Siemens SIMATIC Manual page 242

Principles of analog value processing
4.2 Wiring and connecting transducers to analog inputs
Wiring non-isolated transducer to electrically isolated AI
When connecting non-isolated transducers to electrically isolated modules, the CPU / IM 153
can be operated in grounded or ungrounded mode.
Figure 4-3
242
Wiring non-isolated transducer to electrically isolated AI
S7-300 Module data
Manual, 06/2017, A5E00105505-AJ