Ribbon Cable or Twisted Pairs? - Page 1
Forum section: electronics / projects

author: kronk
author_rank: Newbie
timestamp: December 01, 2020, 10:36:51 pm
content: Hi,I'm building a simple probe station (my first one) and I'm using the Agilent 34970 mainframe with internal DMM and the 34971A 20 channel general purpose multiplexing card. I'm measuring resistances of ~1-100 ohms and also measuring I-V response.I need to connect all 20 channels to the probe card via a 40 pin connector (male on-board so cable needs female). Only need about a meter of cable, 3-4 ft would be plenty.I'm sure twisted pairs are "the best"... but do I need them for these simple DC tests? The switching time is ~100ms, hardly fast, and I can slow down the scan speed if needed to get stable results. Would a ribbon cable do for what I am doing? These measurements do need to be fairly accurate.Thanks!

author: cdev
author_rank: Super Contributor
timestamp: December 01, 2020, 10:59:52 pm
content: Alternate signal leads and grounds?

author: kronk
author_rank: Newbie
timestamp: December 01, 2020, 11:06:40 pm
content: Quote from: cdev on December 01, 2020, 10:59:52 pmAs in use 80 cables, where every other cable is a ground? I guess I could do like two 40 wire ribbon cables with half the channels each?Oh also important piece of information, these are 4 wire resistance tests, and the source and sense channels are coordinated by the instrument.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: December 01, 2020, 11:14:37 pm
content: With the Agilent 34980, I use shielded D-subminiature cable assemblies from the unit to test jig. The cables come from Amphenol:https://cablesondemand.com/d-subminiature-d-sub-cables-1Additionally I use twisted pairs or twisted ribbon cable inside the jig, or to small jigs located further away. Typically, IDC DIN 41612 connectors on the twisted ribbon cable, when there are a lot of signals involved.These setups have proved very reliable over the years. That's my goal: build it once for reasonable cost, and have it simply work.

author: cdev
author_rank: Super Contributor
timestamp: December 04, 2020, 05:11:19 am
content: 80 wire ribbon cable used to be used in newer IDE cables, so its out there and probably affordable. Alternating signal and ground both improves signal integrity and crosstalk isolation. I don't know about very long lengths. I'd just try it out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2020, 06:16:44 am
content: Certainly you'll need 4-wire measurements for resistances in that range. Which you mentioned, so that's covered.How about dynamics, stability, in the V-I measurements? Passive stuff like diodes, or active stuff like transistors? That might suggest termination or damping (a couple high value ferrite beads usually does the trick), hardly a huge concern, just something to beware of.Diodes might be susceptible to RF interference (10s of mV of RF being rectified to ~mV of voltage error?) so there may still be some reason to provide shielding. I don't know that twisted pair would do the job, more likely a shielded (foil or braid) multiconductor cable, or a shielded enclosure, would do. Mainly because you need more than two conductors at a time, for most tests. If you can do all of this setup in a rack or something, that could be nice.Tim

author: cdev
author_rank: Super Contributor
timestamp: December 04, 2020, 06:03:27 pm
content: Clamp on flat ferrite? ferrite beads?I have some which are #43 material and have a flat slot, Like a long very narrow OThey are meant for ribbon cable.They are basically a conventional toroid sleeve, but flattened.