![background image](http://html.mh-extra.com/html/national-instruments/tbx-1328/tbx-1328_manual_710238017.webp)
3.
Use the following formula to determine the total voltage drop (
V
drop
)
in the SH32-32-A cable excitation leads:
For example, if you have a 1 m SH32-32-A cable, 120
Ω
full-bridge strain
gauge, and
V
ex
= 3.333 V, then the calculations from steps 1 through 3 are:
1.
R
TL
= 2 × 0.21
Ω
/m × 1 m = 0.42
Ω
; multiply the cable length by two
to take into consideration both the
V
ex
+ and
V
ex
– lead resistances.
2.
R
SG
= 120
Ω
is the total equivalent bridge resistance as seen from the
V
ex
+/– terminals of the TBX-1328.
3.
V
drop
= 11.6 mV, which is 0.3% of the 3.333 V excitation.
Now calculate the voltage drop across the field signal wires you are
connecting to the TBX-1328. Perform similar calculations for the field
wires as you did for the cable. Resistance can vary depending on the cable
and field wires. Add this error amount to the voltage drop across the
SH32-32-A cable to get a total voltage drop.
You can compensate for this error along with any additional cable lead
resistance introduced by the strain-gauge connection wires. One simple
way of compensation is to calculate the lead resistance, then input it along
with the other strain-gauge parameters into the conversion formula
provided in your software applications, such as LabVIEW and
Measurement Studio. To minimize resistive compensation, move the load
closer to the SCXI module by using shorter cable lengths, or use
heavy-gauge wire to connect to the TBX-1328.
V
drop
R
TL
R
TL
R
SG
+
------------------------
V
ex
×
=