and yes ... i made some more improvements again. and again, in a area which should not seem to influence final noise. but it did
as with the previous looky looky poking around, i had some thoughts that the zener (VR107/108) was a trouble maker. i had some spare zener (panasonic DZ2J series, 4.7v. SOD323, approx Tja = 250
oC). the est current thru the zener is about 4-5mA, power dissipation is approx 0.025w, = Trise of abt 6C
based on the last 3600-7200 samples of plot 2213 (this is a 2Hz plot 7.2k samples), we are looking at 1NPLC plot (yes the super noisy ones), we have a STDEV = 0.392nV (with stock SOT23 zener, STDEV was average 0.45-0.5uV BUT for a flat 3600 sample plot). i thought shit, 7 year old zener, must be noisy as crap!
and the MAX-MIN p-p noise is now 2.9uV as opposed to average over 3.4uV
these cheap SOT23 zeners are suppose to provide 5v to the LTC1050 zero buffer opamp, they are changed now to 9.3v using 2x 4.7v.
come to think of it, there are 2 more 6v SOT23 zener at the LM399, they are suppose to be giving +/- 14v VREF. what the heck !?!?
so maybe i should get some 5v VREF zener (LM336?LM329?) and further enhance the zero buffer. but what should i replace the 6.2v with? there arent any VREF 6.2v zeners.
... 1n829 ?
( if the noise of the 1NPLC is reduced more, it will look like a 10NPLC plot, when i first logged w/o any mods. it would be hilarious if NPLC1 has same noise profile as NPLC10)
*** i guess i was tired, and was poking at the DMM and shorted something. now its not happy with me
... mod -> becomes repair