Low-Resistance Testing Approach Utilizing the Two-Port Short-Circuit Measurement Method
Measuring Ultra-Low Impedances with Precision: A Guide to Minimizing Errors
Measuring impedances on the order of 20 micro-ohms is a challenging task due to factors such as contact resistance, lead inductance, ground loops, cable effects, thermal EMFs, and noise. However, with the right setup and techniques, it is possible to achieve accurate results. In this article, we'll discuss best practices for measuring ultra-low impedances using the Picotest J2102A and Keysight E5061B.
Optimizing the Measurement Setup
The first step in minimizing measurement errors is to optimize the setup. Connect the Device Under Test (DUT) in a 2-port shunt-through mode to measure the s-parameters, from which you derive the ultra-low impedance. This configuration minimizes the impact of lead and fixture inductance, especially at low frequencies. Additionally, the Picotest J2102A can be used as a ground loop breaker, placed in the path such that it isolates the VNA receiver from the source ground, without introducing significant series resistance or inductance.
Cable and Connector Considerations
Using short, low-loss, phase-stable, and low-thermal EMF cables is crucial. Employ phase-stable calibration cables (e.g., coaxial cables designed for VNA measurements). Keep them as short as possible to reduce loss and phase noise. High-quality connectors and ensuring tight, stable connections are also essential, as oxidation or slight looseness can add contact resistance on the order of milliohms, swamping the micro-ohm-level signals. Twist and isolate cables to minimize inductance and EMI coupling.
Calibration and Measurement Technique
Perform a thorough 2-port calibration using a suitable calibration kit to calibrate the VNA directly at the measurement plane. Preferably perform a full two-port SOLT (Short-Open-Load-Thru) or LRRM/TRL calibration to remove systematic errors. If possible, calibrate with the J2102A and test fixture in place to compensate for its parasitics. Use a proper de-embedding or error correction for the fixture, measuring or modeling the fixture (including the J2102A) to de-embed their impedance from the results. This is crucial if the J2102A introduces small but non-negligible offsets. Use low test power levels and narrow IF bandwidth on the VNA to minimize noise and thermal EMF effects. Use averaging to improve SNR without causing DUT heating.
Mechanical and Thermal Stabilization
Rigidly fix the DUT and test fixture to avoid mechanical vibrations, as vibrations can cause fluctuating contact resistances and noise. Allow thermal equilibrium, as thermal EMFs can cause millivolt offsets corresponding to micro-ohm errors. Minimize temperature gradients and gradients in the wiring.
Specific Tips for Ultra-Low Impedance (<100 µΩ) Measurements
Ensure the measurements take proper “force” and “sense” paths to avoid lead and contact resistance effects. The VNA test port cables and the DUT connection should replicate a Kelvin connection as much as practical. Check the J2102A insertion effect, as the J2102A introduces a small parasitic impedance. Characterize its frequency-dependent impedance (using a known standard) and mathematically remove this from the measurement results. Use guard shields and ground planes to minimize capacitive coupling and noise pickup.
Measurement Verification
Validate the setup on known standards or precision low-ohm resistors. Use standards with known ultra-low ohmic values to confirm measurement accuracy. Repeatedly measure reference components to check repeatability. Perform control tests with and without the J2102A to understand its effect and optimize placement.
Summary
To minimize measurement errors in 20 µΩ impedance measurements using the Picotest J2102A and Keysight E5061B:
- Use a well-engineered 2-port shunt-through configuration with Kelvin connections.
- Insert the Picotest J2102A where it breaks ground loops without adding significant parasitic impedance.
- Perform a rigorous, full 2-port VNA calibration with the test fixture and J2102A included.
- Use short, phase-stable cables and high-quality connectors.
- Thermally stabilize and mechanically fix the setup.
- Characterize and de-embed fixture and J2102A parasitics.
- Use low VNA test power, narrow IF bandwidths, and averaging to reduce noise.
- Verify with ultra-low ohm reference standards.
By carefully combining these techniques, you will suppress sources of error and measure ultra-low impedances accurately with your VNA and ground loop breaker setup. The ground loop introduces significant errors if proper care is not taken. The Picotest DTBK01 kit includes most standard sizes and includes de-embedding data to remove parasitics of the PCB from the measurement. In most cases, the J2102A is a better choice for measuring ultra-low impedance (20 μΩ). Picotest offers two solutions to minimize common mode current: the J2102A common mode transformer and the J2113A semi-floating differential amplifier. In this article, a 4 wire Kelvin measurement is used to verify the measurement sample at DC. Picotest cables are made especially for accurate, 2-port impedance measurement. Picotest cables provide the best compromise for ultra-low impedance measurements in terms of loss, flexibility, and shielding characteristics. The 2-port shunt-through measurement is a standard method for measuring milliohm impedances up to very high frequencies (GHz). The Picotest PCK01 is a high-performance cable and connector kit. The common mode current should be minimized to minimize the measurement error. The 2-port shunt-through measurements suffer from non-idealities, including a ground loop. Figure 7 shows the comparison of the measured impedance with and without the Picotest ground loop isolator J2102A while measuring an 88 μΩ resistance. The Picotest J2102A is used to measure impedances at and above 20 μΩ in a 2-port shunt-through configuration. Picotest ground isolators are effective in eliminating ground loops in various instruments and test setups. The J2113A is effective down to DC, while the J2102A is effective at, or even below, 100 Hz.
1. For accurate results in low impedance measurements, science and technology collaborate to create solutions such as using the Picotest J2102A and Keysight E5061B for minimizing errors. This requires the optimized setup, which utilizes a 2-port shunt-through configuration, employs short, low-loss, phase-stable cables, and high-quality connectors, and rigorously calibrates the VNA with the test fixture and J2102A taken into account.
2. To minimize errors associated with ground loops, technology offering ground loop breakers like the Picotest J2102A can be implemented into the precise setup for measuring ultra-low impedances, functioning as an isolator in the path and breaking the ground loop without adding significant series resistance or inductance.
