Previous videos in this series discussed the use of vector network analyzers (VNAs) to measure impedance using one- or two-port configurations. This video discusses another method of measuring impedance–the current injector method. The current injector is a voltage-controlled current source. In the impedance measurement, the current injector is modulated by the VNA, which also measures the resulting voltage at the device under test as well as the modulated current. The VNA calculates voltage/current to obtain impedance.
Although not quite as accurate as the two-port VNA impedance measurement, the current injection technique has advantages including wide range (approx. 1 milli-ohm to thousands of ohms), the ability to measure in-system, and a suitability for measuring low-power devices such as op amps, voltage references and voltage regulators. In addition, current injectors can be used to carry out other types of tests such as non-invasively measuring PSRR, determining power integrity and signal integrity sensitivity, and in generating high-speed load steps. In this video, Steve Sandler discusses each of these current injector measurement capabilities and presents test examples that illustrate how designers can perform and interpret these measurements. Along the way, Sandler offers many test tips to help the engineer obtain the most accurate results.