isolator for reading that analog signal

Hello datttt;

The need for a signal isolator depends on how "clean" your power grid is, and how that affects the signals coming from your sensors to your PLC analog input.

If you have ground loops interfering with your sensors, the signal received will have spikes and will fluctuate uncontrollably, making the PLC program unable to make logical decisions based on a clean input signal.

There are many sites you can look at discussing the need  and use of analog signal isolators, here is one:

https://www.elprocus.com/signal-isolator-working-and-its-applications/

Under "normal" circumstances, where your plant power is clean, no ground loops or switching problems, you wouldn't need analog signal isolators to stabilize your analog sensor readings.

Hope this helps,

Daniel Chartier