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:
- 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
- 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.
- Solder to the copper pour (still electrically floating).
- 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.
Best Answer
'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.