A signal isolator electrically separates input and output circuits to block ground loops and interference — the signal type and range remain unchanged at the output.
A signal conditioner converts raw sensor signals (thermocouple, RTD, 0–5V) into standard formats a PLC or DCS can read directly, with amplification, filtering, and linearization built in.
They are not interchangeable. Use the wrong one and you either fail to fix the noise problem or fail to convert the signal.

