DMMs should be more or less immune to common mode noise.
At least under present circumstances, what I'm seeing doesn't seem to be in complete agreement with that. My 500,000 count DMM does tend to settle if given enough time, but any movement on my part throws it off again.
Let's compare/contrast measurements using BNC cabling versus banana plug cabling:
If I turn off averaging and BW limiting, so as to give the noisest picture on my oscope, here is what my oscope shows both with nothing plugged into it and with a quality 6 inch BNC cable plugged into it (open circuit, so nothing on the other end):
![Beginner level: DIY an accurate picoammeter (measure picoamps!) 1001333-0](https://www.eevblog.com/forum/index.php?action=dlattach;topic=241746.0;attach=1001334;image)
Now, here it is with the same settings, but with DMM probes plugged in instead:
![Beginner level: DIY an accurate picoammeter (measure picoamps!) 1001335-1](https://www.eevblog.com/forum/index.php?action=dlattach;topic=241746.0;attach=1001336;image)
Increasing the voltage/reticule shows that it's more than 100 times worse:
![Beginner level: DIY an accurate picoammeter (measure picoamps!) 1001337-2](https://www.eevblog.com/forum/index.php?action=dlattach;topic=241746.0;attach=1001338;image)
and from the wavelength it becomes clear that it's mains common mode noise that's a strong component of it.
I do think it maybe does make a difference if trying to get single digit picoamp resolution out of the picoammeter. Admittedly, the testing environment can probably be improved yet further, but for now it just is what it is. Less dramatic compare/contrast was done by the tinyCurrent guy. Here is the tinyCurrent noise when using the BNC connector:
![](https://github.com/nfhw/tinycurrent/blob/master/Scope_Shots/output_noise_Coax.png?raw=true)
and here it is using leads:
![](https://github.com/nfhw/tinycurrent/blob/master/Scope_Shots/output_noise_Leads.png?raw=true)
Well, just to be certain, after I get the lunchbox testing enclosure set up and connected to the picoammeter, I'll try doing the picoammeter measurements using a DMM both with regular DMM cabling and also with using coax connected to the DMM via a BNC adapter. I'm pretty sure I'll see some kind of difference, but I'll wait until then before I reach final conclusions.
The very first thing I'm planning to test with the picoammeter is this Vishay load switch:
https://www.vishay.com/docs/66597/sip32431.pdfwhich claims to have a leakage current of just 10pa. My conjecture is that, given the environmental noise, it would be very challenging to accurately measure in the single picoamp digits with just a DMM and DMM probes without the benefit of the new test enclosure and upgraded cabling.