[volt-nuts] Two 4338B high resistance meters fail on the same range - Keysight can't adjust EEPROM of either.

[Dr. David Kirkby]

On 2 May 2018 at 10:37, David C. Partridge <david.partridge at perdrix.co.uk>
wrote:

> My bet is that they've lost the "secret sauce" for updating the EEPROM :(
> So it's measuring as best it can with no calibration adjustments stored.
>
> Dave
>

Dave,

Your comment got me to check something, which reveals something VERY
interesting, and is perhaps the source of the problem! I owe you a beer!

I initially assumed the calibration procedure required some software to
check the performance of the 4339B, and if out of specificaton update the
EEPROM. But upon reading the user and service manuals, I find that's NOT
the case.

The user manual describes how to check the performance, which requires no
software. The service manual describes how to adjust the instrument if the
performance is not right. The adjustment which needs software.

Furthermore, it seems to me the procedure used by Keysight to verify the
performance maybe wrong, although I am not going as far to say it is, as
maybe they are doing something that's not obvious from the calibration
certificate. Looking in the user manual, it would appear one is supposed to
verify the performance on the 10 nA range by setting the output voltage of
the 4339B to 1 V, measure the voltage on a 3458A, set a resistance box to
10^8 ohms and calculate the current, which should not be assumed to be 10
nA unless the 3458A indicated 1.00000000 V, I have not checked the
specification of the 4339B when generating 1 V, but when set to 0 V, it
should output 0 +/- 0.1 V and when set to 10 V it should output 10 +/- 0.12
V. So at low voltages, this is not an accurate voltage source. At higher
voltages it is a lot more accurate. This reflects my experience when using
it to measure close tolerance resistors - measuring them at low voltages
gives poor results, but at higher voltage, the resistances are measured
more accurately.

I am wondering if Keysight are assuming the 10 nA is generated when the
output voltage is set to 1.0 V.

If out of specification, the EEPROM should be updated with software. I'm
assuming that software is written to compute current based on what voltage
the 3458A measures, rather than what voltage the 4339B is set to output.

To answer Illya's questions, the EEPROM is a Xicor X28C64P-20. I can't find
any reputable supplier with those - plenty on eBay, but I am always
concerned about counterfeits on there. Someone suggested a Mouser P/N.
AT28C64B-15PU would probably do, but I decided to buy the EEPROM directly
from Keysight (P/N 1818-4808). I'm glad I did, as it is removed any doubt
about the suitability of the EEPROM. I'm sure if I had bought the EEPROM
from Mouser or eBay, Keysight would have suspected an incompatibility of
the EEPROM.

This has certainty got me thinking about what might be happening, although
the fact Keysight have said they can't update the EEPROM, suggests they
have the software to do it.

Dave