The ware for November 2022 is a Keithley 2110-240. I’ll give Rodrigo F. the win, but I’m curious how he knew it was the -240 version; I did not expect someone to discern the line voltage rating from the photos!
Also, thank you Ian Mason for the lucid explanation of the exposed traces near key signals. Here’s his quoted answer, so you don’t have to look it up in the comment thread:
The reason for stripping resist from over guard rings [is] to ensure that any leakage paths come into electrical contact with the guard ring. If you had, say, a bit of flux residue as a leakage path, if it passed between two pins but over the solder mask then the guard ring would be insulated from it and would have no effect. The whole point of a guard ring is that it’s a (relatively) low impedance path either to ground or to a duplicate of the measured signal – being insulated behind soldermask is anything but low impedance.
It’s tricks like these they never teach you in school. I’m guessing it was a hard-learned lesson for the persons who had to figure out that trick on their own. Thanks for sharing the knowledge!
Edit: I forgot to note that Rodrigo F. should email me to claim the prize!
From memory, I learned that about guard rings from an old instrument manual from the 70s or 80s when they bothered to put a proper “Principles of Operation” section into the documentation. Older manuals from instrument makers, HP in particular, are a goldmine for learning the little bits of theory and practice that are sadly rarely taught.
There is a literature on this stuff. Before the internet, we all kept our own libraries of it. One book which describes guarding is titled Low Level Measurements, published in 1984 by – wait for it – Keithley Instruments, Inc.
Lots of practical advice was described in trade journals by the analog gurus of the day, especially Bob Pease and Jim Williams. I would tear the articles out and keep them. Some of that (EDN) is archived online.
This specific point (no solder mask on guard rings) also appears in the application notes for precision, low bias current op amps made by ADI, LTI, Burr-Brown, National Semi (those two now part of TI of course).
I forgot to add to this post — Rodrigo F., email me for your prize!
First I saw that it was the Keithley 2110 multimeter. Then I saw that it was the option without GPIB (the connector is not mounted) and I went after a model. The search showed 2110-240 and it was my (lucky) guess.