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Background

My project is split over two PCBs for convenience.

The first PCB adapts an input signal and converts it to a low-voltage differential signal. There are 4 channels to be measured, so 4 differential pairs. Signals are 50 kHz, about 1Vrms. Seemed best to use a regular ethernet cable to carry over those signals to the other PCB (the data acquisition board). The cable distance is 1.5—2 metre.

The RJ-45 receptacles I selected have metal outer casing, exposed as through-hole pins. So if I will, I can make the whole ethernet cable's shield be electrically connected to a copper pour on both PCBs, which surrounds the area around the connector on the PCB. I can also elect to connect this copper pour to analog ground.

Options

So I have a multitude of choices:

  1. Don't connect the cable's shielding to the RJ-45 ends of the cable (or use plastic ends). This way the cable shield will be floating with respect to the signals it carries
  2. Use metal ends and connect them to the cable's shielding, but do not solder the PCB's RJ-45 receptacle to the copper pour. This way the cable's shielding will be connected to the receptacles, but still electrically floating.
  3. Solder to the copper pour (still electrically floating).
  4. Connect the copper pours to the PCBs' analog ground - the shielding becomes ground potential.

The PCBs already use the same ground, as they are powered from a common DC source. Option 4 creates another ground path which I'm slightly worried about.

Question

I'm wondering which option is the best from signal integrity standpoint? I want to measure the differential voltages with a 12-bit ADCs, so signal interference and noise could be an issue, hence the question.

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    \$\begingroup\$ Often the shield is only connected to ground at one end. \$\endgroup\$
    – evildemonic
    Commented Apr 2, 2020 at 19:03
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    \$\begingroup\$ More options: Connect cable shield to ground only at transmitting end, leave floating at receiver end, to avoid ground loop. Or connect one end or both ends of the shield via capacitor to ground. But watch out the naming convention, it is not an Ethernet cable since it is not used for Ethernet in your case, it is just CAT-5 or -6 cable with 8P8C connectors, and RJ45 is a convention how to wire CAT cabling pairs to 8P8C connector. \$\endgroup\$
    – Justme
    Commented Apr 2, 2020 at 19:04
  • \$\begingroup\$ @Justme, thanks, my bad. Of course I don't mean real Ethernet with magnetics and all that, I mean CAT-5 or -6 with 8P8C connectors. \$\endgroup\$
    – anrieff
    Commented Apr 2, 2020 at 21:27

1 Answer 1

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'Regular' Ethernet cable (Cat3, 5, 6) is unshielded. That's the U in 'UTP' - Unshielded Twisted Pair.

As for the connector, RJ45 as used for Ethernet provides no grounding at all. This is intentional: the Ethernet magnetics actually completely isolate any DC path between the cable and the electronics, so there is no common-mode path at all, ground or otherwise. This is fine for Ethernet which uses an AC signal, but for an ADC input that presumably needs DC, this would pose a problem.

The shielding you see on the RJ45 connector doesn't actually make a connection to the cable in any way. Instead, its purpose is to suppress common-mode radiation from the board itself and prevent it from coupling onto the transformers (if it's a magjack) and cable pairs.

For your application, HDMI cabling would perform better and provide the common-mode shielding you're looking for. HDMI cable provides 4 shielded pairs with separate drain wires for each shield, plus an overall shield that connects to the shell. You could adapt this to the connector of your choice (not RJ45) or use an HDMI connector if you wish.

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  • \$\begingroup\$ There are 8P8C connectors that have a metal shield, it is connected to cable shield and it will connect to metal shield of 8P8C socket. Shielded ready-made cables do connect the two devices via connectors and cable shield - at least the ones I've tested. Cheap HDMI cables also do not connect all ground pins as separate shields, but what you describe is how HDMI cables should be and how most of them are. However, I strongly advice not to use common connectors for other purposes, as it is too easy to connect incompatible devices and break something. \$\endgroup\$
    – Justme
    Commented Apr 2, 2020 at 20:41
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    \$\begingroup\$ "The shielding you see on the RJ45 connector doesn't actually make a connection to the cable in any way." - this is something we specifically checked. It contacts to the cable's 8P8C connector's exterior, so if we use metal-clad 8P8C connectors on the cable, we can get connectivity all the way from the PCB to the cable shield. Of course presuming we use STP or FTP cable. \$\endgroup\$
    – anrieff
    Commented Apr 3, 2020 at 5:22
  • \$\begingroup\$ STP Ethernet cable is not common. On the other hand I think you’ll find HDMI cable to be easier to source and very cheap for what you get. Another option is SFF 8643 - it’s 4 pairs plus a clock, used for PCIe NVMe drives. \$\endgroup\$
    – hacktastical
    Commented Apr 3, 2020 at 5:30
  • \$\begingroup\$ Someone marked this down. Read carefully what I’m about to tell you: shielded Ethernet cable is a very bad idea for field wiring: it defeats the purpose of the magnetics, which provide galvanic isolation for the link partners. That’s why it’s practically never used. SATA or SFF 8643 cable a cheap, widely available option that does offer shielded twisted pairs, and would be more appropriate for what OP is trying to accomplish. \$\endgroup\$
    – hacktastical
    Commented Jul 31, 2023 at 18:20

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