while spending some time getting used to a used K2015 THD bench DMM, i got the chance to compare some readings with this post
https://www.eevblog.com/forum/testgear/dmm-adc-noise-comparison-testing-project/msg763062/#msg763062(thanks to Alex and TiN). i then discovered an odd noise problem with my used K2015
under 0.1v range, 10NPLC and 1 NPLC seems fine (i even added in a 5 NPLC)
01 NPLC, 19999samples set 1, sampling 5Hz, autozero=1, filter=0,REL=0 [mean = 0.003uV, SD = 0.488]
01 NPLC, 19999samples set 2, sampling 5Hz, autozero=1, filter=0,REL=0 [mean = 0.019uV, SD = 0.500]
01 NPLC, 19999samples set 3, sampling 5Hz, autozero=1, filter=0,REL=0 [mean = 0.030uV, SD = 0.492]
05 NPLC, 19999samples set 4, sampling 2.5Hz, autozero=1, filter=0,REL=0 [mean = 0.034uV, SD = 0.309]
10 NPLC, 19999samples set 5, sampling 1.5Hz, autozero=1, filter=0,REL=0 [mean = -0.065uV, SD = 0.211]
10 NPLC, 19999samples set 6, sampling 1.5Hz, autozero=1, filter=0,REL=0 [mean = -0.086uV, SD = 0.232]
10 NPLC, 19999samples s14, sampling 1.33Hz, autozero=1, filter=0,REL=0 [mean = 0.017uV, SD = 0.236uV] 1v
10 NPLC, 19999samples s15, sampling 1.33Hz, autozero=1, filter=0,REL=0 [mean = 0.011uV, SD = 0.796uV] 10v
1 NPLC, 19999samples s16, sampling 8Hz, autozero=1, filter=0,REL=0 [mean = -1747uV, SD = 61uV] 100v1 NPLC, 19999samples s17, sampling 8Hz, autozero=1, filter=0,REL=0 [mean = -1665uV, SD = 197uV] 1000vwhen i hit 0.1 NPLC, my noise readings are way way off
0.1 NPLC, 19999samples set 7, sampling 10Hz, autozero=1, filter=0,REL=0 [mean = 1.279uV, SD = 155.668uV !!!!
]
before this is a recapping run, all caps are replaced with minimum 105
oC [5k hrs+++] spec caps, and all have been oversized by at least 1.5x-3x. before the above sets of logs, there are over 50 other logs to track the noise post capping. and so far, as it seems, the mean average is getting lower and lower (the logs are taken in reverse set 7 to set 1, when it all started yesterday, the mean was over -0.5uV, the NPLC 0.1/NPLC0.01 are all gone ape in readings).
curiosity 1) with new caps in place, i have never reached near zero mean before. is my guess correct that larger bypass caps do reduce measurement noise performance?
curiosity 2) when these DMM are switching amongst NPLC modes, does the multislope circuit increase in samplings (and error)? i will try to poke a scope in there, but i have no idea how it works. except for this article
http://www.google.com.sg/patents/US5321403 ... only barely grasping some bits)
curiosity 3) if i look at the K2000 schematic page 7 on multislope, i am guessing the logic to the left are the different multislope section, goes thru filter around C179, and i am guessing C171 is the key of the entire multislope? the original K2000 schematic is from some chinese guy
curiosity 4) by looking at the noise charted (see pic, Y scale is in uV, yep uV of noise
? this log is 3998 samples, 0.1NPLC, mean = +13.6uV, SD = 166uV ! ouch !), it is as if 1 of the multislope logic to the left is "broken" or injecting noise? i dont suppose anyone have seen such a problem before? in NPLC modes 1 or more, they are all "quiet". from the pic, there seem to be some kind of saturation point @ 250uV. (when i have more time, i will scope this bit of noise)
does anyone have tips on what else could lead to a extremely noisy low NPLC performance?
all input appreciated
(edit : added NPLC 10 1v/10v/100v)