National Instruments SCXI -1122 - Manual (Page 41)

Theory of Operation Chapter 4
SCXI-1122 User Manual 4-6 © National Instruments Corporation
gauge resistor–to complete the bridge network. Connect this resistor in your SCXI-1322
terminal block between the CH+ and VEX- terminals.
The current output channel is provided for transducers–such as thermistors and RTDs–which
need a current excitation to operate properly. The current output has a value of 1 mA and has a
maximum permissible load of 5 k
Ω
. If you connect loads greater than 5 k
Ω
, the current source
will lose regulation. When connecting several loads which need current excitation, you have two
possible approaches. The first is to connect them all in series (as long as they do not exceed
5 k
Ω
total) and use the two-wire scan mode as shown in Figure 4-2 or use the four-wire scan
mode and have them connected as shown in Figure 4-3.
CH+0
CH-0
CH+1
CH-1
CH+2
CH-2
CH+15
CH-15
IEX+
IEX-
R
1
R
2
R
3
R
15
RT = R1 +...+R15
≤
5 k
Ω
Figure 4-2. Series Connection with Current Excitation
CH+8
CH+0
CH-0
CH-8
CH+9
CH+1
CH-1
CH+15
CH+7
CH-7
CH-15
CH-9
R
1
R
2
R
3
Any R
≤
5 k
Ω
Figure 4-3. Four-Wire Scan Connection with Multiplexed Current Excitation