National Instruments NI 6238 - Manual (Page 31)

Chapter 4 Analog Input
© National Instruments Corporation 4-5 NI 6238/6239 User Manual
Note that AI GND must always be connected to some voltage level.
AI GND is the reference that NI 6238/6239 devices measure against.
The NI 6238 and NI 6239 are isolated devices 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 6238/6239 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