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Home > Archive > Electrical Engineering > May 2007 > industrial signalling and noise immunity
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industrial signalling and noise immunity
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| jack007 2007-05-15, 8:25 pm |
| I deal with alot of industrial 4-20mA. the cabling from tranducer/
transmitter to output is is always twisted pair, shield c/w a drain
wire.
1. twisted pair is so induced current signals (from magnetic fields)
cancel, correct?
2. shielding (foil etc), acts a faraday cage to the wiring within
essentially 'shielding' the charges in the signal wire copper from
outside e-fields influence, correct?
3. what is the drain wire for?
4. would there be negative outcome with using an overall shielded
triad cable c/w drain wire, isolating one wire, in the stead of the
twisted pair (ie if the contractor accidently pulled triad and not
twisted pair)?
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| jack007 2007-05-15, 8:25 pm |
| On May 15, 5:29 pm, jack007 <brendan.fin...@gmail.com> wrote:
> I deal with alot of industrial 4-20mA. the cabling from tranducer/
> transmitter to output is is always twisted pair, shield c/w a drain
> wire.
> 1. twisted pair is so induced current signals (from magnetic fields)
> cancel, correct?
> 2. shielding (foil etc), acts a faraday cage to the wiring within
> essentially 'shielding' the charges in the signal wire copper from
> outside e-fields influence, correct?
> 3. what is the drain wire for?
> 4. would there be negative outcome with using an overall shielded
> triad cable c/w drain wire, isolating one wire, in the stead of the
> twisted pair (ie if the contractor accidently pulled triad and not
> twisted pair)?
Sorry for 1) i should say it is differential and cancels
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| Floyd L. Davidson 2007-05-16, 3:25 am |
| jack007 <brendan.finley@gmail.com> wrote:
>On May 15, 5:29 pm, jack007 <brendan.fin...@gmail.com> wrote:
>
>Sorry for 1) i should say it is differential and cancels
The signal is differential, the noise is common mode and
cancels.
Using a twisted pair (fed with a balanced circuit)
improves noise reduction compared to non-twisted pair
due to greater common mode rejection.
If the contractor "accidentally" pulled non-twisted when
twisted was specified, the contractor should be required
to correct the problem by pulling twisted pair. If that
is not what was specified, consider a new contract to
get the job done correctly. There is no substitute.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@apaflo.com
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