A Low Cost OSHW Voltage Calibration Reference Project

[z01z]

The paper and the link mentioned here might be of interest.

[rhb]

The kitchen oven and freezer would be more convenient than water. There is also the question of reducing the excursions with time as shown in a Fluke paper about the 7001 in the reference teardown thread.  If you have hysteresis, you need to do that.  Consider demagnetization.

Once the curve flattens out, it becomes difficult to determine the deviation from a straight line.  The computation becomes very sensitive to numerical precision.

A physical model for the drift and the hysteresis would be very useful.  Variations due to stress imply piezoelectric effects so far as I can see.  Is there literature on that in solder joints for example?

A man was walking down the street when he saw another man run out, stomp his foot and then go back to leaning against a building.  As he approached, the man did this several times.  When he reached the man, curiousity got the better of him. 

"What are you doing?" 

"Keeping away tigers." 

"But there are no tigers around  here." 

"Works very well, doesn't it?"

[rhb]

One might consider building a temperature chamber with a pair of Peltier devices so one could raise or lower the temperature as desired.  That would make acquiring data on hysteresis  fairly simple.  Run a group of LM399s with the heaters off and measure differential voltages among the set for a variety of thermal cycles.  Put CPU heatsinks and fans on the Peltier devices to improve heat transfer.

That was an interesting paper, thanks.  There ought to be more data available.  If you get any of the references from the thesis I'd like to have them.

Reg

[cellularmitosis]

hello,

update:

the lm399 has "survived" a week of operation and the measurements are substantially the same. ie;

Tamb = 29 degC
Vref = 7.00390V (main reference)
Vhd = 0.560V (heater in temp. sensor diode mode)

so it does look like a valid mode of operation.

has anybody duplicated the experiment?
has anyone seen an LM399 being used like this?

best regards.

-zia

I definitely plan on duplicating locally!  As soon as get around to it    

[Andreas]

Hello,

you cannot calculate 1/f noise (uVpp below 10 Hz) from noise density data (uVeff above 10 Hz).
Below the 1/f corner frequency the noise density increases (by 1/f).

The stray on LM399 noise is very large.
Good devices have down to 4uVpp 1/f noise (0.1 .. 10Hz)
Not so good devices are above 10uVpp.

Below 0.1 Hz (there where it hurts most) the noise is not specified.
This is the area of popcorn noise with jumps with ppm-Level for some minutes or hours.

best regards

Andreas

[Kleinstein]

To get 4 good LM399 one would need to buy considerably more than 4 units. So there would not be that much cost advantage compared to the LTZ1000 (if not using super high grade resistors).

The LTZ1000 1/f noise corner can be well below 10 Hz. This means the 0.01 Hz to 1 Hz noise can be considerably better than 0.1 -10 Hz noise. For the LM399 the 1/f corner is higher and thus 0.01 Hz-1 Hz noise is expected to be similar to 0.1-10 Hz noise. So even if one averages enough LM399s to get the same 0.1-10 Hz noise level, the LTZ would be still better at 0.1 Hz. Better performance of the LM399s at 10 Hz is not that relevant.

It would be rather difficult to get the LM399 to a stable 25 C. To get good stability without the internal heat, if would likely need a two layer temperature regulation. With just heaters this would be more like 50 C as a minimum. Going  below environment is tricky due to possible condensation.

[Andreas]

Of course I tested that with a very good device.

now tell me which of the randomly choosen measurements have the heater on and which have the heater off 

All measurements with 0.5uV / div and 1 sec / div.

[Andreas]

that was not the question.
This is a comparison between heater on and heater off.
So some measurements are with heater on others with heater off.

[Andreas]

where is your factor 2 in difference for the heater on or off?

this may be valid for the noise density of the wideband noise.
The difference in 1/f noise is much smaller.
But you cannot decide it by a single measurement since the stray from 10 sec to 10 sec window is also very large.
Perhaps you get 20% increase by "heater on" in average. but the stray between measurements can be a factor of 2.

A not so good example (LM399#2 with 1uV/Div) (that does not mean that this is the worst case)
and some typical examples one with one edge of popcorn noise (which is filtered mostly away by the high pass filter).

most of my LM399 are good old National Semiconductor.

Also measured a newer LTC LM399 (last picture also with 1uV/Div).

By the way: #3 and #4 are with heater off.

with best regards

Andreas

[cellularmitosis]

Hmm, so the lower heater temperature benefit is now only a question of possibly slower ageing rate?

[rhb]

Collecting data with heater on and off should allow determining the coefficient of the thermal noise for the devices as well as the coefficients of the 1/f noise.  The different slopes of the amplitude spectra make them nearly orthogonal and thus easily separable.

If one used an oven, one could derive a more refined model for the thermal noise by making measurements at multiple temperatures.

I've not yet considered the problem of popcorn noise.  My 8648C arrived yesterday and my 5386A on Monday, so I'm comparing them at the moment while I reorganize my lab/closet.  Next up is to reference each in turn to my GPSDO and trim them and the 8560A .  After that I need to build a distribution system for the GPSDO reference signal.  But shielded and filtered AC power comes first.  I may well go over the top and encase everything in steel from the IEC socket on the instruments all the way to the entrance panel.

I ordered 3 AD584 "precision" voltage references from eBay for $3 each.  These are the J parts with 3000 ppm initial, 30 ppm/C and 25 ppm/yr or thereabouts.  I'll set them up with the well aged LM399s and collect differential readings to see how well I can predict the output voltages at 2000 hours from the readings over 1000 hours and the output at 4000 hours from the readings over 2000 hours.

[rhb]

Not actually what I've been talking about, but very similar and  dead simple to do.  The noise is a boxcar in time so it is a sin(f)/f in frequency. That is very different from the 1/f decline and the growing amplitude of the aliased thermal noise.  I'll try to get a simulation put together in the next few days of what the aliasing looks like to measurements with an integrating ADC with PLC=10.  Thermal and 1/f can be done with L2, but popcorn noise requires a sparse L1 pursuit.  You build a large dictionary of possible popcorn events of various lengths and amplitudes and then find the set which best fits the data.

Unlike thermal and 1/f  noise, it might be possible to remove popcorn noise, at least under certain circumstances.