Siemens Simantic ET 200S - Page 145

Analog electronic modules
4.3 Basics of Analog Value Processing
ET 200S
Manual, Edition 10/2005, A5E00514527-01
4-15
Analog Input Modules
In the case of the analog input modules there is galvanic isolation:
• Between the logic circuitry and backplane bus
• Between the load voltage and the channels. The following differences exist:
– No isolation: Link between M
ANA
and central grounding point
– Isolation: No link between M
ANA
and the central grounding point (U
ISO
)
Analog Output Modules
In the case of the analog output modules there is generally galvanic isolation:
• Between the logic circuitry and backplane bus
• Between the load voltage and M
ANA.
Note
Ensure that this potential difference U
ISO
does not exceed the permitted value. If there is
a possibility of exceeding the permitted value, make a connection between terminal M
ANA
and the central grounding point.
Connecting measuring sensors to analog inputs
Between the measuring lines M- of the input channels and the reference point of the
measuring circuit M
ANA
there can be only a limited potential difference U
CM
(common-mode
voltage). To ensure that the permitted value is not exceeded, you must take different steps
depending on the whether the sensors are isolated or non-isolated. The steps you have to
take are described in this chapter.
Generally speaking, however, when connecting two-wire measuring transducers for current
measurement and when connecting resistance-type sensors, you should not make a
connection from M- to M
ANA
. This also applies to inputs that have been assigned parameters
correspondingly but unused.
Note
For the analog input modules 2AI U, 2AI RTD, and 2AI TC, you must short-circuit unused
analog inputs.
Abbreviations used
The meanings of the abbreviations in the figures below are as follows:
M + Measuring line (positive)
M - Measuring line (negative)
M
ANA
Reference potential of the analog measuring circuit
M Frame connection
L + Rated load voltage 24 VDC
U
CM
Potential difference between inputs and reference potential of the measuring
circuit M
ANA
U
ISO
Potential difference between M
ANA
and central grounding point