Author Topic: Cable Shielding  (Read 2223 times)

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Offline logictom

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Cable Shielding
« on: May 01, 2011, 03:52:33 pm »
What variations of cable shielding exist and when should each be used?
I know of and understand twisted pair but that's about it  ???

The reasoning behind the question; I'm looking at building a basic ECG circuit but was wondering whether to purchase a cable for the job, such as the pic below, or build my own.
There's three connections for the electrodes but six connections at the other end. From what I understand so far, two cables are used for measurement and the third as a reference but I seem to be hitting a brick wall with the use of the other three connections.
If the cable is shielded and the electrode has one connection where does the shielding connect/terminate at the sensor end?

 

Offline DaveW

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Re: Cable Shielding
« Reply #1 on: May 01, 2011, 04:12:54 pm »
Well twisted pair isn't really shielding. Shielding is where there is a foil like conductor surrounding the wires. This is normally connected to ground and allows any interference to be conducted away to ground. The most common surrounds all the cables and protects from outside interference. I would guess that what's being done in that ecg lead is that every wire is individually shielded, which has the advantage of preventing cross talk between the signals
 

Offline logictom

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Re: Cable Shielding
« Reply #2 on: May 01, 2011, 04:21:59 pm »
Well twisted pair isn't really shielding. Shielding is where there is a foil like conductor surrounding the wires. This is normally connected to ground and allows any interference to be conducted away to ground. The most common surrounds all the cables and protects from outside interference. I would guess that what's being done in that ecg lead is that every wire is individually shielded, which has the advantage of preventing cross talk between the signals

Thanks for the clarification Dave. Sorry yeah I understand its not shielded, I meant to ask about methods for shielding against interference rather than shielded cables specifically, if you get my meaning. Coax would be what I meant to say in the original statement.

So in this case the shielding would just be unterminated at the sensor end? Does terminating the shielding effect performance as I always noticed with a TV coax if the signal was strong you could get away without connecting the sheilding
 

Offline DaveW

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Re: Cable Shielding
« Reply #3 on: May 01, 2011, 05:09:22 pm »
Twisted pair would be effective if you're looking at a differential signal, but for a single ended signal it won't help. If you're using shielding that's only terminated at one end it won't be quite as effective as terminated at both ends (higher impedance connection), but still reduces interference, may well be enough for this application. As you say using coax should give you the same effect, with some decrease in flexibility
 


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