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The noise of the ADRs RefAmp evidently is about 50% of the LTZ1000, which several others also have confirmed.
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As my 34465A showed much higher stability / lower noise when I replaced its LM399 reference by a LTZ1000 clone PCB, my idea is to replace the LTZ1000 in my 3458A with an ADR1000 (*).
Maybe its noise figures will also go down, best by a factor of 2.
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I would like to remind that TiN some time ago made a similar experiment on one of his 3458A, by implementing 4 parallel LTZ references, which would also yield 50% noise of a single LTZ.
I'm not aware what his final result was.
Frank
(*) my 3458A already has a modified FW, with CAL? 2,5 = 6.50000V (7V reference lower limit).
Thanks to Poul-Henning Kamp for your advice how to correct the checksum.
Hello Frank,
in your post you mentioned that you modified your 3458a's firmware to accept the lower voltage value of the ADR.
Would you mind sharing the process? I like to try it as well...
BR,
Max
Hello Max,
It was not sufficient to only change the minimum limit for the CAL? 2,5 parameter, i.e. the minimum internal voltage reference, to 6.5000V, but it was also necessary to modify all those lower ACAL limits, which the lower reference voltage also affects.
This can be identified by comparing the auto-calibration constants of a 3458A with two different reference voltages, like a regular LTZ1000, and one with a different LTZ chip, or with an ADR1000 on board, and slightly faked calibration reference values.
TiN kindly helped me by testing my first version on one of his 3458As, and sending me dumps of all his calibration constants. With the ADR1000, he found out, that the ACAL procedure gave system errors, due to several lower limit violations.
So the procedure is to search for the location of these additional, crucial constants CAL? XXX,5 inside the firmware (=> 13 EA), and then lower them by the approximate difference by the ADR1000 voltage, about 6.667V, and the lower limit of the LTZ1000, i.e. CAL? 2,5 = 7.000V, i.e. each new minimum value has be calculated to about -6% each.
The displayed minimum values CAL? XXX,5 are not represented directly in the firmware, instead they are a calculated product of several internal cal parameters, so that's a bit tricky to find them in the hex file.
For that you need an appropriate hex editor, which is also capable of calculating the DOUBLE numbers back and forth, by taking care for the correct endianess and correct byte-order inside the single or 6 fold EPROMs, so that you can now determine the new lower limits.
I've done that successfully on my 2nd firmware version, which I also run in my own 3458A.
TiN was again so kind to test it and he confirmed that now the basic calibration processes, CAL 0, CAL 10 and CAL 10000 are successful, as well as the ACAL ALL procedure.
Next step would be to optimize the ADR1000 reference board for lowest noise performance, maybe also using less noisy OpAmps inside the 3458A, and then check against another external ADR1000 reference, or another 4x averaged zener reference, if the 3458A then shows a lower noise performance as well.
I don't know, how much further hints or spoilers you want to receive, just let me know.
Frank