The Effect of Circuit Impedance

Author: N.H. Crowhurst

Without shielding magnetic induction is more troublesome in low-impedance than in high-impedance circuits

Without the use of shielding, magnetic induction is usually more troublesome to low-impedance circuits than to high-impedance circuits, because a magnetic field will induce the same voltage in a loop regardless of its impedance. Suppose that the magnetic field induces 6 microvolts in a given circuit loop. If the circuit has a high-impedance, the audio voltage level may be 60 millivolts, which gives a margin of 80 db (a voltage ratio of 10,000:1). If the circuit has an impedance of only 50 ohms, however, the audio is likely to be about 2 millivolts. A. transformer with a step-up ratio of 30:1 (an impedance ratio of almost 1000:1) could step up this voltage to the same 60 millivolts as that in the high-impedance circuit. As well as stepping up the 2 millivolts of audio, however, the transformer would step up the undesired 6 microvolts to about 180 microvolts. Hence, with the same magnetic field, the high-impedance circuit gives a margin of 80 db (10,000:1), while the low-impedance circuit only gives a margin of 50 db. (312:1).

The opposite is true with electric induction. If the surface on which charge is induced by a nearby voltage has a high-impedance connection to ground, the charges will not have time to leak away and voltages will appear with them, causing interference. If, on the other hand, the surface is connected by a low-resistance path to ground, the charges will leak away rapidly without causing this interference. This means that low-impedance circuits are less susceptible to electric induction than high-impedance circuits.

Electric induction