Over the past year, I have been building a battery powered 10v reference based on TiN's KX-LTZ1000 board. This was not really a project out of need, but just a project I wanted to build and experiment with. I ended up building three of the KX boards, the first one I had accidentally revered the polarity from the 15v power supply feeding it. The power supply was not current limited and spiked 20 amps as the input filtering 47uF tantalum caps smoked. I replaced all three caps and two current limiting resistors and it came back to life, but the voltage shifted a bit so I was skeptical (after 6 months of testing turns out this board is rock solid). Then I built a second KX board, this one looked very promising but I was seeing a voltage jump of a few PPM randomly, I fought this for a month and never did figure out what the deal was, I suspect its the LTZ1000 chip itself. Since I had all the parts to build a third, minus another LTZ1000, I decided to try a third time. Third time was the charm! Unfortunately I had calculated and spec'ed a Vishay VHD200 for the 7->10v buffer based on a slightly different voltage, but I was able to compensate in the trimming section.
The KX ref and buffer board would be placed in an oven held at 35C, to stabilize the buffer resistors mostly. Rather than post duplicate photos and info here are some links to the build already posted.
Initial KX build;
https://www.eevblog.com/forum/metrology/kx-reference/msg1230341/#msg1230341Experimenting with draft shields on the LTZ1000 can and leads;
https://www.eevblog.com/forum/metrology/kx-reference/msg1230535/#msg1230535The oven build begins (oven controller schematic attached here);
https://www.eevblog.com/forum/metrology/kx-reference/msg1236890/#msg1236890Planning and building the portable enclosure;
https://www.eevblog.com/forum/metrology/kx-reference/msg1244824/#msg1244824Completed enclosure and external power supply;
https://www.eevblog.com/forum/metrology/kx-reference/msg1263564/#msg1263564Temperature Coefficient concerns;
https://www.eevblog.com/forum/metrology/kx-reference/msg1301155/#msg1301155KX #2 build (possible jumpy chip);
https://www.eevblog.com/forum/metrology/kx-reference/msg1317082/#msg1317082Temperature Coefficient testing and resolution + reasoning to buy a new 7.5 digit DMM;
https://www.eevblog.com/forum/metrology/kx-reference/msg1369941/#msg13699417v reference noise testing;
https://www.eevblog.com/forum/metrology/kx-reference/msg1402418/#msg1402418Building a worthy 10v buffer (schematic included on post #1);
https://www.eevblog.com/forum/metrology/ltz1000-10v-buffer/msg1435737/#msg1435737All of this has taken a bit longer than a year to get to this point. Its built, I'm satisfied with the performance and now its time to make use of it. Late last year I contacted a fellow forum member, Ryan (CalMachine), who also was building some KX references along with his own design who also happened to have access to traceable standards at his work. The conversations started off just as wanting a second pair of eyes to verify my results but being a voltnut he went far beyond my expectations.
With fully charged batteries my reference can run on battery power for 7 days powering the KX and Buffer boards or 72 hours with the oven on (at normal room temp). The idea was to ship the reference to Ryan in Indiana with the reference powered on (oven off) so the LTZ1000 would not be subject to extreme cold temp swings during transport this time of the year. The oven is insulated well enough there is a 5C rise in internal temp from ambient from the KX board and buffer being powered alone.
Final test prior to shipment;
https://www.eevblog.com/forum/metrology/usa-calibration-club/msg1470038/#msg1470038Ryan went out of his way, comparing my reference to no less than three HP 3458's, two recently calibrated units and one with option 002. He also spent some time recording some data with the oven off, on and during warming. After a few days he made a small adjustment to my 10v trimmer and netted the 10v within 0.2ppm from his 732B which is keep in agreement with his work standards. After about two weeks, the reference was sent back powered on, there was some delay in shipping as it arrived exactly 7 days after it was sent. It was still running on battery power which had 10% capacity left.
Knowing the voltage as I sent the reference and knowing what Ryan measured on his calibrated equipment and knowing the amount of change he adjusted I should be able to calculate what my new readings would be if everything remained stable during the return trip. After letting it stabilize in my lab overnight here is a quick summery;
As measured by my Keithley DMM7510 prior shipping;
10.000000
As measured at ENI Labs;
9.9999637 - 3458 option 002, cal'ed at Fluke Std Labs 5 moths ago
Dif;
0.0000363 (3.6 ppm low)
Ryan adjusted the buffer trim upward by 37.7uV to the reference now measured;
10.0000014 (0.14ppm high)
I should now “theoretically” measure;
10.000038
My real measurement now is;
10.000039
I love it when the math checks out, this gives me a very high level of confidence the reference remained stable during its transport back and I now have a very good realization of 10v.
At ENI Labs (using my supplied test leads for consistency);
Back on my DMM7510, new as of 8 months ago;
And on my 34465A (calibrated by Keysight 6 months);
Ryan sent along some data, Ill try to put some of this together in my next post
Attached are some general notes I put together regarding the build and operation.