National Instruments NI USB-9219 - Manual (Page 15)

© National Instruments Corporation 15 NI USB-9219 User Guide and Specifications
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 USB-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 for lead wire resistance. Refer to
Figure 14 for an illustration of the connections.
Figure 14. Connections in 2-Wire Resistance and Quarter-Bridge Modes
Half-Bridge and Full-Bridge Modes
Half-Bridge and Full-Bridge modes use the internal voltage excitation to
set the input range of the ADC and return voltage readings that are
proportional to the excitation level. The internal excitation voltage is
nominally 2.5 V but it varies based on the resistance of the sensor. Refer to
the
Specifications section for more information about excitation levels.
In Half-Bridge mode, the HI input is referenced to EX–. In Full-Bridge
mode, the ADC reads the HI and LO inputs differentially. Refer to
Figure 15 for an illustration of the connections.
Figure 15. Connections in Half-Bridge and Full-Bridge Modes
1 The dotted line represents the portion of the circuit that is connected only in
Full-Bridge mode.
LO
R
Wire
Wire I
ADC
I HI
NI USB-9219
HI
LO
EX+
EX– NI USB-9219
R1
R2 R3
R4
ADC
Wire
Wire
1