Tektronix DMM Series of digital multimeters available from TekMark Australia maintains the convenience of two leads for resistance measurements but delivers the measurement performance of 4-wire methods.
Using two wires to measure resistance is convenient, but can cause measurement error. One can virtually eliminate this error by using four leads and a multimeter with separate source and measure terminals.
Unfortunately, adding additional leads and connections makes the measurement more complicated. A new concept from Tektronix enables 4-wire resistance measurements with just two leads.
Measuring resistance with four wires
Managing two leads can be challenging enough, especially when measuring small components in tight spaces. But using four leads to check a small solder joint, flex connector or chip resistor can be difficult.
Switching lead configurations can lead to swapped banana plugs and measurement mistakes. Also, changing from voltage probes to Kelvin leads and back takes time.
Using two wires to measure voltage does not seriously impact measurement accuracy. Unfortunately in the case of resistance measurements, lead resistance can be a source of significant inaccuracies.
When performing a resistance measurement, the multimeter switches a current source into the measurement loop. The current is driven through the unknown resistance and the multimeter measures the resulting voltage drop.
If there are only two leads, the source current travels on the same path used to measure the voltage drop. The measurement leads are not perfect conductors and have some series resistance of their own.
By driving the current through the measurement leads, one can see not only the voltage drop across the unknown resistance, but also the voltage drop for each lead. The resulting measurement combines the resistance of the positive lead, the unknown resistance and the resistance of the negative lead.
If four leads are used for resistance measurements, the source current and the voltage measurement can be separated. The meter terminals are called ‘Source’ for the current supply and ‘Sense’ for the voltage input.
The series resistance in the source leads does not affect the current flow while the measure/sense leads have almost no current flow because of the meter’s high input impedance.
This means there is no I x R voltage drop in the measure leads. Only the voltage drop across the unknown resistor is measured due to the source current flowing through it.
At the far end of the leads, clips and probes that maintain separation between source and measure signals can deliver 4-wire performance right to the component under test.
A line of clips and probes is available, which brings four wires right up to the point of connection. They include test probes, alligator (Kelvin) clips and tweezers. All of these accessories may be used for measuring 4-wire resistance or voltage.
The 2x4-wire technique available on the Tektronix DMM Series of digital multimeters simplifies resistance measurements without having to change cable configurations or work with complicated cables.
Introducing 2x4 wire ohms measurements
The patented split terminal jacks on the Tektronix digital multimeters maintain the convenience of 2-wire resistance measurements, but deliver the measurement performance of a 4-wire method.
The jacks are completely compatible with standard 4mm banana plugs. But on the inside, each jack is split into two contacts: one source and one measure.
Specially-designed test leads have two conductors per lead with one source and one measure. The leads align with the contacts inside the jacks and carry the separate source while measuring signals over the full length of the leads.