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Which creates more interference, an open loop or a closed one?

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Triode

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To test a capacitive touch system I would like to generate interference in wires laying along side my wires to the sensing pads. I have a function generator that can go from -10V to +10V. Is the worst case scenario an open line acting like an antenna, but with no current flowing. Or a loop back to ground, so that it is fluctuating in voltage and current?

Thanks!
 

RadioRon

Well-Known Member
Usually a loop is worse. Most of the coupling in the side-by-side scenario is magnetic. It is somewhat dependent on the impedances involved. If there is a relatively low impedance load on your touch system, then magnetic, but if the impedance loading those wires is high, then I would swing over to considering the E field coupling to be the stronger interferer. However, a loop with a termination resistance might still be effective for both, and you can tune the termination resistor to get the worst case effect.
 

Triode

Active Member
Thanks, I was able to verify that the shielded cables make a huge difference. The odd thing is that I found that if you have another cable along side the unshielded cable and you touch that, even somewhere that isn't near the sensing cable, it also increases the capacitance. It seems like that would just be like a really weak capacitor added in series and too far from the measurement to show up, but it was only a little weaker than touching the first wire.
 

ci139

Active Member
there was one(=1) occasion where some 20m (66ft) - side to side - parallel run of L,N,GND and shielded data cable generated about ~125V/MΩ in cable shield
the shield was N/C at far end 100m(300ft) -- the data transmission was NOT affected by this "hot plugging flasher indicator" setup (RS422)
 
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