Hi Frank,
Have you looked at LT5400? http://www.linear.com/product/LT5400 Networks of 4 resistors, 2ppm / 2k hours long term ratio stability. And not that expensive really.
A unique part as far as I have seen. On paper they look very interesting for making ratios, provided you can get lucky with the values available. But look, you can make the temperature controller divider using R4=(10+1) in series and R5=(10+1) in parallel. = 11k and 0.9090909k, equivalent to 12.1k + 1k. Perfect!
JohnWhat for?
My design goal was < 1ppm/yr., and I achieved that without any schmuck devices or exotic PCB slots.
Use ordinary precision wirewound resistors and solid thermal management. That's it.
Stability is now < 1ppm / 4yrs.
Frank
Hello all volt nuts - has anyone already experience using the LT5400-6 and LT5400-7 as matching resistor network [0.2ppm/K matching temperature drift] ... - one can easily fabric the 12.5k:1k0 and 12k0:1k0 ratios 12k5 = 5k0 + 5k0 + 1k25 + 1k25 or 12k = 5k0 + 5k0 + 1k0 + 1k0. The LT5400-x networks are low cost.
Another idea I got in mind - has anyone already considered to replace the Op Amp controlling the heater element with a low power (galvanic isolated) microcontroller that is executing a PID algorithm ? How close could we get in delta mK - would a sensor based on a thermo-couplebe accurate enough ?
I found that you can use the resistor to fine-tune the temperature coefficient in the LTZ1000 circuit, so start with 400 K and put the circuit in a temperature controlled environment (I used a beer fridge that could heat/cool ) Then you might change it to e.g. 470k to reduce the TC below even 0.05ppm/K.
This will also take care of any concerns about opamp TC.
Hello,
when looking up to my TC-measurements (LTZ1000A) that I did, I cannot see how the TC could be bettered by such a measure. (especially above 20 °C environment temperature).
Most of the TC above 20 degrees is related to some kind of hysteresis. There is no linear relation to the environment temperature which could be compensated in my case. The hysteresis might come either from the temperature gradient, from PCB, the LT1013 in plastic case or some effect on the LTZ1000A.
The best measure would be to heat the whole reference to a constant temperature in my case. But with battery operated device this will not be possible.
The diagrams show output voltage (divided by 2) over ambient temperature of the reference and over the internal temperature sensor near the LTZ1000A on the PCB.
Temperature gradient is 0.1K / minute.
With best regards
Andreas
Hello,
when looking up to my TC-measurements (LTZ1000A) that I did, I cannot see how the TC could be bettered by such a measure. (especially above 20 °C environment temperature).
Most of the TC above 20 degrees is related to some kind of hysteresis. There is no linear relation to the environment temperature which could be compensated in my case. The hysteresis might come either from the temperature gradient, from PCB, the LT1013 in plastic case or some effect on the LTZ1000A.
The best measure would be to heat the whole reference to a constant temperature in my case. But with battery operated device this will not be possible.
The diagrams show output voltage (divided by 2) over ambient temperature of the reference and over the internal temperature sensor near the LTZ1000A on the PCB.
Temperature gradient is 0.1K / minute.
With best regards
Andreas
JD, are you using the LTZ1000 or the LTZ1000A?
What I do not understand: The absolute change in Uref is - 0.08ppm/K only, although it should be -50ppm/K.
Have you really compensated the T.C.?
I plan on using a 16-bit PWM instead of a divider resistor for the temperature control set-point--
If the Zener is making about 7.2V, then a 16-bit PWM should give me about a 0.05-deg-C step for each count-- thus allowing very fine grained control.
I have not looked properly at hysteresis so I may have missed it.
The red line shows the temperature of the LTZ "module"; it is self-regulating to an extent so the excursion is a lot less than the module "ambient" (fridge) temperature it is in. The black line is the 3458a internal temperature which tracks that of the room.
What I am wondering, is which device has less thermal hysteresis-- the LTZ1000, or the LTZ1000A? Does anyone have data on this?
Of course there are the other bits that need to be addressed as well-- I want my 10V reference to have battery backup for an extended period of time (for shipping the device "hot" to/from a cal-lab that has a JJA).... I am leaning towards the CALB 40Ah if I finally decide to go with the Peltier-cooler method...
Yes-- the Peltier system will need to breath outside air in order to function correctly. My plan is to build a special carrier with inlet/outlet vents for airflow from the fan that cools the hot-side of the Peltier device. The carrier will act as the "shipping container".
Hmmmm... That's a good thought-- but are those things allowed on cargo planes-- (I know that some of those you mentioned are not allowed in the cabin of a passenger airliner)?
Maybe the easiest thing to do is restrict the time of year to ship the reference off for calibration to the more temperate seasons-- (avoiding winter and summer-- perhaps in the fall when it is dryer)... This would only happen once a year anyway...
I don't know how radical I need to get in cooling the LTZ1000--- maybe just running it at 25C die temperature (+/- 0.001C) will be sufficient, and further chilling might be wasted because of no further decrease in drift rate. All of that remains to be determined after construction and testing. My plan is for a die temperature of 0C, but that may be "overkill", and if 25C *is* sufficient, then the Peltier device will have much less work to do, and the power needs from the battery will be more reasonable.
I am going to contact them to see if they would give us "Volt-Nuts" a break on calibrations-- (no certificate of compliance, just a "before" and "after" if an adjustment is involved).
.. Well, after 2 weeks of this torture, the readings in the 3458A suddenly dropped 0.25ppm (when it had been solid, with no discernible drift, for the first part of the year). ...
and the 3458A dropped another 0.25ppm! [The 3458A has never recovered-- even though the humidity has been around 8% for months now, it is still low by 0.5ppm-- so I think it was permanently affected]. ...
My Fluke 732B did not suffer at all through this (as far as I can tell-- I don't yet own a JJA)-- the important resistors in the 732B are all hermetically sealed.
I don't know where in the country you are, but Sandia Labs has a Cal-Lab that is humidity controlled (one of the few in the USA), and their uncertainty for Zener reference calibrations is 0.017ppm !!!
I'm reminded of the book "Foundations of Mechanical Accuracy"
I did not think about the hysteresis problem-- is there any way of determining if that is the cause, and if so, is there a way to fix it (put the reference board in the freezer or something)?
I have an idea to fix this calibration cost problem for us "Volt-Nuts". If it turns out to be feasible, I will announce the solution here and on the "Volt-Nuts" list.