Sinking and sourcing
Sinking and sourcing in electronic circuits relate to driving capabilities at the output of an electric circuit. To drive a point in an electric circuit to a desired value of voltage means to supply the required electric current to do so, overcoming electric resistances or other sources affecting (connected to) that point (sources may include voltage and/or current sources).
In a more general sense in science and technology, sinking and sourcing are used to describe processes which absorb (sink) or generate (source) something. For example, a heat sink takes up heat from electronic components that generate heat, heat sources.
Definitions
- Sinking - the current driving capability of a circuit, to draw a current toward the ground or zero voltage, or to a lower voltage.
- Sourcing - the current driving capability of a circuit, to draw a current from the power supply or the higher voltage in the circuit.
- Fan - out - Fanout is the capability of a digital circuit output to drive several logic gates connected to it, both in sinking and sourcing an electric current. For example, a fan-out of 10 indicates that that output can be connected up to 10 inputs of other logic gates.
- Fan-in - the current drawn by a digital circuit input when activated into the logical one or zero state.
Analogy
This is akin someone trying to keep a board at a desired fixed location in the sea, against the forces of the wind and waves: At times, she may be required to apply a force to pull it toward shore (sinking), at other times - to push it away from shore (sourcing).
Applications
- Sinking - a logic circuit output, when connected to the inputs of one or more logic gates, and the output is in logical Low (zero) state, has a capability of providing a current to the ground or zero, so the voltage at that point will indeed correspond to the Low state.
If the sinking capability of a circuit is exceeded (for example when too many inputs of other gates are connected to it), then the voltage at that point may be too high, higher than that corresponding to a Low state. The circuits then may malfunction.
- Sourcing - a digital circuit output, when connected to the inputs of one or more logic gates, and the output is in logical High (one) state, has a capability of providing a current to the power supply, so the voltage at that point will indeed correspond to the High state.
If the sourcing capability of a circuit is exceeded (for example when too many inputs of other gates are connected to it), then the voltage at that point may be too low, lower than that corresponding to a High state. The digital circuits then may malfunction.
- Various types of logic family may have different values of current sourcing and sinking.
- Sinking and sourcing also apply to analog circuits, such as operational amplifiers, and mixed circuits, such as comparators.
References
- National Semiconductor Corporation, F100K ECL 300 Series Databook and Design Guide - Family Overview