Wire/Cable suggestion for mV level signals - Page 1
Forum section: electronics / projects

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 02, 2025, 04:53:18 pm
content: Looking for suggestions for the following:I need to measure DC signals in the range of ~10mV or so, from inside a thermal chamber to an external 34461A fixed to 100mV range, located about 3 feet away.Even using 100 NPLC's, I am getting 100-300uV of noise which manifests as spikes in the curves I derive, which are basically Rds(on) measurement vs. Vgs, and also Rds(on) vs. Temperature -- standard stuff of datasheets, right? Right now the measurement setup is basically a bunch of nonshielded grabber leads that plug into the banana input jacks of our meter, wires flying around everywhere.I need to make this setup neat and as noise-free as possible. I think the best way to do this is with a shielded, twisted pair going to the meter. I looked at twinax connectors and cables, but literally the only PCB mount twinax connectors in stock I can find anywhere are from Samtec, are proprietary, and Samtec requires you to buy custom built cables that interface with this twinax connector ( P/N CJT-T-P-HH-ST-TH1 ) none of which come off the shelf in the lengths I need. They do not sell just the cable connector interface, only completed cable assemblies.Can anyone suggest a PCB mount twinax connector + cable connector which I can use to make my own twinax cables of custom length? The meter-end of said cable assembly will just split off into the - and + inputs to the meter.Alternately: I don't necessarily require twinax, just a PCB connector and cable system that has a shielded, twisted pair, can thru hole or SMT onto a PCB, and is equal to or smaller than the footprint of a standard BNC coax PCB connector. Sounds simple but I've been Digikey-ing and Mouser-ing for hours now in the United States and I cannot find anything appropriate which boggles my mind.Thanks for reading

author: tooki
author_rank: Super Contributor
timestamp: April 02, 2025, 06:22:02 pm
content: You don't necessarily need to shield all the way to the PCB, and you don't need to use twinax. (Indeed, it's unclear to me why you want to make a PCB at all, as it doesn't solve any problem, and significantly reduces options.)I think you're way overthinking this. First thing I'd try is simple shielded wire. Convention puts the shield on the negative banana. If that's not good enough, then use shielded 2-conductor cable and connect the shield to ground on one end (for example to the thermal chamber enclosure) and banana on the other end.I assume you're not measuring high-frequency AC, so you don't need to worry about impedance.

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 02, 2025, 06:34:11 pm
content: I need to make a PCB because the DUT is actually a semi-complex power IC, and needs to be communicated with via PMBUS in order to turn on the various switches of the power stage; requires its own bypass caps for various LDOs and analog/digital supplies too. Can't say more than that...I didn't consider I could just take each sense point separately and shield them; so basically I would have a kelvin sense point for Drain ( single SMA connector ) and another kelvin sense point for Source ( another SMA connector ), shield them to the same place ( like you said, thermal chamber metal or earth ground ) ... this will require two coax cables. Does that sound about right?I guess I was pushing the requirement for a twisted pair because ... I wanted to be sure common mode interference would be taken out of the equation. We've had issues with AM/FM radio stations interfering in our lab here when doing mV level measurements before (no, we do not have full faraday cage around entire lab) and I wanted to perhaps over-engineer a solution to not be stuck with something noisy. So shielding both + and - sides of the meter interface conductors, without them being magnetically bound or twisted together, should be sufficient?

author: tooki
author_rank: Super Contributor
timestamp: April 02, 2025, 07:01:06 pm
content: Oh, so you mean a PCB-mount connector on the DUT? You can use a simple coax connector, or just a plain old connector of any kind and connect the shield to a normal pin. (Look at how, for example, cassette deck heads connect to their circuit boards. Those are tiny signal levels, too.)I am not an expert on this, but I think whay you suggest should be fine. I mean, look at how Analog Devices, for example, designs the noise test ports on its linear regulator evaluation boards, like this one:https://www.analog.com/en/resources/evaluation-hardware-and-software/evaluation-boards-kits/dc2637a.htmlThey use simple BNC jacks with the shield on ground. Those are intended to demonstrate how little ripple comes out on the output of an extremely good regulator.I'm not sure what you mean by Kelvin sensing here. It's just normal voltage measurement, no? Can you share a diagram of the measurement setup you need?What you might want to consider is double-shielded coax cable, so that you have really good shielding to keep out your interference.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 02, 2025, 07:15:29 pm
content: Using coax for each wire is overkill but should work but will require a lot of space and will be a pain to connect/disconnect...If you want shielded pairs, I would make use of media cables/connectors such as SATA/USB/HDMI, as opposed to the BNC-style twinax cables/connectors you find on some test equipment. Much cheaper, more compact, and easier to work with.

author: fourfathom
author_rank: Super Contributor
timestamp: April 02, 2025, 08:29:57 pm
content: Bandwidth, measurement speed? Appropriate filtering can help a lot.

author: moffy
author_rank: Super Contributor
timestamp: April 02, 2025, 09:27:39 pm
content: Inexpensive shielded CAT6 cable just use a single pair.

author: Poroit
author_rank: Frequent Contributor
timestamp: April 03, 2025, 03:24:30 am
content: How about using 24G (or smaller) thermocouple wire with Connectors and Panel Jacks (Omega, Marlin or similar)https://au.omega.com/pptst/SPJ.htmlhttps://marlinmfg.com/connectors-and-stripanels/mini-2-pole-jacks-plugs/

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 07:47:31 am
content: Spikes in the measurement could be from contact problems at some connectors. Especially low grade BNC cables are prone to issues.Another possibility is EMI from mobile phones. Normal radio stations cause a more contineous background.For common moder noise, I would consider a CM filter at the DMM, a few turns through a ferrite core or maybe just a clip on ferrite.Thermocouple wire is not a good idea, as it is made to have high thermal EMF.A sensible choice is ethernet cable (twisted pair + shied) - some even come with good isolation. With low impedance signals the insolation would not be that important.

author: coppercone2
author_rank: Super Contributor
timestamp: April 03, 2025, 10:07:14 am
content: try this with the back panel grounding on keysighthttps://www.pomonaelectronics.com/products/cables/coaxial-and-rf-cables/double-banana-plugs-shielded-balanced-lineyou can replace banana ground with fork or get banana to fork adapter if you don't want to modify cable (also from pomona)I made a nice shielded cable but its rather expensive to get good braid, unfortunately. But microvolts = money

author: smoothVTer
author_rank: Regular Contributor
timestamp: April 03, 2025, 04:30:35 pm
content: I think this is a great option. I like how the shield comes out on a different banana plug.Thank you. A lot of times I don't even know the words to use on a google or Digikey/Mouser search, so this cable never came up as an option.