Introduction to Four-Terminal Sensing

Four-terminal sensing, also known as Kelvin sensing, is a highly accurate method for measuring low resistances in advanced printed circuit boards (PCBs). This technique is particularly useful in applications where precise resistance measurements are critical, such as in high-current circuits, power electronics, and precision instrumentation.

In traditional two-terminal sensing, the resistance measurement includes the resistance of the test leads and contact resistances, which can introduce significant errors when measuring low resistances. Four-terminal sensing eliminates these errors by using separate pairs of current-carrying and voltage-sensing leads.

How Four-Terminal Sensing Works

Four-terminal sensing works by using four separate connections to the device under test (DUT). Two of these connections, known as the “force” or “current-carrying” leads, are used to supply a known current to the DUT. The other two connections, known as the “sense” or “voltage-sensing” leads, are used to measure the voltage drop across the DUT.

By using separate pairs of leads for current and voltage measurement, the voltage drop across the test leads and contact resistances is effectively eliminated from the measurement. This allows for highly accurate resistance measurements, even for very low resistances.

Advantages of Four-Terminal Sensing

1. High accuracy: By eliminating the effects of test lead and contact resistances, four-terminal sensing provides highly accurate resistance measurements, even for very low resistances.

2. Improved repeatability: Four-terminal sensing reduces the influence of variations in test lead and contact resistances, resulting in more consistent and repeatable measurements.

3. Wide measurement range: This technique can measure resistances ranging from micro-ohms to several ohms, making it suitable for a wide range of applications.

4. Reduced measurement errors: Four-terminal sensing minimizes errors caused by temperature variations, lead resistance changes, and contact resistance fluctuations.

Applications of Four-Terminal Sensing in Advanced PCBs

Power Electronics

In power electronic applications, such as switch-mode power supplies and motor drives, accurate resistance measurements are essential for ensuring proper current sharing among parallel components and minimizing power losses. Four-terminal sensing allows for precise measurement of low resistances in high-current paths, enabling designers to optimize circuit performance and reliability.

Precision Instrumentation

Precision instrumentation, such as digital multimeters and LCR meters, rely on accurate resistance measurements to provide reliable results. Four-terminal sensing is often employed in these instruments to measure low resistances with high accuracy, ensuring the integrity of the measurement data.

High-Current Circuits

In high-current circuits, such as battery management systems and welding equipment, accurate resistance measurements are crucial for monitoring the health of electrical connections and detecting potential faults. Four-terminal sensing enables precise measurement of low resistances in high-current paths, helping to ensure the safe and efficient operation of these systems.

Implementing Four-Terminal Sensing in PCB Design

PCB Layout Considerations

When designing a PCB for four-terminal sensing, several layout considerations must be taken into account to ensure accurate and reliable measurements:

1. Kelvin connections: The force and sense leads should be connected as close as possible to the DUT to minimize the influence of lead resistance and contact resistance.

2. Lead routing: The force and sense leads should be routed in parallel and kept as short as possible to reduce the effects of lead inductance and capacitance.

3. Ground planes: Adequate ground planes should be provided to minimize the influence of ground loops and electromagnetic interference (EMI) on the measurement.

4. Component placement: Components should be placed strategically to minimize the distance between the DUT and the four-terminal sensing connections.

Test Point Design

Test points for four-terminal sensing should be designed to facilitate accurate and repeatable measurements. Some best practices for test point design include:

1. Kelvin Test points: Use dedicated Kelvin test points that provide separate connections for the force and sense leads.

2. Test point size: Test points should be large enough to accommodate the test probes and ensure reliable contact.

3. Test point spacing: Adequate spacing should be provided between test points to prevent shorting and minimize the influence of adjacent components.

4. Test point labeling: Test points should be clearly labeled to indicate their function and facilitate accurate measurements.

Best Practices for Four-Terminal Sensing Measurements

Test Equipment Selection

When performing four-terminal sensing measurements, it is essential to use appropriate test equipment to ensure accurate and reliable results. Some factors to consider when selecting test equipment include:

1. Measurement range: The test equipment should have a measurement range that covers the expected resistance values of the DUT.

2. Accuracy: The test equipment should provide sufficient accuracy for the desired measurement resolution.

3. Test lead compatibility: The test leads should be compatible with the test equipment and the DUT, and should be designed for four-terminal sensing applications.

4. Calibration: The test equipment should be regularly calibrated to ensure accurate measurements.

Measurement Techniques

To obtain accurate and repeatable four-terminal sensing measurements, several techniques should be employed:

1. Test lead compensation: The resistance of the test leads should be measured and compensated for to minimize their influence on the measurement.

2. Contact resistance minimization: The contact resistance between the test probes and the DUT should be minimized by ensuring clean, secure connections and using appropriate test probes.

3. Averaging: Multiple measurements should be taken and averaged to reduce the influence of noise and random errors.

4. Temperature control: The temperature of the DUT and the test environment should be controlled to minimize the effects of temperature variations on the resistance measurement.

FAQ

1. Q: What is the difference between two-terminal and four-terminal sensing?
   A: Two-terminal sensing measures the resistance of the DUT, test leads, and contact resistances, while four-terminal sensing eliminates the influence of test lead and contact resistances by using separate pairs of current-carrying and voltage-sensing leads.

2. Q: What are the advantages of using four-terminal sensing for resistance measurements?
   A: Four-terminal sensing provides high accuracy, improved repeatability, a wide measurement range, and reduced measurement errors compared to two-terminal sensing.

3. Q: In which applications is four-terminal sensing commonly used?
   A: Four-terminal sensing is commonly used in power electronics, precision instrumentation, and high-current circuits where accurate resistance measurements are critical.

4. Q: What are some important PCB layout considerations for four-terminal sensing?
   A: PCB layout considerations for four-terminal sensing include using Kelvin connections, proper lead routing, adequate ground planes, and strategic component placement.

5. Q: What are some best practices for obtaining accurate four-terminal sensing measurements?
   A: Best practices for accurate four-terminal sensing measurements include selecting appropriate test equipment, compensating for test lead resistance, minimizing contact resistance, averaging multiple measurements, and controlling the temperature of the DUT and test environment.

Conclusion

Four-terminal sensing is a powerful technique for accurately measuring low resistances in advanced PCBs. By using separate pairs of current-carrying and voltage-sensing leads, this method eliminates the influence of test lead and contact resistances, providing highly accurate and repeatable measurements.

Implementing four-terminal sensing in PCB design requires careful consideration of layout, test point design, and measurement techniques. By following best practices and using appropriate test equipment, designers can ensure accurate and reliable resistance measurements in a wide range of applications, from power electronics to precision instrumentation.

As PCB technology continues to advance, the importance of accurate resistance measurements will only continue to grow. By understanding and applying the principles of four-terminal sensing, designers can create high-performance, reliable electronic systems that meet the demands of today’s increasingly complex applications.