7
Voltage Drops
The voltage output of the SCXI-1124 can drive up to 5 mA. When you use the SCXI-1124 in
voltage mode with the TBX-1325, a small amount of voltage drop will develop across the
positive/negative channel wires in the SH48-48-A cable and across any other wires you may
have connected to this channel. This voltage drop is due to the wire resistance and the current
flow in these leads. Voltage drop is not a concern if you are using the SCXI-1124 in the current
mode.
To reduce errors in your output voltage, first you need to calculate the voltage drop across the
SH48-48-A cable. This voltage drop results in an error in your measurement and depends on the
cable length and on the load impedance and configuration. Solutions to this problem include:
•
Locate the load closer to the SCXI-1124 by using shorter cable lengths.
•
Use heavy-gauge wire to connect to the TBX-1325.
•
Add a high-impedance input buffer near the load.
•
Calibrate your system to account for the resistive losses of the cable.
The SH48-48-A cable has a resistance of 0.21
Ω
/m. To determine the amount of error introduced
by the cable, do the following:
1. Calculate the total resistance (R
TL
) of the cable based on the cable length.
2. Determine the amount of current (I
L
) being drawn from the SCXI-1124.
3. Use the following formula to determine the total voltage drop (V
drop
) in the positive/negative
channel leads:
V
drop
= R
TL
x I
L
For example, if you have a 1 m SH48-48-A cable and a 5 mA load, then your calculations from
steps 1 through 3 would be:
1. R
TL
= 2 x 0.21
Ω
/m x 1 m = 0.42
Ω
; notice that you multiply the cable length by 2 to take
into consideration both the positive and negative channel lead resistances.
2. I
L
= 5 mA, which is the total current draw from the SCXI-1124.
3. V
drop
= 2.1 mV
⇔
0.042% of full scale at 0 to 5 V range.
You then need to calculate the voltage drop across the field signal wires you are connecting to
the TBX-1325. For example, 30 m of 22 AWG stranded copper wire has about 1.5
Ω
of
resistance. Thus, driving a load 15 m away at 5 mA will produce an error at the load of 7.5 mV.
Add this error amount to the voltage drop across the SH48-48-A cable to get a total voltage drop
of 9.6 mV.
