Surface electrodes are those that are placed in contact with the skin ofthe subject. Also in this category are certain needle electrodes of a size that prevents their being inserted inside a single cell which a microelectrode is.Surface electrodes, other than needle electrodes, vary in diameter from 0.3to 5cm, with most being in the 1 cm range. Human skin tends to have a very high impedance compared with other voltage sources. In any event, one must treat surface electrodes as a very high impedance voltage source- a fact that seriously influences the design of biopotentials amplifier input circuit.