low noise AC measurement cable that is cheap? - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 15, 2014, 02:03:44 pm
content: I have almost built a preamplifier (using low noise amplifier, good resistors, batteries, metal box) which connects directly into a multimeter.I wanted to drill a hole into the equipment that I am testing, solder in the wire leading to the preamplifier, reassemble the box and turn it on.But what kind of cable should I use from the preamp to the DUT?The first thing that came to mind was twinaxial, but I am not sure.I want some kind of cable that I could buy cheaply and cut, not a dedicated expensive transfer standard type cable, this is to ensure that I can properly seal up whatever is being tested with howeve rmany screws it has and solder it directly to the circuitboard that is being tested.

author: nickm
author_rank: Regular Contributor
timestamp: April 15, 2014, 03:07:32 pm
content: Balanced microphone cable works well. You get a shield which is very important for low noise and then the inside conductors are twisted to give you give CMRR. For even better CMRR you can get quad microphone cable which you connect the opposite conductors together at each end. Any stray interference will be induced in the pairs more evenly if they are the two opposite conductors than if the pairs consist of single conductors. You can get L-4E6S for about $0.50/ft.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 15, 2014, 03:11:28 pm
content: you are saying that if the output of this quad microphone cable looks like a square to short out the diagonals in order to increase CMRR?1 23 4so connect 1/4 and 3/2?I assume that the quad cable is also twisted, right?I will pick some up.what connector would you use for this cable on the preamplifier side? I figure just leave it soldered in? easy enough to replace it in my lab.I have some large diameter braid too, if this cable is not good enough then I could always just throw additional braid over it (I am going to be making this measurement not very many times)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 15, 2014, 05:41:16 pm
content: Before designing a defense, you need to identify the enemy. What are you trying achieve here? Environmental sealing? Mechanical robustness? Shielding? One issue with cables is microphonics, and there are some very special cable designs that address that. Does the machine vibrate? Should you be using a proper hermetic feedthru? Not enough info, but lots of things to consider.

author: nickm
author_rank: Regular Contributor
timestamp: April 16, 2014, 03:27:04 am
content: Yeah you have the wiring right. That particular cable comes with the pairs color coded so you connect the blues together and the whites together. Keep in mind it increases the cable capacitance so if you're trying to put high frequency through it you might have trouble. As far as connections it depends on what you put in your preamp. That cable is made for 1/4" phone jacks or XLR cables so those would be a good choice if you want to be able to connect/disconnect. Directly soldered is fine too as long as you remember to add some sort of strain relief.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 16, 2014, 04:40:32 am
content: yea its for audio, i got the cable thnks

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 16, 2014, 05:19:22 pm
content: Quote from: Conrad Hoffman on April 15, 2014, 05:41:16 pmno, this isj ust for testing something that I built in a box, I want to take all internal measurements when its fully assembled. Maybe putting a tiny 30 ga twisted pair through a vent hole is enough but I plan on just drilling a hole for this audio cable and slapping some copper tape on it after the measurement is done. nothing fancy