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I'm designing a simple board with a differential ADC. The inputs are from TRS jacks that will (usually) carry balanced audio signals.

The datasheet, and general best practice, advises keeping analog and digital grounds separate. This makes sense to keep the digital ground currents away from the analog circuitry where they would contribute to noise.

I wonder then, where should I attach the screen from the TRS jacks? If these are balanced audio, the screen should contain only noise (RF and potentially ground loop hum) so it might make sense to dump it with the digital ground. On the other hand, it's not unusual for an unbalanced phono plug to get shoved in the jack, and now the screen is also the reference point for the signal so should be kept quiet.

What's best practice in this situation? And does the answer change if I'm certain if the equipment will only ever be used for balanced connections versus trying to design for either balanced or unbalanced cases?

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Quite often, the screen is connected to chassis to (a) dump any interference noise to the chassis and (b) blow any fuses should the screen become live. Of course, assuming chassis is metal, and with a mains ground connection. Failing this, PSU 0V is probably the next best.

It's also not unusual in stage and/or semi-pro gear which may be connected to god knows what, for that connection to be made via a "ground lift" switch, to prevent ground loops where the screen is grounded at both ends to different sides of the studio/theatre/etc. Given a choice, the screen should be grounded at the receiving end of a line, so that it is close to the receiving end's ground potential. Leave the ground lift in the "connected" position where possible (see link above).

With regard to plugging into an unbalanced jack: that effectively shorts ring and sleeve together. The ring forms the audio signal return. If the sleeve introduces noise or ground loop hum from chassis, the ground lift switch should take care of that.

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  • \$\begingroup\$ The audio world: where getting zapped from your mic is just a simple switch away. I wish I knew more about how they get all of that stuff to place nice with each other. \$\endgroup\$ – Chris Knudsen May 8 at 20:38
  • \$\begingroup\$ ...funny how you just updated that answer. \$\endgroup\$ – Chris Knudsen May 8 at 20:39
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In my opinion, a differential input should be able to cope with a little common mode noise on the screen and it should easily cope with low frequency common mode noise. However, higher frequency common mode noise might be a problem so I’d want to connect the screen to the receiver’s local analogue ground via a capacitor, maybe 100 nF. This also prevents earth fault currents flowing through the cable and causing problems.

What's best practice in this situation?

Design the balanced receiving circuit with some care and, to avoid earth loops, connect the screen to analogue earth via a capacitor. Plus, it makes sense to also put a TVS on the screen to good old fashioned PSU or digital ground. If you are designing to meet any form of surge withstand, this makes sense.

For an unbalanced signal I’d adopt the same method and use the two inners for (a) the signal and (b) the return. This ensures best resilience to noise and hopefully, the unbalanced source drive impedances (send and return) will be low and similar.

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