This FAQ explains what power ground, functional ground and analogue ground does.

Functional Ground

Functional Ground is the EARTH connection through the DIN rail that the ED-5xx and ES-5xx is attached to. This Functional Ground (EARTH) on the ED-5xx is on PIN 5 of the BLACK power connector. It is directly connected to the FUNCTIONAL EARTH metal tab on the DIN rail clip. Thus the FUNCTIONAL EARTH electrically connects the metal DIN
rail to PIN 5 of the BACK power Connector. The customer can choose to wire or not wire to PIN 5 on the Black power input.

Power Ground

Power Ground is the low side (0V) of the power input. The GND on the schematic is connected directly to PIN 1 and PIN 4 of the BLACK power input connector and is thus connected to the low side of the customers power supply.

Signal Ground

Signal Ground is applicable to those ED/ES products which have a serial gateway or serial port. (ES 020/357/246/257/701/279/313/320/368/842 and ES-5xx range) This is the ground associated with the data transmission signals- it is typically connected to the cable sheath. It is import to connect this even in twisted pair situations.

Analogue Ground

Analogue Ground is applicable to the ED analogue range (ED-549, ED-560) and is the ground for the analogue input circuitry. It is isolated from power ground to reduce noise and prevent ground loops.  The analogue measurements are not directly dependent on the AGND connection (voltage mode measures the voltage between the AIn+ and the AIn- terminals, and current mode measures the current flowing through the AIn+ and AIn- terminals) – but there are limitations on how much voltage there can be between the AGND and AIn terminals (see the manual for details).

AGND is the terminal that should be used for the shield connection of analogue signal cables, if present.  For current mode, AGND can either be connected to the negative side of the sensor power supply or left disconnected. AGND is meant to be used as a connection for a cable shield or drain wire.  AGND is isolated from GND and power GND.