Siemens S5-135U/155U System Manual page 471

The following figure shows the connection of sensors to the module. For
measurement with respect to ground, all the M- connection points are
interconnected internally on the module.
U E2 +
–
+
U E1
U E1/2: Input Voltage
U ISO: Potential Difference
: Equipotential; this potential is established by the sensor reference potential
(external reference potential)
Figure 9-30 Connecting Sensors to the 466 Analog Input Module (Measurement with Respect to Ground)
Differential The differential measurement is a method of compensating for interference
Measurement
on the line. Each signal source is assigned its own signal reference line. The
differential measurement between signal line and signal reference line thus
compensates for the interference acting on both lines.
You must also short-circuit the unused channels with this measuring method
(jumper between M+ and M-).
Differential measurement is required:
System Manual
C79000-G8576-C199-06
Analog Input Module
M+
M+
MUX
M–
–
U ISO
if the sensors are at different potentials;
if various signal sources are not close to each other;
if there is a need to measure signals with high precision;
and when a high level of interference is expected.
Analog Input/Output Modules
0 V Bus
9-89