National Instruments NI 9219 - Manual (Page 15)

© National Instruments Corp. 15 NI 9219 Operating Instructions
lead wires have the same resistance. A gain of 2x is applied to the
voltage across the negative lead wire and the ADC uses this voltage
as the negative reference to cancel the resistance error across the
positive lead wire. Refer to Figure 8 for an illustration of the
connections.
Figure 8. Connections in 3-Wire RTD Mode
2-Wire Resistance and Quarter-Bridge Modes
In 2-Wire Resistance and Quarter-Bridge modes, connect the
two ends of the resistor or gauge to the NI 9219 across the HI and
LO terminals. These modes source a current, which varies based on
the resistance of the load, between the HI and LO terminals. The
resulting resistance is computed from the voltage measurement.
2-Wire Resistance and Quarter-Bridge modes do not compensate
LO
EX+
EX–
RTD
Wire
Wire
Wire
I
x2
ADC
I
I
NI 9219