Beckhoff EL3773 Documentation page 127

◦ Beckhoff terminals/ boxes (and related product groups) always feature electrical isolation
between the field/analog side and the bus/EtherCAT side. In other words, if two analog
terminals/ boxes are not connected via the power contacts (cable), the modules are effectively
electrically isolated.
◦ If channels within a module are electrically isolated, or if a single-channel module has no
power contacts, the channels are effectively always differential. See also explanatory notes
below. Differential channels are not necessarily electrically isolated.
• Analog measuring channels are subject to technical limits, both in terms of the recommended operating
range (continuous operation) and the destruction limit. Please refer to the respective terminal/ box
documentation for further details.
Explanation
• differential (DIFF)
◦ Differential measurement is the most flexible concept. The user can freely choose both
connection points, input+/signal potential and input-/reference potential, within the framework
of the technical specification.
◦ A differential channel can also be operated as SE, if the reference potential of several sensors
is linked. This interconnection may take place via the system GND.
◦ Since a differential channel is configured symmetrically internally (cf. Fig. SE and DIFF module
as 2-channel variant), there will be a mid-potential (X) between the two supplied potentials that
is the same as the internal ground/reference ground for this channel. If several DIFF channels
are used in a module without electrical isolation, the technical property V
voltage) indicates the degree to which the mean voltage of the channels may differ.
◦ The internal reference ground may be accessible as connection point at the terminal/ box, in
order to stabilize a defined GND potential in the terminal/ box. In this case it is particularly
important to pay attention to the quality of this potential (noiselessness, voltage stability). At
this GND point a wire may be connected to make sure that V
differential sensor cable.
If differential channels are not electrically isolated, usually only one V
channels are electrically isolated this limit should not apply, and the channels voltages may
differ up to the specified separation limit.
◦ Differential measurement in combination with correct sensor wiring has the special advantage
that any interference affecting the sensor cable (ideally the feed and return line are arranged
side by side, so that interference signals have the same effect on both wires) has very little
effect on the measurement, since the potential of both lines varies jointly (hence the term
common mode). In simple terms: Common-mode interference has the same effect on both
wires in terms of amplitude and phasing.
◦ Nevertheless, the suppression of common-mode interference within a channel or between
channels is subject to technical limits, which are specified in the technical data.
◦ Further helpfully information on this topic can be found on the documentation page
Configuration of 0/4..20 mA differential inputs (see documentation for the EL30xx terminals, for
example).
• Single Ended (SE)
◦ If the analog circuit is designed as SE, the input/reference wire is internally fixed to a certain
potential that cannot be changed. This potential must be accessible from outside on at least
one point for connecting the reference potential, e.g. via the power contacts (cable).
◦ In other words, in situations with several channels SE offers users the option to avoid returning
at least one of the two sensor cables to the terminal/ box (in contrast to DIFF). Instead, the
reference wire can be consolidated at the sensors, e.g. in the system GND.
◦ A disadvantage of this approach is that the separate feed and return line can result in voltage/
current variations, which a SE channel may no longer be able to handle. See common-mode
interference. A V
wired' through the input/reference potential.
EL3773
effect cannot occur, since the module channels are internally always 'hard-
CM
Version: 2.5
Commissioning
(common-mode
CM
is not exceeded in the
CM,max
is permitted. If the
CM, max
127