The simplest and most effective method for eliminating a ground loop is to add a wideband common mode transformer to the measurement, such as the Picotest J2102B Common Mode Transformer. The transformer for a low impedance PDN measurement must have very wide bandwidth, low loss and tight coupling, otherwise, the results will not be accurate over the measured frequency range. It is also important to maintain the 50 Ohm transmission line impedance through this transformer. The J2102B accomplishes this with ease.The J2102B paired with the Picotest PDN Cable is the best general solution. The J2113A is a better choice if you need to measure below 3kHz and below a maximum frequency of 500kHz. Due to the limited CMRR, the J2113A is not recommended below 1mOhm. If other cables are used with either solution, there may be significant degradation in measurement accuracy below 1MHz due to poor cable shield resistance and isolation.