Common-Mode Voltage And Reference Ground (Based On Differential Inputs) - Beckhoff EL3692 Manual

5.4.6 Common-mode voltage and reference ground (based on
differential inputs)
Common-mode voltage (V
inputs and is measured/specified against reference ground.
Fig. 143: Common-mode voltage (V
The definition of the reference ground is important for the definition of the permitted common-mode voltage
range and for measurement of the common-mode rejection ratio (CMRR) for differential inputs.
The reference ground is also the potential against which the input resistance and the input impedance for
single-ended inputs or the common-mode resistance and the common-mode impedance for differential
inputs is measured.
The reference ground is usually accessible at or near the I/O device. Locations for this can be terminal
contacts, power contacts/power line or just a mounting rail.
Please refer to the documentation regarding positioning. The reference ground should be specified for the
device under consideration.
For multi-channel I/O devices with resistive (= direct, ohmic, galvanic) or capacitive connection between the
channels, the reference ground should preferably be the symmetry point of all channels, taking into account
the connection resistances.
Reference ground samples for Beckhoff IO devices:
1. Internal AGND fed out: EL3102/EL3112, resistive connection between the channels
2. 0 V power contact: EL3104/EL3114, resistive connection between the channels and AGND; AGND
connected to 0 V power contact with low-resistance
3. Earth or SGND (shield GND):
◦ EL3174-0002: Channels have no resistive connection between each other, although they are
capacitively coupled to SGND via leakage capacitors
◦ EL3314: No internal ground fed out to the terminal points, although capacitive coupling to SGND
EL3692
) is defined as the average value of the voltages of the individual connections/
cm
)
cm
Version: 3.1
Commissioning
131