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In a hobby project I am using shielded cable to connect capacitive sensor's electrode (see https://electronics.stackexchange.com/questions/189916/capacitive-touch-not-working-at42qt1011 ) to the AT42QT1011 (there are two cables ending with electrodes going in separate directions). Usage is proximity (millimeters), electrodes are insulated.

However, I left the shield disconnected (using only wires inside) as connecting to ground will make strong capacitive coupling with the ground - something which will render the sensor useless.

The question is, where to connect the shield? It is not particularly annoying that IC senses thru the shield, but what is the right thing to do?

(I have already settled on using particular cable, but may be there are better choices for cabling capsensors?)

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To be honest, the best way to do this is not to run the capacitive sense signals over any distance. Local cap sense ICs connected over distance via a digital bus is the way to do this most robustly. Anything else and you're having to fight making the outside of the cable insensitive to touch without placing a large capacitive burden on the cap sense signal itself.

There are ways around this (including capacitively coupling the shield to ground) but none are as reliable as just having the cap sense IC where you need it and most involve a tradeoff against proximity sensing. The increased cost normally pays itself back quickly in system robustness and reliability.

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  • \$\begingroup\$ Does the width and length of the copper trace that connects the sensor pad to the IC, matter? And similarly, does it matter if that same copper trace is inthe vicinity of other copper traces which carry other analog signals? (Assuming complete absense of a gnd place near or below the afformentioned traces) \$\endgroup\$
    – Geo
    Feb 17 '21 at 22:26
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    \$\begingroup\$ Yes to all the above. You want the lead-in trace to be narrow to avoid capacitively coupling to its surroundings (i.e. not becoming a sensor itself). That said, you don’t want it so narrow that it’s resistance starts causing a voltage drop along it. For most capacitive sensing with metal traces, this isn’t a limiting issue, but it can be with other materials at higher drive currents. You want the trace short for both these reasons - lower resistance to some extent, but mostly because longer traces make better antennas. You also want it away from other traces to avoid crosstalk. \$\endgroup\$
    – stefandz
    Feb 17 '21 at 22:34
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Using a shielded cable and leaving the shield unconnected (floating), you still have a large capacitance. One inside wire will have a capacitance to the shield, and from the shield to the second wire, you end up having two series-connected capacitors. If a small cable capacitance is the main issue of designing the cable into your circuit, then the best choice will be an unshielded cable, maybe just two twisted wires.

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  • \$\begingroup\$ That is, in other words, if it is not a problem, can I leave the shield floating? \$\endgroup\$
    – Roman Susi
    Oct 5 '15 at 7:10

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