[volt-nuts] 732A and 3458A stability

[Randy Evans]

I ran a stability test on my  new 732A and 3458A units.  I had both
connected together (using copper wire leads) and I randomly ran
measurements of the 3458A reading of the 732A 10 VDC output.  I did an ACAL
DCV before each reading and used NPLC of 100 and 8 digit readings , and
recorded the room temp, 3458A internal temp , and the MIN/MAX/MEAN/STDDEV
of the 3458A readings, and the thermistor value of the 732A.

The room temp varied from 22.1 to 27.9 C over the several days of
 measurements.  The 3458A internal temp was, on average, 13.7 C higher than
the room temp.  The end result was a temperature sensitivity of 1.15uV per
degree C of the 3458A internal temp, or .115ppm/C.  I am assuming the 732A
is relatively stable over the changing temperature range (the thermistor
varied from 3666.6 to 3670.7 ohms) and attribute most of the variance to
the 3458A.

The absolute value of the readings are around 10.58 VDC but neither
unit is calibrated.  I assume they can be calibrated but I wanted to make
sure they are stable first.

Do these numbers look reasonable and do they indicate any issues with the
units?

Thanks,

Randy Evans
___
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

[volt-nuts] 732A vs. 3458A stability

[Frank Stellmach]

Hello Randy,

your stability measurements of 732A versus 3458A depend on the 
integration time.
It is not possible that way to distinguish, which of both instruments 
contribute more noise.


For NPLC 100, you may expect around 0.1ppm noise, i.e 1µV/10V, at NPLC 
1000, maybe 0.01ppm.
(mains frequency for NPLC determination normally is measured 
automatically in the 3458A)


For noise figures, see DCV datasheet of 3458A, or have a look at real 
measurements in the legendary LTZ1000 blog here, should be similar what 
you may find with the 732A:


http://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/180/
http://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/270/

The 732A may also be less noisy, if you measure its outputs running on 
batteries only, and disconnect the mains.



There is no reasonable explanation for the observed drift of the 1V and 
1.018V outputs.


These voltages are derived by simple resistive dividers, and any leakage 
currents should influence the measurement instantaneously and 
statically. And both outputs should behave exactly the same, looking at 
the schematic.


If the 10V range of the 3458A is really stable, i.e. 1µV noise at 10V 
from the 732A, then simply measure the 732As 1V outputs on this same 10V 
range on the 3458A, which will still resolve 100nV.
Maybe, these 1V will also be stable in this configuration, and then your 
3458A has some trouble with its 1V range.


Then, you may check the guard function, if there's a difference with the 
switch set to open, or to Low, and connecting guard to the 732A or not .

Maybe you have some odd ground loops in your cabling.

Well, maybe thermoelectric voltages may be a problem in the 1V ranges.. 
hope you use good connections.. blank copper cables may be oxydized, for 
example, giving 500µV/°C vs. copper.


Maybe the 1.000V jack is not ok internally? Some degradation or bad 
solder joint somewhere?


Does this drift stabilize somewhere after some time?

Frank

___
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.