Canned oscillator as clock jitter was effecting the accuracy, but can be swapped out, been building that area to make it fairly painless to alter to a well built crystal.
Micro can be SMD, have the footprint for the TQFP footprint off screen on the top left, but wanted to keep it as close to the original schematic as possible to start with. Also have an EEZ BBM3 template off to the left, but all things in good time.
Giant snake traces is to keep both heat away from the reference area, which does not nicely fit next to the crystal as it would leave it too near the power supply area. and keep digital from crossing the analog stuff, the input mux is switched between conversions so no noise on these traces should couple.
Needing to fit the input amplifier i the middle right part due to me using these constraints is another part for the snaking.
Playing with replacing the UART opto's with a digital isolator, ends up with roughly the same power dissipation, but should allow much faster transmission rates, so hopefully free up a few cycles for potentially setting the DAC or other stuff. at present I cannot keep it isolated and fit in where I was hoping, will keep playing to see if I can fit it, but for now surrendered and stuck it in my second choice.
Unless anyone can suggest much more, all that is left right now is that input amplifier. and the possible placement of J400/401, but for now there should be enough test pads in the area for someone to bodge in there higher end reference.
For the LTZ1000 reference, its output is ~7.1 compared to ~7.0, even doubled this is still within the control range of Q7, which should be good up until ~14.5V, is there any reason why we would need the Jfet in this case.?
Also as a secondary point, everything on the board so far can run at +-18V, so there is headroom if it is really needed.
For the option of an external 10V reference, thinking I will just break out test points for the supplies to Q7/Q3 in the reference, so if anyone wants to, they can drive them with a higher bodged on voltage otherwise just fitted with a jumper.
And double checking pays, LTC2057 is mainly available in an MSOP package, so that should make it easier to pack together the input amplifier.