Ultra-low Impedance Testing Using the 2-Port Shunt-Through Measurement Technique

Measuring ultra-low impedances (20 micro-ohm) is a challenge for any power distribution network (PDN) design engineer. The 2-port shunt-through measurement is the standard method for measuring milliohm impedances up to very high frequencies (GHz). Unfortunately, this measurement includes an undesirable ground loop related to the instrument grounds and test setup cabling. The ground loop introduces significant errors if proper care is not taken. This application note shows how to measure as low as 20 μΩ using the Picotest J2102A as a ground loop breaker.

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What is a PCB?

One of the major concepts in electronics is the printed circuit board or PCB. It’s so fundamental that people often forget to explain what a PCB is. Electronics have come a long way. A journey that started with vacuum tubes and relays got its way into silicon and ICs. It became more prevalent in consumer goods. Therefore, there is always pressure to reduce the size and manufacturing costs of electronic products. This drove manufacturers to look for better solutions.

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Measuring PCB, Cable and Interconnect Impedance, Dielectric Constant, Velocity Factor, and Lengths

Controlled impedance printed circuit boards (PCBs) often include a measurement “coupon”, which typically includes sample traces, 6 inches long and constructed as part of the printed circuit board panel.  They are measured to assure the PCB stack-up and the accuracy of the PCB transmission lines. These coupons, as well as the actual PCB signal traces, interconnects and cables are typically measured using a Time Domain Reflectometer (TDR).  A TDR is generally a large, expensive instrument that includes a high-speed edge pulse and a sampling oscilloscope.

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