1Gbps Ethernet : use shield for slow signal?

Question

Short version : Is it OK to use the shield of an Ethernet cable to carry a very slow analog signal (100Hz bandwidth is great, 1Hz is OK)? Nb : I don't care if I get some >1MHz noise on my analog signal.

Long version : I'm working on a subsea robot, with very limited space for connectors (sub-sea connectors are very bulky, and choice is very limited (excepted if you accept to buy the very expensive ones)). Also note that there are only a few physical sizes, and once there is the maximum number of wires for a given size, adding one more is a lot larger (about +100% surface).

I have 13 signals I would like to group into a single connector if possible : 8 for 1Gps Ethernet (ie 4 twisted pairs), a supply (0V, 24V), a RS232 connection, and an analog signal (0-5V).

I would like to use this 13 pin connector. On the dry side, there is no shield around the twisted pairs, and there are 13 wires. On the wet side, there are only 12 wires + shielding for the 4 twisted pairs (the shield is connected to the "missing" wire).

Can I use the shield as a 13th cable, to carry my 0-5V analog signal? (retrieving it at the other end is no challenge).

My main concern is will this affect the integrity of the Ethernet signals?

The analog signal (output of an DAC + protection circuitry) is set manually by the user through the user interface : it doesn't changes often, and it doesn't need to change fast (I can add a low pass filter (RC), with cutting frequency 100Hz ideally, but I can go down to 1Hz if needed). I don't care if there is high frequent noise on this signal (any noise >100Hz doesn't matter).

Should I add extra decoupling capacitors between the analog signal/shield and the signal ground?

NB : there is no other connector meeting my requirements with 13 true wires, so if this don't works out, I will need either 2 connectors, or one big one. In both cases, this means abandoning some "nice to have" features of the robot. On the other hand, all the signals on this connector are for "must have" features, so if it's risky, I'd rather go for a bigger connector than having problems.

EDIT : To clarify some points that might not have been explicit enough :

• the cable is split in 3, to go to 3 different of the shelf devices, each using some of the signals (ethernet (with PoE) for one; 0V, 24V and analog for a second; 0V, 24V and RS232 for the 3rd).
• the 3 devices at the end cannot be easily modified to accept other protocols, and I haven't found any better combination to fit our needs. In addition, we plan to retrofit the electronics on some existing ROVs
• adding electronics into the cables is possible, but is a last resort solution. Among the requirements of this project is to get rid of all electronics in cables. If we have to choose, we will probably go for the bigger connector rather than adding electronics in the cable
• the analog signal is very slow, rather strongly driven (>10mA source/sink capabilities). It doesn't need to be precise (a few hundreds mV low frequency noise is OK). And "higher" frequency noise (>100Hz) is of no concern at all. On the subsea side, the current drawn is <1µA.
• for other devices we design ourself, we have far more freedom to optimize the wiring (and we do so), but for these devices, I have no viable alternative

Answer 1

Can I use the shield as a 13th cable, to carry my 0-5V analog signal? (retrieving it at the other end is no challenge).

Essentially: no. The whole point of that shield is that undesired stray fields couple into it, and not that it's a good conductor; two things making it undesirable for transporting an analog signal.

Generally, transporting an analog signal next to both a high-speed and a high-current cable over long distances is a bad idea. Full stop – the standard way to do this is not doing it but converting to digital at the source.

The logical way here is to convert your analog signal to digital underwater, and transport that over Ethernet. If you can live with 100 Hz bandwidth, that's not even close to being a problem.

Same, by the way, for your RS232, but I can see that if that is for a safety-critical aspect of operation, you'd rather have a more complicated cable arrangement than transport the serial data via e.g. telnet over IP over Ethernet.

Whether or not the device speaking Gigabit ethernet has a suitable ADC, or USB or another serial, both of which you could connect to a cheap square-centimeter board with a microcontroller doing the analog-to-digital conversion, I don't know.

So there is no room for added electronics excepted by adding an additional waterproof container (bulky, heavy and expensive).

What is bulky, heavy and expensive are submersible cables, and connectors, not the electronics needed to support this, usually. I think you're working on preconceptions here, not on actual technical considerations!

I have 13 signals … a supply (0V, 24V)

Depending on the length of your cable, and the amount of power you, you might also get supply voltage using Power over Ethernet (PoE), specifically standards-compliant 802.3bt Type 3 PoE. 50 to 100 m usually quite doable for powers up to 51 W with the usual Cat5 or better cabling.

That way, you'd reduce your cabling needs to one submersible Ethernet Cat5 cable.