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Impedance

Electrical impedance is effectively resistance in an AC circuit (see Ohms). Impedances are significantly more complicated than simple resistance because they are frequency conscious and because they can apply to the behaviour of an entire circuit rather than a single element.

There are load impedances, having the effect of a virtual resistor across the input terminals of a piece of equipment; and source impedances, having the effect of a resistor in series with the output terminals. A high input impedance will draw very little current from a circuit – it will only load it lightly; a low output impedance can source (supply) plenty of current.

These impedances are usually designed to be fairly constant for the frequency range of the equipment but they may be very different at the LF and at HF extremes (particularly when the range extends to RF).

Quoted impedances are for the normal signal path and are thus the differential-mode impedances of balanced circuits. The impedance to ground – the common-mode impedance – may be very different. Impedancecm has to be identical for each leg of a Balanced Line.

Matched impedance lines have the source and load impedance equal. The cable and any connectors used will typically also have matched characteristic impedances. Matched impedance circuits transfer the maximum power and are important for very wide bandwidth applications.

High-speed data, video, RF and digital audio circuits use this Transmission Line technique. They are intended for single-point to single-point connections, unless active or passive splitters, or bridging techniques are employed. Correct Termination of a Transmission Line must be observed at all times.

Unmatched impedance lines use a low impedance source and a high load impedance. This technique transfers the maximum signal voltage, not power. This is not a Transmission Line approach and so multiple loads can be fed from one source without problems.

Analogue audio, including loudspeakers, use this approach. In general the load should be >10x the source impedance though with some dynamic microphones the ratio is lower. Only when analogue lines are >>1km, now very rare, do they need to be treated as Transmission Lines.

Line impedances are characterised largely by the type of cable and connector* that it is practical to use. Coax has a lower characteristic impedance compared to pair cable.

Baseband video and SPDIF 75 Ohms

Radio antenna feeds 50 Ohms

Classical (long distance) analogue audio 600 Ohms

AES-3 digital audio 110 Ohms

Ethernet 10BaseT approx 100 Ohms

Ethernet 100BaseT 150 Ohms

*[BNCs can be found with both 50 Ohms and 75 Ohms nominal Impedance. Despite many rumours to the contrary the centre pins are the same size and the connectors can be (and frequently are) mixed without any problems for normal video use. For longer runs of SDI, HDSDI and UHF radio mics it is wiser to use the correct type. 50 Ohm BNCs have a ring of white dielectric visible inside the sleeve connection – 75 Ohm BNCs have none.]

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