha ain't gonna get high resolution accurately and consistently if he makes bad decisions about references.

Thermal noise in a 1 Hz B/W is -174dBm, IIRC. Figure it from that.

That's at 290K, BTW

Yes, and the data sheets made it DAMN hard to figure out where the 1/f knee was. Everybody rates their amps in different ways, trying as hard as possible to obscure the real 1/f performance. But, AD had a graph of noise out to 1000 seconds on the AD706, with a guaranteed test performance. Amazing they don't charge $100 for the part if they have to test each one for 1000 seconds.

There were some tricks we found with the LM4140 also, such as running it at the top of the rated voltage spec (5 V) leads to a bunch of 1/f noise, but if you run it off ~3.5 V it gets much quieter. This was found entirely by experimentation.

Jon

I assume that was tongue-in-cheek.

On Monday, June 25, 2012 12:42:52 PM UTC-7, (unknown) wrote: [about accurate voltage divider 10:1]

Lower part count if you put 3 in series, and three in parallel to get that ratio. Or, just use a resistor array with six elements (but that requires laser-trim of six or more elements to get the off-the-shelf accuracy high).