Kleinstein, ap, floobydust, martinr33Thank you for feedback. I've updated output stage as discussed.
PNG-schematics FX Rev.02Also I've dig into stashes and got enough parts to build some crude prototype. Sanity check in LTSpice before we go blowing stuff up:
Few hours later...to my surprise it even worked right off the bed.
Critical resistors used are custom VPG VHD200 (20K/7.663K,
measured <1ppm/K TCR), and trim S102K 330R + PTF 30R.
Rest are random PTF's. KX LTZ output connected to opamp ADA4522-4 input via 3.01K-1uF-3.01K network. Current drawn from reference is <300nA.
Output transistors are NPN+PNP pair
STD830CP40.
Currently I've hooked it to 3458A to log some stability/noise ballpark.
FX output stage with ADA4522-4 and KX channel 6 ref, which is +7.13665V output. Simulation values very well match to real output value.
Live measurement data:
Last year data from Fluke 732B vs another 3458A, 24 hour, NPLC 100 to comparison:
Also output is trimmed (R3 on LTSpice schematics) using S102K 330ohm + PTF56 30 ohm 50ppm/K. Per calculation shift of this resistance network to 100ppm will cause output to change 1.3ppm.
I'm pretty happy with the result, given it has zero shielding, no air drafts protection.
Both LTZ reference and prototype amp powered from Fluke 792A battery pack (+12.5/-12.5V SLA).
This is where this build will eventually go, to make it a transportable 10V standard to use for 3458A/5700A calibrations.