Sinking and sourcing 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).

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
  Fanout
  In digital electronics, the fan-out of a logic gate output is the number of gate inputs to which it is connected.In most designs, logic gates are connected together to form more complex circuits. While no more than one logic gate output is connected to any single input, it is common for one output...
  
   is the capability of a digital circuit
  Digital circuit
  Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...
  
   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
  Digital circuit
  Digital electronics represent signals by discrete bands of analog levels, rather than by a continuous range. All levels within a band represent the same signal state...
  
   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
  Logic family
  In computer engineering, a logic family may refer to one of two related concepts. A logic family of monolithic digital integrated circuit devices is a group of electronic logic gates constructed using one of several different designs, usually with compatible logic levels and power supply...
  
  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.