Please search for yourself, this time
I expected this type of comment - it was inevitable...
Sorry, den Spruch konnte ich mir diesmal nicht verkneifen.
Regarding the non-A version: Would not the better thermal coupling of the non-A lead to longer stabilization times and reduce the immunity to external temperature deviations? Or is the temperature control (an I controller as I understand it?) good enough so it does not matter either way? Combined with your foam isolation, would that not result in a much longer stabilization time?
The LTZ1000A stabilizes its oven temperature within about 3 sec, the LTZ1000 requires about 30sec. I provided an overview table for all of my different references in this thread.. I don't repeat that sentence again..
Thermal stabilization of the complete circuit, i.e. including the 5 precision resistors is virtually identical between A and and non-A version, i.e. within 1h.
Guard: Interesting, first time I see this. It seems that precision DMMs (my 6.5 digit does not have a guard terminal) interpret "Guard" differently than in high impedance (e.g. pH probe) measurements: With those, you drive exactly one signal line guard to the level you measure it to be. In DMMs the guard seems to enclose the whole input section, not just one signal. That probably makes sense as they have an excellent isolation to PE (earth) and their guard can be driven to some convenient potential (e.g. LO input).
Well, higher grade instruments have exactly the same guarding scheme, look how the HP3458A, Fluke 335, 720A, 752A are built.
It's not that complicated, and as it's a passive Guard in each case, this might not be as effective.
I see an improvement in noise immunity, anyhow.
I just upgraded my "cold" travelling reference LTZ #3, which has an LTZ1000 inside.
I tested it's warm-up behavior after a 56h off-time @ 21°C room temperature. It returns to its initial value to +/- 0.1 ppm within 1h, see first diagram.
In the longterm stability diagram, you can see two hysteresis shifts due to low temperature storage (<0°C).
At the end, there was a temporary shift of + 0.3ppm directly after assembling LTZ#3 into its new outer case. After one week, its reference value returned to its recent one within 0.1ppm, as well after the 56h off time.
So it's ready for a comparison @ Adrian.
I have to say I am a bit demotivated by the lowish 10 ppm uncertainty I could get from my local cal lab. After all, setting up several LTZ1000 boards is quite a (expensive) effort. On the other hand, simply switching on my DMM6500 (still in cal) will give me a 32 ppm uncertainty reading a 7.2 V source.
- Martin
At first, build your LTZ reference. Then you might either ship it to Adrian for a volt-nuts level calibration, <1ppm "unofficial" uncertainty, or you might join our Cal-Club and make an exchange with either of us. It's a rather simple procedure, I guess, although you're living in Switzerland. You need an anchor reference in your lab, to identify possible shifts during transport. That might be your DMM in first place. Another LM399 reference would be better.