Siemens Simatic S7-400 - Manual (Page 260)

Analog Modules
5-44
S7-400, M7-400 Programmable Controllers Module Specifications
A5E00069467-07
Abbreviations and Mnemonics Used in the Figures Below
The abbreviations and mnemonics used in the figures below have the following
meanings:
M +: Measuring line (positive)
M –: Measuring line (negative)
M
ANA
: Reference potential of the analog measuring circuit
U
CM
: Potential difference between inputs and reference potential of
the M
ANA
measuring circuit
U
ISO
: Potential difference between M
ANA
and chassis ground
Connection of Isolated Measuring Sensors
The isolated sensors are not connected with the local ground potential (local
ground). They can be operated free of potential.
With isolated sensors, potential differences might arise between the different
sensors. These potential differences can arise as a result of interference or the
local distribution of the sensors.
To ensure that the permissible value for U
CM
is not exceeded during use in heavily
EMC-affected environments, connect M– to M
ANA
in modules with an M
ANA
connection.
M+
M–
U
ISO
M+
M–
M
ANA
Connection required
for modules with
M
ANA
Chassis ground
Isolated
sensors
Figure 5-7 Connecting Isolated Sensors to an Isolated AI
Note
Do not connect M– to M
ANA
when connecting two-wire transmitters for current
measurement and when connecting resistance-type sensors. This also applies to
inputs which are not used.