Hi MisterDiodes,
there were four people in the LTZ thread, who virtually did the same measurement, like you, I assume, i.e. modifying these both resistors by some percentage, and measuring the LTZ1000 output deviation.. this varied from 74 (Andreas) to 105 (somebody on bbs38hot).
So 74 was worst case.
I just mentioned that, because even using these worst case T.C. numbers, the effect and the optimizing of the resistors T.C. are of 2nd order importance only ..
The residual T.C. will always be below 0.05ppm/k, and may be disappointingly too high, if you set your money on expensive ultra - low - T.C. Vishays, or T.C. matched sets.
The LTZ itself, i.e. up to now undisclosed / not yet explained effects, will play an important role, so a T.C. trimming to << 0.05ppm/K might be accomplished otherwise, e.g. by this T.C. compensating resistor
What interests me, in this context, how did you characterize the T.C. of your LTZ1000 devices, as most of the gear accessible to us amateurs has much higher T.C.?
Do you know a possibility to accomplish that, say trimming to 0.01ppm/K , w/o the aid of a JJA standard?
Frank
Hello Dr. Frank - I want to say first off I appreciate your thoughtful comments.
Not to be shy, but I spoke to my client about what they want revealed about their testing process, and I have to decline an -exact- description of equipment used. What I can describe is a "Black Box" test jig (capable of measuring dozens of test voltages by various methods) built by the facility, and literally 10's of millions of dollars$$ in calibration gear. Let's leave it that lots of 732b's that are regularly measured against the facility JJ-array, along with DMM's the likes of 3458a's and such. The typical uncertainty at this test jig we used is about 0.3ppm or better, and we are > 95% confident we can accurately measure a +-1ppm voltage shift over a several day time span.
The test we performed on LTZs had a basic 13k over 1k resistor ratio, and we had several resistors that could be added in series to the 13k ranging from 0ohm to 1.40ohm via jumper block. The basic 0-ohm dummy resistor was added to 13k to get a baseline reading, then add in 1.20 Ohm resistor, wait 48~ 72 hrs, look at Vout change. Everything else being equal. Then go back to 0 ohm resistor, wait, then add in 1.25 Ohm resistor to 13k and wait 48 hours again. And so on.
In our tests the 10ea. LTZ performed within all datasheet spec for heater resistor sensitivity of an attenuation of slightly over of a factor of 100 to see a 1ppm change in output voltage by this method. IT SHOULD BE NOTED though that the heater ratio stability is greatly influenced by the heat flow into and out of the LTZ device. You can get different results based on A) A particular LTZ and its age / stress level and B) How thermally insulated the LTZ is (or not) to ambient. HINT: You can over-insulate an LTZ to the point where the heater circuit does not function well, and that's just as bad as drafty or no insulation. This effect can show up as what looks like an unstable heater resistor ratio problem.
Remember, the LTZ is a power out vs power in device. You will get a stable Vout only when the circuit has good, solid thermal stability and the heater circuit can servo to setpoint temp correctly.
RE: Add TC compensation resistor. We Looked at this once, but here's the caveat to chasing 0ppm TC, especially on LTZ1000A - You can do that, but we found in long run that was never profitable to try to characterize the LTZ circuit over time (and over several units) and adjust that TC resistor individually for each LTZ. The real trouble comes 5 years later when you realize the die has stress-relieved itself, and that TC resistor that worked originally now really isn't doing the job.
If you're playing with just a one-off LTZ circuit on a well-aged LTZ die, this might be fun to try for entertainment - but at least in my limited experience the correct TC compensation resistor value doesn't last forever.
We found other ways to compensate for slight TC adjustment if our circuit is running warmer or cooler than anticipated - and only if required by customer specs. Normally we are after a very -quiet- voltage source for the equipment we are building. Yearly drift rate or extreme low TC over say a few 10's degrees C is not the primary requirement. Normally the datasheet-spec TC of an LTZ on a well-built circuit board and enclosure will do just fine as-is. We use only LTZ1000a units.
The advice to hobbyist: The more tests the merrier. 3458a's (and similar DMM's) are great instruments for getting an initial approximation, but if you collect 732b's and 7-decade KVD's and even null meters you can get even better approximations - especially when you run critical tests on battery power. Or at least get rid of all local switcher noise. If you are chasing PPM-level absolute values traceable to NIST or whatever government lab you like, keeping your equipment calibrated often is the best approach - you generally want a Z540- type calibration so you can see what your actual equipment measures at - A typical regular low-cost calibration will just tell you if your equipment is within spec or not. Again: Low-noise Battery powered tests are your best friend, and yes on the production line tests I see these methods are used as well on very critical test points.
My other suggestion: Keep track of your 732b calibrations, and keep track of how well the predicted drift matches the actual Z540 cal information when you get a freshly cal'd 732b back from Fluke. That measurement data over time will really help that 732b become more valuable as it gets more stable over time.