How many power supplies for Analog Inputs?

I am fairly new at using PLC's and have just completed the level 1 course for the ControlLogix and I have a question regarding wiring of the 1756-0F8H Analog Input card w/HART. Many of the documents I've seen depict a single power supply for each channel, is this required, or can all the channel be run from 1 supply?

I've created a program to stroke a couple valves open and close, and have been monitoring the AI feedback from the valve top. Both valves cycle fine for awhile, but then one decides to stop, and can be restarted by removing and reconnecting one of the AI feedback wires. Wasn't sure if this was caused by using the same power source on the AI feedback or a possible ground loop.

The process industry norm is to run multiple analog channels from the same power supply.

I haven't done CLX, but it can't be so deviant from industry norms as to require a separate power supply for each analog channel.

Thanks for the reply danw. I had thought using a single supply would be enough since the RTN on the card are being used as well. Anyway, I think I found what was giving me issues. The valve had stopped working a short while ago, so I reviewed the wiring and the drain/shied wire was grounded to the field device, but left hanging at the card. The field device may not be grounded very well, because after grounding the drain to the chasis of the PLC, the valve began cycling by itself again without cycling the AI feedback.

weedman15 said:

I reviewed the wiring and the drain/shied wire was grounded to the field device, but left hanging at the card. The field device may not be grounded very well, because after grounding the drain to the chasis of the PLC, the valve began cycling by itself again without cycling the AI feedback.

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Careful! Normal grounding practice is to ground on one end only, not on both ends.

Thats what I had thought, to prevent ground loops. Luckily, this is only a test rack and not a process. I just thought I would try grounding on the chasis and it seems to be working so far. I may run it over the weekend and check the grounding on the rack and/or remove the ground at the field device to see what difference it may make.

Just adding to the thread... I find confusion with grounding on field wiring installations to be routine. The right way is to ground at one end, at the control panel. At the instrument the shield should be removed and insulated with heat shrink or just electric tape so it cannot make contact.

But sometimes I find:
1. Grounded at the instrument, not the panel.
2. Grounded at both ends.
3. My favorite: When grounded at only one end, at the other end the shield has been carefully stripped back and then wrapped around the cable.

Item #3 means I ask the installer if he is planning on watching TV with that antenna.

Usually: Improper grounding introduces noise. Improper commoning of DC 0-volt signals causes things to not work properly.