Earthing

A key principle in avoiding a number of stability and signal quality problems is in the strategy used to return currents to a common point. As far as possible each point where current is returned to the source should be taken to a single point in the circuit. Where this is not possible, care must be taken to provide low impedance returns. Two configurations shown below should illustrate the difficulties that arise when this principle is not observed.

If there are heavy currents from the output stage that pass back to the source through a common connection, small voltages will be generated along the return wires due to the (small but significant) resistance of the physical wires. These are shown as R1, R2, R3. In the first diagram, these voltages will appear at the common connection from each stage, and will therefore be subtracted from the output voltage of each stage to be fed into the next stage. This results in feedback and possible oscillation (or in the case of power suplies, ripple on the regulated output). The strategy used by the second diagram is to connect the return wires from each stage directly to an common point near the source. This means that any currents flowing from the last stage will not geenrate feedback voltages to the earlier stages.

It is not always possible to avoid daisy-chaining of the return wires. In such a case feedback would need to be managed more carefully, by reducing the wire resistances and possibly more advanced techniques.