Measurement Circuitry
Resistance measurement in conductive paths employs a technique that separates the current sourcing leads from the voltage sensing leads. Kelvin probing eliminates the influence of lead and contact resistance by ensuring that no current flows through the measurement circuit.
Voltage Precision
The test instrument drives a known current through two outer probes while two inner probes measure the voltage drop directly at the point of contact. Since the voltmeter has high input impedance, the voltage drop across the kelvin probing leads themselves becomes negligible. This allows for the detection of subtle defects like cold solder joints or thin plating that a standard two wire test would miss.
The physical probes are often designed as concentric tips or closely spaced parallel pins to fit onto a single test pad.
Application Range
Reliable data acquisition occurs most frequently in the range of milliohms to micro-ohms. Kelvin probing stops being the preferred method when the resistance of the path is high enough that lead resistance becomes statistically irrelevant. At very low levels, thermal electromotive forces can introduce errors, so some systems use alternating current or pulsed direct current to null these effects.
The technique is essential for verifying the integrity of high current busbars and ground planes in heavy copper boards.
