[volt-nuts] 3458A Questions - The Sequel

Frank Stellmach frank.stellmach at freenet.de
Sat Oct 29 13:06:27 UTC 2011


congratulations on your instrument!

At this occasion, I opened and photographed my unit, finding out, it is 
younger than assumed, 19th of June 2000, ser. SN 2823A25xxx. Price 2 
years ago was comparable to yours.

Mine has got the same board revisions, especially the processor board:

I simply plugged in the 4x 32k SRAMs, nothing further was necessary, and 
got Opt. 01.

I would not bother agilent with the readout of the Dallas NVRAM, they 
only replace PCBs and they do nothing else (see CLIP manual). You have 
to read out and replace the Dallas parts on your own.

The instability of 1ppm compared to the 731s seems to be too high, but 
you did not mention the timescale, the NPLC parameter, and the 
environmental conditions under which you run the unit and the refs.

I assume the instability is coming mainly from the 731s, which are not 
temp. stabilized.

If the ambient temp. changes by 1°C only, you must perform an ACAL DCV 
on the 3458A to bring it back exactly to its calibration. That drift is 
due to the DAC / range resistors, not the LTZ ref, which will stay constant.

In contrast to DCV, OHM calibration itself will change by 0.5-1ppm per 
Kelvin environmental temperature change!

Using NPLC 100, 8 digits, I get DCV short termstability on the order of 
0.1ppm (sample to sample), and < 1ppm stability over one year, compared 
to two external LTZ1000 and a 5442A.

Both the 3458A and the 5442A are shut off if not used, but the LTZ 
references are running continuously.

The temperature of the reference on the 3458A was modified to run on 
60°C or so, compared to the drifty default 90°C, by adding an 100k ww. 
resistor in parallel to R411, 15k. By calculation, this should reduce 
drift to 1ppm/year for continuous operation (opt. 02 ;-), but with 
reduction of max. ambient temp. to about 30°C.

I finally located the 3458A in our basement, where I have an 
environmental temperature constantly between 20° and 22°C over all 
seasons. The 3458A is intentionally NOT located in a closed, but in an 
open rack, to get better internal air circulation.

The internal temperature rise is much lower that way (to about 37-40°C, 
i.e. 17°C above environmental).
It is also much more constant,  which is extremely important for making 
stable and precise measurements, especially for OHM.
<1ppm OHM transfer stability over hours can be reached, although this is 
not specified.


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