Author Topic: Screen Ground on a PoE application  (Read 466 times)

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Offline TwinScrollTopic starter

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Screen Ground on a PoE application
« on: June 28, 2020, 07:08:19 pm »
Hi

Just after peoples opinions/suggestions on a screen ground location/path.

Please see the attached image, The setup will be powered by a PoE switch - it is connected to an Audio Encoder (AXIS C8033).
The Encoder creates a local 12V supply and this is used to supply (in my case) a microphone (the mic works between 9-18VDC)
The output from the microphone is balanced line level (consumer) and via a 1:1 transformer this is routed into the audio encoder as an unbalanced input.



My question is with regards to screen ground connection point for the multi-core cable from the microphone.
The cable is physically (2 x) 22AWG twisted pairs, each individually foil wrapped and the cable has an overall screen braid (with drain wire).
1 of the pairs is microphone 12VDC & 0VDC and the other pair is for the balanced audio output from the microphone.

The enclosure for the encoder / transformer is plastic so a case screen is not possible.
If I use the 0VDC from the encoder for the screen ground I am curious if this is going to be a bad idea.... any noise I pick up will have a long path through the encoder internals and I presume back down the PoE power wires.
Presenting this noise to the encoders 0VDC feels wrong?
Should I be thinking along the lines of maybe using a common mode choke on the DC power wires? and dare I say no screen?  :-\

Best Regards
TS
« Last Edit: June 28, 2020, 07:20:34 pm by TwinScroll »
 

Online Simon

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Re: Screen Ground on a PoE application
« Reply #1 on: June 28, 2020, 08:20:14 pm »
As an audio application does it matter that much? The 0V of the encoder is not a bad place to start.
 

Offline David Hess

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Re: Screen Ground on a PoE application
« Reply #2 on: June 28, 2020, 10:04:31 pm »
The screen should be connected to a low impedance point near the input amplifier which will usually be common or the negative supply but could be the positive supply.  This is the same point that a metal shield would be connected.
 


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