Floating Analog Inputs and Isolation

Hello Guys,

A couple of questions about analog inputs:

As I understand it, separate power supplies (separate from the PLC supply and Digital I/O) are sometimes used for analog sensors to reduce noise (contactors, relays and other switched devices can cause noise on +24V DC lines). Under these circumstances, do you use the separate power supply to power the analog module as well? Or just the sensors themselves?

Also, as I understand it, isolated analog modules have the common for the analog measurement circuit separate from the common for the module power supply. These, however, may need to be tied together if there is a risk of the analog common drifting too high with respect to the module common. Under what circumstances would you want the analog measurement circuit (sensor connected to + and - inputs) to be completely floating with respect to the module/PLC power supply?

Thanks!

Try a 1meg resistor from the input to ground to bias an unconnected input to read zero. Keep in mind that a floating channel is not 0 volts but rather an undefined voltage. In order to see 0 volts,amps, a 0 volt signal (such as gnd) should be connected to the input. A pull up resistor could be used to detect a broken wire, bad card, etc.

just connect it as it should, inside the modules should be protection for floating.