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SCXI-1321 Terminal Block Installation Guide 12 ni.com
Figure 5. Full-Bridge Nulling Circuit
Shunt Calibration
Shunt calibration provides a method of adjusting for gain error to improve
accuracy. The shunt-calibration circuitry configuration places a shunting
resistor in parallel with the resistive element connected between EX+ and
CH+ (element R4) of the Wheatstone bridge gauge, as shown in Figure
5.
Shunt calibration circuits of each channel are independent from each other,
although they are controlled together in software; therefore, when SCAL is
engaged on a channel, all the shunt switches of the channels are closed.
When SCAL is disengaged, all the switches are open. At startup or reset,
all switches are open by default. You can control SCAL with the NI-DAQ
function
SCXI_Calibrate_Setup. Set the Cal_Op parameter to 2 for
engaged or
0 for disengaged.
You can use LabVIEW to take measurements from channels with the shunt
resistors connected by using the SCXI channel string
ob
x
! sc
y
! md
z
! shunt
w
.
where
x is the onboard channel number (0 for single chassis systems)
y is the chassis ID (1 by default)
CH+CH+
CH–
EX+
EX–
+
–
+
–
R
SCAL
301 kΩ
R
Tr im
10 kΩ
R
Null
39.1 kΩ
(Screw Adjusts Potentiometer)
R
L
R
L
R
L
R
4
(gauge)
EX+
CH+
CH+
CH–
EX+
EX–
R
Tr im
SCXI-1121 SCXI-1321 Transducer
V
EX
R
L
CH–
R
1
(gauge)
R
2
(gauge)
R
3
(gauge)