Commissioning
EL3783
155
Version: 1.1
5.7.5
Common-mode voltage and reference ground (based on
differential inputs)
Common-mode voltage (V
cm
) is defined as the average value of the voltages of the individual connections/
inputs and is measured/specified against reference ground.
Fig. 183: Common-mode voltage (V
cm
)
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 terminal/ box, e.g. at the terminal contacts, power
contacts (cable) or a mounting rail. Please refer to the documentation regarding positioning. The reference
ground should be specified for the device under consideration.
For multi-channel terminals/ boxes 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. 0V power contact: EL3104/EL3114, resistive connection between the channels and AGND; AGND
connected to 0V 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
5.7.6
Dielectric strength
A distinction should be made between:
• Dielectric strength (destruction limit): Exceedance can result in irreversible changes to the electronics
◦ Against a specified reference ground
◦ Differential
• Recommended operating voltage range: If the range is exceeded, it can no longer be assumed that the
system operates as specified
◦ Against a specified reference ground
◦ Differential
