In-Circuit Impedance Measurement Using Current Sensing

A novel approach to in-circuit measurement of resistors, capacitors, and inductors was evolved. This approach utilizes current sensing, making possible the measurement of in-circuit components which may be directly shunted by impedances as much as two to three orders of magnitude lower in impedance than the unknown. A minature clip-on probe was developed to facilitate measurements of components mounted on printed circuit boards. A compact impedance comparator was constructed which measured in-circuit components within approximately 2 to 4-percent accuracy over the following ranges: for resistance, 1.5 ? to 1 M?; for capacitance, 100 pF to 60 ?F; and for inductance, 400 ?H to 60 H. Impedances as low as one to three orders of magnitude smaller than the measured element impedance, depending on the probe used, shunted the unknown when the stated accuracy was measured. Much of the quoted inaccuracy is due to the ±3-percent resistance and ±0.1-percent linearity tolerances of the reference resistor which was used for all measurements.