Shielding isolated signals - Page 1
Forum section: electronics / projects

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 17, 2023, 01:51:49 am
content: What's the best approach in your experience for shielding signals that are isolated?Say you have a device the chassis of which is isolated from some part of the circuit, and you have connectors that transport signals from the isolated side. What would you connect the connector/cable shield to?

author: Someone
author_rank: Super Contributor
timestamp: February 17, 2023, 02:32:52 am
content: Depends on the signals and the aggressors....... how long is a piece of string."Low Level Measurements Handbook" would be the starting point.AES48 is one end solution but it assumes both ends of the shield can dump current without problems.

author: ajb
author_rank: Super Contributor
timestamp: February 17, 2023, 02:39:38 am
content: We run isolated RS-485 interfaces on XLR connectors. The convention is to tie the cable shield to isolated common, but leave the connector shell isolated from the cable. When mated, the connector shell electrically connects to the equipment chassis. Isolating the connector shell from the cable reduces risk to people handling the cables if there’s a hazardous common mode voltage on the bus due to a fault (typically one end of the bus is not isolated). The shielding this arrangement provides is good enough for a few hundred kbaud.Ultimately you have to consider what you’re isolating against, but I can’t think of many good reasons to isolate an interface where directly connecting the cable shield to the enclosure wouldn’t defeat the purpose. RF coupling them of course is an option. If you really need a continuous shield then I guess you’re using something like an insulated BNC connector and an internal shield

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 17, 2023, 03:48:24 am
content: Quote from: ajb on February 17, 2023, 02:39:38 amYep, that's my thought as well.It would be a cable with multiple pairs each with its own shield, then there's the outter shield for the whole cable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2023, 05:52:31 am
content: Isolated -- with respect to what?At RF, there is no such thing as isolation. At best, you might distinguish free (radiating) EM fields, from near fields, from currents flowing in wires (with voltages in cables*, themselves propagating as well), and all of these get a bit murky as you go up and up in frequency.*That is to say, the fields between given wires in a cable.Generally, if you have a cable exiting a circuit, it will have capacitance (or the higher-frequency variations thereon) to that circuit, at the very least equal to the isolation barrier's capacitance, plus whatever strays of the isolated section's ground plane and the cable and etc. to their surroundings (the rest of the circuit, the enclosure, the environment).You can (and should, if this is the case!) have the shield grounded to the local isolated section, and then only shield currents (common mode) interact with the main circuit. Which is the domain of 'Y' caps* (perhaps with some ESR, to bridge the barrier and provide some termination or filtering impedance) and ferrite beads (on the cable, to dampen standing waves on the shield).*In the abused sense of, a capacitor crossing an isolation barrier; when this is between mains and earth or SELV, a Y type safety capacitor is specifically needed.It wouldn't make much sense to ground the shield to the main circuit and/or enclosure, while still having the signals within be isolated from them. Certainly there's little benefit at AC, but there might be reasons at low frequencies still; consider the triaxial cable typically used with an electrometer.There could still be the case where, the circuit within is isolated from its enclosure, for... perhaps dubious reasons (i.e. there are likely better overall / system-level solutions), but this happens from time to time, often when grounding isn't well thought out. The above example of audio equipment is a familiar one -- well, XLR jacks that is, which are often used in AV, professional lighting, etc. equipment all the same. Due to poor wiring (read: hastily assembled, or a faulty venue), equipment can even become live; or perhaps because grounding isn't available -- or not possible, say because a crossed-wire fault can't be traced in timeand the show must go live-- and so things may be better (safer), at least in that immediate case, when insulated, or left floating.These also tend to be the domains of one-side-open shields, as the ingress of RFI from the open shield may be more tolerable than the alternative, a ground loop dropping across the signal path. (There is a design solution for this -- use differential inputs, including on coax connections -- but one that is sorely underutilized in the industry, go figure!)Tim