Minimizing Voltage Drop Loss when Cabling

Voltage drop 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. Instruments with remote sense capabilities can compensate for voltage drop 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.
  • Use the thickest wire gauge appropriate for your application. NI recommends 18 AWG or lower.

To reduce noise picked up by the cables that connect the instrument to a load, twist each wire pair. Refer to the following table to determine the wire gauge appropriate for your application.

Caution Use wire that is thick enough to avoid overheating if the output current from the power supply or SMU were to short circuit.
Table 9. Wire Gauge and Noise
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 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:

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.

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.
  • Keep the current return path as close as possible to the current source path by using twisted pair cabling.

To reduce the susceptibility of low currents to noise and other unwanted interfering signals:

  • Use shielded cables, such as coaxial cables.
  • Connect the outer conductor of the shielded cable to the common or ground terminal of the channel.

To reduce the effects of leakage currents:

  • Use shielded cables, such as triaxial cables.
  • Connect the Guard terminal to the inner shield of the cable and Output LO to the outer shield.