Isolation

A ground loop can occur if more than one ground connection is made to a single control signal. Because grounds are seldom at the same potential, an unwanted current will be generated and interfere with the control signal. Signal isolators break the ground loop current path and maintain the integrity of the measurement.
Noise Filtering

Isolators incorporate low pass filters that eliminate high frequency EMI/RFI and unwanted signals from power lines, generators and motors.
Linearisation

Many sensors output a signal that is not linearly related to the engineering value being measured. For example, a thermocouple used to measure temperature has a nonlinear millivolt output. A thermocouple input signal isolator translates this to a standard robust linear signal such as 4 to 20 MA.
Signal Conversion

Industrial applications use a wide array of sensors to measure temperature, flow, length, speed etc. These signals may then need to be converted into a form usable by the instrumentation to which they are connected. Any sensor signal (thermocouple, RTD, DC voltage, DC current) can be converted to any standard process signal.