@mawyatt
Regarding getting the PCB 'right', I'm going to try bypassing this issue (maybe replacing it with a different issue) by using this bunch of gold plated banana plugs I just ordered from here:-
https://www.ebay.co.uk/itm/294120874481?hash=item447af861f1:g:zDYAAOSwF8Zgdrrpwhich look ideal for mounting onto a piece of that 3mm thick copper heatsink spreader plate I mentioned.
I removed the other two items from my shopping basket for now (and hopefully for the foreseeable future if my experiment works out), reducing my expenditure by almost 35 quid. At just £4.89 from a UK seller promising delivery by next Monday, it seemed well worth taking a punt.
They can hardly prove worse than my home made 4mm ex-GPO banana plug shorting link's ~3μV performance. If they do prove to be worse, then
we will have
all learnt a valuable lesson about these particular fine looking gold plated banana plugs, won't
we!
Having just retested with my home brewed shorting link, the problem seems to be a small temperature difference between the Hi and LO voltage sockets generating different thermocouple voltages between the shorting link's banana plug tips.
Given enough time to regain thermal equilibrium, the voltage drops down to ~ -900nV. Auto-zero just confuses the issue by turning a -2.5mV into a 2.5mV reading or -8mV into a 3mV reading. Obviously a minor re-calibration at the zero end of the scale is being called for. I get less confusing results if I just use 100PLC without auto-zero.
If I reverse the shorting plug, I see a reading of ~-8mV which starts slowly ... er, no, actually this time it
very slowly increased to -9.5mV! Time to swap it back round again... Gah! my banana plug shorting link now seems to be acting like a 6mV galvanic cell with just a soupcon of Seebeck effect thrown in for seasoning
One thing to keep in mind when testing the SDM3065X is, despite its very low tempco at more sane voltage levels, it (and presumably the 3055 to a lesser extent) doesn't take kindly to sudden room temperature transients.
I've been running thermal stress testing of my temperature stabilised Rubidium oscillator project all day. Starting at 19 deg C for the first couple of hours before turning the radiator on (and a 2.5KW room heater for the first half hour) to zoom the temperature up by 5 or 6 degrees before finally deciding, at 27 deg, that enough was enough by shutting the radiator off and flinging my hobby room door wide open to cool down to a less stressful temperature.
Within just ten minutes, that 9.4μV reading you can see in the attached photos I took shortly before, was dancing within less than a μV either side of the zero mark. Talk about becoming familiar with your test instrument's limitations and quirks!
EDIT: BTW, the object dangling by top left corner of the bench meter is that 3mm thick copper heatsink spreader I'm going to use as a shorting bar (it was actually recovered from the cpu fan I'm using to regulate the LPRO's base plate temperature to within 100mK of 36.1 deg C that can be seen in the next three photos).
For added interest, I've included photos of my Rubidium oscillator project. The ambient temperature readout is about 1 degree low on the actual room temperature. I've had to compensate for about a 2 deg self heating effect which will vanish once the BMP280 module is thermally bonded to a heatsink exposed to the room air in the final build (the thermally isolated over-sized aluminium enclosure I fabricated for the project). At present it doesn't follow room temperature very well but it's enough to compensate for increased cooling rate demands at higher ambient temperatures.
Final note: I reversed the shorting plug and the reading went back up to 9.5μV or so after a few minutes after which I reversed it again and observed it stabilise close to the zero volt mark once again. It seems my shorting plug is not quite conforming to the rules of thermodynamics as I'd naively expected.
Never mind, perhaps that cheap set of goldplated banana plugs with a copper shorting bar will do better.