National Instruments NI USB-9219 - Manual (Page 14)

NI USB-9219 User Guide and Specifications 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 12 for an illustration of the
connections.
Figure 12. 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 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 13 for an
illustration of the connections.
Figure 13. Connections in 3-Wire RTD Mode
LO
HI
EX–
RTD/
Resistor
Wire
Wire
Wire
I
ADC
I EX+ Wire
NI USB-9219
LO
EX+
EX–
RTD
Wire
Wire
Wire
I
x2
ADC
I
I
NI USB-9219