Hello,
LTZ1000A + LT1013 are on their way. Several of them.
Meanwhile I read and read, and re-read again the datasheet as a kid waiting for Santa.
My main problem now is the resistor set.
I asked a quote to Mr Pettis but first I want to understand exactly (well...) what I'm doing. Because budget is not infinite... and I could privilege building "many" references over "ideal" ones.
I understand that R1 (120) is very critical as it sets the current through the zener. I guess it has been chosen very near the sweet spot. I understand that changes in this resistor (either due to temp or long term drift) would obviously change it's voltage drop and thus affect the output voltage... So for this resistor, the best tempco, humidity coefficient, long term drift, etc. that one can get will pay in the end. So far so good (but see below).
Then there are R4 and R5. They set the operating temperature. For this the datasheet indicates to use the lowest temperature consistent with the environment, adding 10°C for LTZ1000A. "Consistent" here means (if I understand correctly) take a margin for the ±10°C variation due to production. So all this implies making it work at
Ta_max + 10°C [production variability worst case] + 5°C [extra safety] + 10°C [if LTZ1000A]
From this information and the note below the circuit, one might conclude that the exact ratio is not *that* important, provided the ratio is as stable as possible. So if I had 1% resistors with exactly the same tempco (and other drifts) [Of course I understand that probably no such thing exists] , is better than 0.01% resistors with very different tempcos.
Then there are the two 70k resistors (R2, R3). Their drifts impact 300 (500 for R3) times less. So drift for those does not look like a super critical parameter. What I don't know is how critical is their nominal value (as 70k is a rather hard to find value).
So to resume, if I had not an infinite wallet to bet:
1) Put as much as possible in R1
2) Make sure R4 and R5 are the same kind, their values must respect the ratio giving the desired temperature, and selected for same "drift due to the elements". . This can be tricky, so better put some money in those too.
3) if money is left, bet on R2 and R3. Else just put some cost reasonable metal film, "about 70k" each.
So, is this a question? I hope so, and not a flame war. To summarize, and avoid unnecessary casualties, I think what I'm asking is
1) The rationale of resistor selection when in a budget is correct? (in a limited budget)
2) What is all this 13k stuff anyway? I'm aware that there are discussions about the 13k value, for example, discussions whose origin I don't know, but apparently it's not the datasheet?
3) is 70k important. Would 50k do? and 72k?
4) bonus track: why HP used 111 for R1 and 15k/1k for R4/R5
My guess is they changed the operating temperature with R4/R5 and then selected a new sweet spot with R1, but most people here knows better for sure...