National Instruments DAQ M Series - Manual (Page 29)

Chapter 4 Analog Input
© National Instruments Corporation 4-5 NI 6236 User Manual
Note that AI GND must always be connected to the ground-reference point
of the circuit being measured. AI GND is the ground-reference point that
NI 6236 devices use to reference their isolated front end.
The NI 6236 is an isolated device with isolation ratings up to
60 VDC/30 Vrms. This allows for current measurement at high voltage
levels provided that the common-mode input range requirement is satisfied.
For example, in Figure 4-4, the node connected to AI GND can be at 50 V
above the earth ground.
Figure 4-4. Current Measurement at High Voltage Levels
Analog Input Ground-Reference Settings
The NI 6236 device measures the voltage generated across the current input
sense resistor during current input measurement. This voltage difference is
then routed into the instrumentation amplifier (PGIA) differentially, as
shown in Figure 4-5.
+
–
AI +
AI –
Isolation
Barrier
AI GND
+
–
Vcm = 50 V