Cabling and Current-Resistance Loss
Current-resistance loss is introduced by the cabling wires that connect the power supply or SMU to the load terminals. The voltage drop due to current-resistance loss is determined by the resistance of the cabling wire (a property of the wire gauge and length) and the amount of current flowing through the wire. Devices with remote sense capabilities can compensate for current-resistance loss by measuring the voltage across the load terminals with a second set of leads that do not carry a significant current.
To minimize voltage drop caused by cabling, keep each wire pair as short as possible and use the thickest wire gauge appropriate for your application. The lower the American Wire Gauge (AWG) rating, the thicker the wire. NI recommends 18 AWG or lower.
To reduce noise picked up by cabling connecting a power supply or SMU to a load, twist each wire pair. Refer to the following table to determine the wire gauge appropriate for your application.
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Caution Use wire that is thick enough to avoid overheating if the output current from the power supply or SMU were to short circuit. |
| AWG Rating | mΩ/m (mΩ/ft) |
|---|---|
| 10 | 3.3 (1.0) |
| 12 | 5.2 (1.6) |
| 14 | 8.3 (2.5) |
| 16 | 13.2 (4.0) |
| 18 | 21.0 (6.4) |
| 20 | 33.5 (10.2) |
| 22 | 52.8 (16.1) |
| 24 | 84.3 (25.7) |
| 26 | 133.9 (40.8) |
| 28 | 212.9 (64.9) |
Calculating Maximum Voltage Drop
When cabling a power supply or SMU to a constant load, be sure to account for voltage drop in your application. If necessary, adjust the output voltage of the device or, if available, use remote sensing.
Use the amount of current flowing through the cabling wires and the resistance of the wires to calculate the total voltage drop for each load, as shown in the following example:
Example
Operating within the recommended current rating, determine the maximum voltage drop across a 1 m, 16 AWG wire carrying 1 A:
V = I × R
V = 1 A × (13.2 mΩ/m × 1 m)
V = 13.2 mV
As illustrated in the preceding example, a 1 m, 16 AWG wire carrying 1 A results in a voltage drop of 13.2 mV.
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Note When calculating voltage drop for a pair of wires, multiply the voltage drop by two. Thus, the total voltage drop for a pair of wires in the previous example is 26.4 mV. To compensate for the voltage drop across the wire pair and ensure the correct power is supplied to the load, increase the output voltage of the power supply by 26.4 mV, or if available, use remote sensing. |
Cabling for Low-Level Measurements
Low-level measurements require tight control over system setup and cabling. Long cables and large current loops degrade source and measurement quality even in low-noise environments.
To maintain measurement quality, always limit the length of the cables involved in your system setup. Also, keep the current return path as close as possible to the current source path by employing twisted pair cabling.
To reduce the susceptibility of low currents to noise and other unwanted interfering signals, use shielded cables (for example, coaxial cable). Connect the outer conductor to the common or ground terminal of the channel.