Measuring Cable DC Resistance with the Advanced Cable Tester v2
When testing USB, HDMI, or DisplayPort cables, verifying cable resistance is an important part of ensuring reliable power delivery and overall cable performance. Excessive resistance can indicate problems with a cable’s construction and can lead to voltage drop, inefficient power delivery, or excessive heat during operation. For manufacturers and engineers, measuring resistance provides a way to identify these issues before cables reach the field.
The Advanced Cable Tester v2 evaluates resistance at the pin, wire, and, for USB cables, GND + shield connection levels through its Direct Current Resistance (DCR) test. By measuring resistance across specific conductors and connections, the cable tester helps identify where a cable's construction may fall outside its required specification criteria.
How the Advanced Cable Tester v2 Measures Cable Resistance
The Direct Current Resistance (DCR) test evaluates the resistance of individual conductors and connections within a cable. Depending on the cable and test configuration, the Advanced Cable Tester v2 can measure the resistance of individual power and ground pins, wires from one plug to the other, and, for USB Type-C cables, the SBU and CC lines when present. High-speed data lines are excluded from DCR measurements.
For power delivery, the test verifies that the VBUS and GND conductors meet the resistance requirements needed to carry the cable’s specified current. Excessive resistance can result in greater voltage drop, known as IR drop, and can prevent a cable from delivering power as expected. By measuring resistance at both the individual pin and end-to-end wire levels, the cable tester helps manufacturers identify excessive resistance in specific conductors or connections and verify that the cable meets its electrical requirements.
Four-Wire Resistance Measurement
The Advanced Cable Tester v2 implements a four-wire measurement technique to accurately measure low resistance while minimizing the influence of resistance from the test leads and connections. A known current is passed through the cable while a separate pair of sense connections measures the voltage difference across the points being tested. Using Ohm’s Law, the Advanced Cable Tester v2 calculates resistance from the measured voltage and applied current.
The measurement method varies depending on whether the cable tester is measuring wire or pin resistance:
Pin-Level Contact Resistance
Pin-level resistance measures the resistance of an individual pin or shell contact rather than the entire conductor path. To measure it, the current sink and voltage-sense point are connected through the same pin, while the current source and second voltage-sense point are connected to a group of pins. The cable tester tests each pin in that group and selects the lowest resistance measurement as the most representative reading.
A failing pin-level measurement can indicate a problem localized to that specific contact rather than a defect in the cable’s conductor. Comparing the pin-level result with the corresponding wire-level measurement can help isolate the source of the problem. For example, a passing wire-level result combined with a failing pin-level result may indicate a contact-level issue rather than a problem with the cable’s internal wiring.
Whole-Cable Wire Resistance
Whole-cable wire resistance measures the resistance of a conductor path from one end of the cable to the other, rather than the resistance of an individual contact. To measure it, the Advanced Cable Tester v2 buses all pins assigned to the same net on each side of the cable and applies current across the full path. For example, all four VBUS pins on one connector can be sourced together while the corresponding four VBUS pins on the other connector are used for the voltage measurement.
Bussing the pins in parallel reduces the overall resistance of the net, which is reflected in the expected resistance limits for the wire-level measurement. Shield contacts may also contribute to the measurement, although their effect on the overall resistance is typically minimal.
A failing wire-level measurement is more likely to indicate a problem with the conductor path itself, such as an inappropriate wire gauge, excessive cable length, poor internal connection, or another design or manufacturing defect. Since the measurement evaluates the full path rather than an individual contact, it is less affected by variations in the resistance of any single pin.
Factors Affecting Resistance Measurements
Cable length is not an independent factor in the DCR pass/fail criteria since cable designs typically compensate for length by using different conductor sizes: shorter cables may use thinner wire, while longer cables may use thicker wire to maintain resistance within the specified range. As a result, cables of different lengths can still meet the same resistance requirements when designed appropriately.
Contact quality, connector wear, and cable construction can have a more direct impact on measured resistance. The resistance of an individual contact can vary between insertion events, while wear, surface contamination, and other contact conditions can further affect the reading. To account for this variability, the Advanced Cable Tester v2 takes multiple sense-point measurements for each pin and selects the lowest value rather than relying on a single measurement.
Accounting for Internal Paddleboard Resistance
A cable's internal paddleboard is typically encapsulated in plastic after manufacturing, and therefore can't be measured directly. To account for this, the cable tester uses multiple sense points to identify the measurement point with the least additional resistance from the paddleboard’s internal traces and connector contacts.
The lowest reading corresponds to the sense point closest to where the current and voltage-sensing paths meet near the connector pin. As a result, the measured resistance may be slightly higher than the resistance of the bare paddleboard itself. However, this approach provides a practical and reliable method for identifying manufacturing defects without requiring direct access to the cable’s internal circuitry.
GND + Shield Link Tests
The USB Type-C specification requires that GND and shield be tied together within each plug on a Standard-A to Type-C cable so the shield braid can carry some of the return current and help control electromagnetic interference. The Type-C side handles this cleanly, since GND and shield are bonded directly on the plug's internal paddleboard. The Standard-A side plug design keeps GND and shield as separate connections, and physically tying them together within that plug is difficult. As a result, many cables only have this bond made on the Type-C end.
A standard continuity check can read as "continuous" whether the bond is on the Standard-A side, the Type-C side, or both. The Advanced Cable Tester v2 resolves this with the GND+Shield A-Side Link test, a measurement built specifically to confirm whether the Standard-A side actually has its own bond, rather than relying on the connection at the opposite end of the cable.
If a link test fails, the most likely cause is a missing or poor GND-to-shield bond on the Standard-A side specifically, since that's the harder connection to manufacture reliably. If the shell contact resistance is unusually high, the pass/fail calculation could become unreliable, in which case the Standard-A plug should be physically inspected.
Reading a DC Resistance Report from the Advanced Cable Tester v2
Once you understand what pin, wire, and (for USB Type-C and Standard-A cables) link measurements are evaluating, interpreting the Advanced Cable Tester v2’s DCR report is straightforward. The report uses a consistent set of columns to present the results, regardless of the cable type or module being tested.
Each row in a DC Resistance report includes:
- Status: The pass/fail result for that specific row.
- Group: A high-level category for the measurement, such as VBUS pins or GND/Shield.
- Label: A human-readable name identifying the specific pin or wire being measured.
- Sources: The pin or pins used as the current source for that measurement.
- Sinks: The pin or pins used as the current sink (and sense point).
- Expected Min (Ω) and Expected Max (Ω): The acceptable resistance range for that row, drawn from the governing cable specification.
- Measured (Ω): The actual resistance value recorded during the test.
Below is an example of a passing DCR test on a USB-C cable:
Conclusion
Accurate DCR testing provides manufacturers and engineers with a reliable way to evaluate cable construction, identify contact and conductor issues, and verify that cables meet their required electrical specifications. By combining four-wire measurements with pin-, wire-, and link-level testing, the Advanced Cable Tester v2 provides the detailed measurements needed to identify potential defects and validate cable performance before products reach the field.
For more information on our tools, including those that support I2C, SPI, CAN, and USB, please email us at sales@totalphase.com or request a demo.