National Instruments NI 9219 - Manual (Page 14)

NI 9219 Operating Instructions 14 ni.com
4-Wire Resistance and 4-Wire RTD Modes
4-Wire Resistance and 4-Wire RTD modes source a current, which
varies based on the resistance of the load, between the EX+ and
EX– terminals. The measured resistance is computed from the
resulting voltage reading. These modes are not affected by lead
wire resistance because a negligible amount of current flows across
the HI and LO terminals due to the high input impedance of the
ADC. Refer to Figure 7 for an illustration of the connections.
Figure 7. Connections in 4-Wire Resistance and 4-Wire RTD Modes
3-Wire RTD Mode
3-Wire RTD mode sources a current, which varies based on the
resistance of the load, between the EX+ and EX– terminals. This
mode compensates for lead wire resistance in hardware if all the
LO
HI
EX–
RTD/
Resistor
Wire
Wire
Wire
I
ADC
I EX+ Wire
NI 9219