Hi guys. A curiosity here. I am working on a PCB design for a piece of kit which will handle fast rise accurate (50nS) but slow frequency timing pulses between pieces of equipment well spaced out from each other. Pulses are 5V and 12V. This is on a large vessel at sea, (cross channel ferry size). Grounds on these vessels are notoriously "non-earthlike". In a worst case they can measure more than 40V difference from place to place and are very noisy.
The PCB is powered from an external switched mode PSU at low voltage, 5V/12V. Circuitry draws so little current there has to be a hi wattage load resistor across it to guarantee a PSU will stabilise and drop to the correct voltage. It is enclosed in a rack mounted metal case in a server room in the bowels of the ship and connects to a PC nearby as well as to equipment in other areas of the ship. Inputs are all opto-isolated so effectively low impedance differential mode. Outputs are not opto-isolated and could go to other parts of the boat.
The inputs take care of themselves, just keep them isolated totally. Outputs, while single ended, have also taken care of themselves in the past so I don't see them as a problem. I have the incoming PSU/circuitry 0V, case ground and PCB plane grounds to consider. I'm not sure what is the best way to handle this from the point of view of signal integrity, with a low voltage external supply and rack mounting like this safety is less of an issue. I am tempted to just connect them at a single point, or maybe attach the case to the others via a resistor.
Can anyone offer any advice?
The PCB is powered from an external switched mode PSU at low voltage, 5V/12V. Circuitry draws so little current there has to be a hi wattage load resistor across it to guarantee a PSU will stabilise and drop to the correct voltage. It is enclosed in a rack mounted metal case in a server room in the bowels of the ship and connects to a PC nearby as well as to equipment in other areas of the ship. Inputs are all opto-isolated so effectively low impedance differential mode. Outputs are not opto-isolated and could go to other parts of the boat.
The inputs take care of themselves, just keep them isolated totally. Outputs, while single ended, have also taken care of themselves in the past so I don't see them as a problem. I have the incoming PSU/circuitry 0V, case ground and PCB plane grounds to consider. I'm not sure what is the best way to handle this from the point of view of signal integrity, with a low voltage external supply and rack mounting like this safety is less of an issue. I am tempted to just connect them at a single point, or maybe attach the case to the others via a resistor.
Can anyone offer any advice?