Illya,
that all looks much more reasonable and solid.
Anyhow, now one can really see the good quality of your measurements, in relation to the reference noise.
I don't know, where to find an official definition of this 'box method'.
As implemented by Vishay for the determination of the T.C. of their resistors, they probably use 3 temperature points only, like -55°C, +25°C and +125°C.
Their German representative once measured my VHP202 at 25, 60, 100 and 125°C only, and calculated the average T.C. between these points.
G.R. measured the new 120 Ohm resistor samples between: 23=> 35 =>45 => 35 => 23°C.
They let the DUTs settle for about 24h at each single point, before they take the measurement.
However, they got totally puzzled values, even with different signs for the T.C., and that was due to this big hysteresis of this special value.
Therefore, our method of
continuously measuring the reference voltage en passant, i.e. while the temperature slowly changes, and then to draw a box around the horizontal and vertical limits of the measured graph, gives much better and more information about the DUT.
You only have to take care, that not too big a hysteresis or shift occurs, which often happens, when the temperature is changed too fast.
In a physical sense, the measurements have to be made in or near the 'equilibrium state'.. and it's always a problem to identify this correctly.
This box method is a kind of averaging, and necessary only, when a non-linear or hysteretic behaviour is present.
Yep, if you were able to change the temperature of your 3458A by 5°C, you'll get a real good measurement of its T.C. for DCV, w/o ACAL.
I assume, the root cause of this T.C. is located mainly in the A/D ASIC, i.e. these internal gain resistors, which you were very familiar with, I think
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif.pagespeed.ce._hfWAz_QHO.gif)
This famous cal constant 72, related also to the AN18 stability problem, should directly depend on the environmental temperature (which is not considered in the AN18).
What do you think?
I have measured this CAL? 72 regularly, but could not find a real drift over time (fortunately), but a negative dependency over temperature (
TEMP? CAL? 175), of about -0.3ppm/°C.
The graph over temperature has a lot of scatter, because the temperature sensor is on the opposite site than the A/D board.
But these -0.3ppm/°C are in good accordance with the DCV T.C. of +0.4ppm/°C, which my wife measured for my 3458A..
The difference in value might to be explained by the T.C. of the internal LTZ1000A reference.
Frank