Banana Jack to Coaxial 4-Wire Kelvin Clip Adapter Board

[figurativelythedevil]

Hey y'all, I just finished assembling the first prototype of an idea I've had bopping around in my head for a while. It's probably overkill, but I had fun making it. Recently I've seen folks talking about 4-Wire measurements and the best way to do them, which sent me down a rabbit hole looking at various styles of "kelvin clips", many with coaxial cabling for each input. I really like this style, but I didn't really like the way I saw it being implemented; usually by people stripping back the coax, twisting the braid, and soldering it to a board next to the center conductor. It works perfectly fine that way of course, but if you want to use those clips anywhere else it's not really viable. So I designed my own board with four BNC PCB connectors to connect to the individual force & sense leads and tie all the shields together, without having to butcher the BNC leads that come on those kelvin clips, so if you want to use them on other gear, you can.

At first, I was hoping to make it all on one board, but the size of the BNC connectors I wanted to use, this didn't really work out. I was also a bit concerned with board flex when using one board, so I settled on two stacked boards. To connect the boards, I found some nice gold plated brass 4-Pin .1" headers from Mill-Max that were the perfect height for the 5mm board clearance I wanted. The board is really simple, both are four layer boards so I can keep force & sense on different layers and maintain shielding all the way up to the banana jack, with vias around the periphery to tie the ground fill on every layer together (this is mostly for looks though, as the ground layers are tied together well enough from 24 plated through holes used by the BNC connectors, pin headers, and the mounting holes). I used KiCad for the board layout, and I had to make my own footprints for everything which luckily seems to have turned out perfectly.

If y'all are interested, I'll upload all the files to github so you can build one yourself, but be wary, it's a bit of a pain to solder, so you'll definitely want to get your hot plates or pre-heaters ready. I'd also really like to order a blue mask version with ENIG and some graphics maybe, so if y'all are really interested, I'd be open to running a small group-buy to offset the fanciness cost.

Photos below, but for the high res ones, here's a link to an imgur album I made, which also has some more commentary! Thanks for reading!
https://imgur.com/gallery/B6nNp7m

EDIT 22/09/2021: Created a public github repository! Includes KiCad project files, gerbers, and bill of materials.
https://github.com/figurativelythedevil/Banana-Jack-to-Coax-Adapter

[Electro Fan]

Very nice project!
Your adapter expands the possibilities since some kelvin clip leads come ready made with BNC connectors. 
Nice work and thanks for posting!
- please keep us posted on your possibilities for building/selling some more

[JohanH]

This looks really nice. Github would be appreciated. Maybe soldering would be easier with these kind of connectors?

[JohanH]

Are you connecting the shield to anything when measuring? Modern meters with banana plugs don't have their chassis ground easily exposed to connect to.

To make better use of the shields, maybe this type of cable with a separate alligator clip for the shield would make sense? But these are horribly expensive (just an example I found).
https://www.amazon.com/Precision-TL-889A-Kelvin-Clips/dp/B005T89N0Q/ref=sr_1_4

Maybe add a separate cable and clip to one of the screws on the board. Easy to connect it anywhere and try if it matters for noise (ground of circuit being measured or actual safety ground).

[bdunham7]

What is the part number of the Pomona swage banana plugs? 

[figurativelythedevil]

This looks really nice. Github would be appreciated. Maybe soldering would be easier with these kind of connectors?

Probably! I don't have a super specific reason for using the ones I did, I just like the quality of the Pomona ones. Nickel plated brass body, PTFE dielectric, gold plated brass center conductor. 👌

And github is coming! I'll get it all uploaded tonight and post the link in this thread.

Are you connecting the shield to anything when measuring? Modern meters with banana plugs don't have their chassis ground easily exposed to connect to.

To make better use of the shields, maybe this type of cable with a separate alligator clip for the shield would make sense? But these are horribly expensive (just an example I found).
https://www.amazon.com/Precision-TL-889A-Kelvin-Clips/dp/B005T89N0Q/ref=sr_1_4

Maybe add a separate cable and clip to one of the screws on the board. Easy to connect it anywhere and try if it matters for noise (ground of circuit being measured or actual safety ground).

I plan on experimenting with attaching the shield to my multimeter ground, and maybe guard, but I'm not sure yet. Initially I had planned on adding another hole on the board to attach a ground/guard lead to so one could plug it in wherever, but I ended up deciding against it for the time being. I figured that those who had multimeters with guard terminals would most likely have more extreme setups with serious test fixtures, or be using meters already equipped with triaxial inputs. As it stands it would be pretty simple to tap off the shield right now as the M3 screws on the corners are all in plated through holes connected to the shield planes, so I could pretty easily run a wire around to the back where my meter has an exposed grounding point, but I'd be concerned running a wire that distance would itself induce noise. I'm really unsure lol. I've also played around with the idea of adding another binding post to the front connected internally to ground/guard, but I'm hesitant to start drillin holes on my meter lol.

If you (or anyone else) wants a set of boards, I'd be willing to send them out. I won't include BNC jacks or banana plugs as they're not cheap, but I have more than enough mounting hardware to throw in an envelope with the boards. I have three boards that I can send out, and I'd love to send them to some folks who would find them useful or at least have a set-up where some more serious experiments can be done with them to see if the project is worth perfecting or not. I'll also be uploading everything to github for those who want to tweak or order their own boards.

What is the part number of the Pomona swage banana plugs? 

3257! I was hoping they would have a gold plated BeCu option, but sadly not. Nickel plating is fine I guess.

[figurativelythedevil]

Created a github repository for this project! All the KiCad project files are there, as well as the gerbers and bom
https://github.com/figurativelythedevil/Banana-Jack-to-Coax-Adapter