Where to connect cable's shielding?

Thread Starter

tpny

Joined May 6, 2012
220
I have a sensor connected to my pcb at a distance of 15 meters. The connection has 3 wires:
1. power (12vdc)
2. gnd
3. signal (0~5vac)

Power and ground are tied to pcb's 12V dc power supply. Signal is tied to an op amp.

The wires are inside a cable with some mesh shielding. Should I connect this shielding to something - power supply 12vdc output's ground? Power supply 110vac input's earth prong? The piece of metal plate my pcb is sitting on? Leave it unconnected (which is what I have right now)...
 

Ron H

Joined Apr 14, 2005
7,063
I have a sensor connected to my pcb at a distance of 15 meters. The connection has 3 wires:
1. power (12vdc)
2. gnd
3. signal (0~5vac)

Power and ground are tied to pcb's 12V dc power supply. Signal is tied to an op amp.

The wires are inside a cable with some mesh shielding. Should I connect this shielding to something - power supply 12vdc output's ground? Power supply 110vac input's earth prong? The piece of metal plate my pcb is sitting on? Leave it unconnected (which is what I have right now)...
What is the frequency of your signal?
 

Thread Starter

tpny

Joined May 6, 2012
220
why does low frequency call for connection to signal ground? Thanks! (Signal ground being the dc ground coming out of my 12vdc power supply output right?)

Is my power supply's dc ground (at output) tied to the main's neutural (at input)?

Right now I have the shield unconnected and it's not problem since my signal is pretty big. I'm concerned that connecting the shield to the power supply ground (at the output) might somehow cause it to be I don't know..less stable? (Because my delicate circuit shares that same ground.) What if the shield spans 500 meters? Is the supply's dc ground boundless in its ability to turn everything tied to it 0v?
 

Ron H

Joined Apr 14, 2005
7,063
why does low frequency call for connection to signal ground? Thanks! (Signal ground being the dc ground coming out of my 12vdc power supply output right?)
Without seeing your schematic, I wouldn't want to speculate how your power supply is connected to your circuit board.
Is my power supply's dc ground (at output) tied to the main's neutral (at input)?
I have no way of knowing.

Right now I have the shield unconnected and it's not problem since my signal is pretty big. I'm concerned that connecting the shield to the power supply ground (at the output) might somehow cause it to be I don't know..less stable? (Because my delicate circuit shares that same ground.) What if the shield spans 500 meters? Is the supply's dc ground boundless in its ability to turn everything tied to it 0v?
A floating shield can pick up noise and capacitively couple it to the signal wire. Grounding the shield prevents this.
 

Papabravo

Joined Feb 24, 2006
21,159
The shield is grounded at ONE POINT ONLY to avoid making it into a loop antenna which can be incredibly sensitive to noise which is capacitively coupled to it. The best place to connect a shield is as close to the power supply as possible. The connection to Earth should also be made at ONE POINT ONLY as close to the power supply ground as possible.
 
Hi Guys, sometimes please don't connect the shielding GND directly into your board's signal GND, especially HDMI cable. Connect it with your power supply's earth GND (earth prong). Or connect it with your product's case/chassis GND.
 
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