@ macbeth, thats why now i like to delete my own old posts. but i do leave some relevant ones.
@ TiN, i think 3458a are so well designed, i doubt they have noise problems like keithley. well at least you have real copper hats
. and i bet 3458a dont have AGND lines that measure 0.3ohm per inch
. OTOH, if you keep autocal-ing, doesnt the 3458a work normally? but as it is, i think it does more good for impeding air currents.
today i tried some more tinkering. and i found something interesting. my version of PCB have these things missing (gif 1). i wonder what is the purpose of the JFET sitting in between?
i then scoped that line. ref C254 pin3 U132, vs U166 pin6. 10v range input is open, and input is shorted. amongst the signal, is also what seem to be a bunch of self noise as the opamp self amplifies
there is also the observation of pin6s of U132/166. when i try to read a battery. the yellow signal is suppose to be to A/D? but the noise, and there i spotted like somehow, the autozero interfering with the A/D signal, tiny spikes. this noise coincides with the way the NPLC/AZERO cycles the MUX (see the jpg of Q104)
and i also spotted a somewhat 1 inch trace AGND, and that somehow is 0.32ohms. making some noise to sit onit. this AGND is a new discovery running amongst the A/D MUX, which needs to ref right back to VREF, and here yet another AGND flapping around at around 0.25ohm between MUX and VREF. using analog scope, at 1mV/div, the probing this net turns a solid trace into a fluff, but the actual p-p cannot be seen properly (lousy scope i have)
i am tempted to add some kind of bypass, but it will just wreck the MUX signal for A/D. anybody have anyideas what to do with such kind of signal line? if magically, ferrite beads could appear in the copper line, would that be a possible solution?
the blue trace U132, is suppose to compensate AD711, but at the same time it injects the compensation signal as noise
sometimes i think, why is it so hard for them to put proper AGND plane? it is so weird they did the planes for the 2 digital sections. but did nil for analog. but in any case, im glad i got this unit for cheap
edit**
A/D MUX shortening of AGND return path, test plot. warm up 1.5hrs, 100mV/1NPLC/AZERO=on
plot 1446
3600samples STDEV 125.92nV, p-p 918nV
500samples STDEV 119.25nV, p-p 659nV <--- quite a large drop in p-p noise
plot 1516
3600samples STDEV 122.91nV, p-p 822nV
500samples STDEV 123.25nV, p-p 755V
looks like STDEV is under 130 for good? and p-p noise is under 1uV for good?