Hi all,
having read through a good part of this thread, I'd like to build a few (1-3, depending on cost) LTZ1000 references as well. I don't usually do precision stuff, best I have right now is a recent DMM6500 6.5 digit bench DMM.
What I'd like to do is
- build 1-3 units (depends on resistor cost)
- let them run a few 100 hours
- bring them to the local cal lab for a precise measurement (and repeat that say every year cost permitting).
I have a few questions! It seems that the peak of this activity was the massive group buy
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg1837169/#msg1837169 of branadic's board in September 2018. I hope there's still someone around...
- Are the design files of this group buy (schematic/PCB in eagle for KiCad format) somewhere available?
- How useful in real life is the 10 V output (besides aesthetics and the extra callenge)? It seems this adds complexity, extra error sources (amplifier) and potentially lots of extra drift (boost divider).
Let's say I want to measure an unknown 5 V reference. I can either hook up the LTZ1000 7 V to the DMM6500 first to see where it stands or I can connect both unknown 5 V and LTZ 7 V to the DMM in ratio mode (not sure what's better).
- How important is an output buffer to the zener voltage? I'd connect mostly DMM inputs (10M) to the output and also the cal lab probably will do that. A buffer adds error sources and cost. AN86
https://www.analog.com/media/en/technical-documentation/application-notes/an86f.pdf p. 46 lists an LTC1150/LT1010 as possible buffer, but I don't think I need the large current drive and would feel better without chopper artifacts.
- What power supply scheme are you using? The branadic circuit has an LT1763 ultra low noise LDO on board, generating +12 V. As the supply would run continously, an abandoned linear lab supply is probably excessive. Switch mode wall wart too noisy. So a small mains transformer/rectifier might be OK (12 V rectified giving some 18 V)? Or pre-regulate to maybe 14 V to reduce power in the LTZ box?
- What about ground, earth and shielding? Guarding seems not necessary as everything is low impedance. A mains supply with unshielded transformer would AC couple the mains voltage to the output, so a 1 nF...1 uF capacitor from supply GND to Earth (PE) would make sense. This also rules out 2 pin supplies, as they don't have a PE connector. What about the LTZ1000 box itself? Connect to PE (how)? Connect (via capacitor) to supply GND? Anodized aluminium is a pretty good isolator, did anyone bother trying to properly contact the anodized housing parts?
- Precision resistors: I asked Edwin and he might be able to supply a set. Apart from that, are there any realistic sources? 70 k and 12.5 k seem pretty exotic values. I think this is the main killer for the entire build.
Thanks! Martin