Digital logic family is a group of logic gates constructed using passive devices like a resistor, transistor, diodes, etc. Such devices have compatible logic levels and supply voltage with certain characteristics.
Classification of the digital logic family
According to the components used, there are different types of logic families. Some of the logic families include Resistor-Transistor logic(RTL), Diode-Transistor logic(DTL), Transistor-transistor logic(TTL), Emitter coupled logic(ECL), PMOS, NMOS, and CMOS circuits.
They are classified into several types as shown below.
Characteristics of the digital logic family
It is the time interval between the application of the input pulse and the occurrence of the output. It is an important characteristic of the digital logic family. If the propagation delay is less, then the speed at which the IC operates will be faster.
Let THL is the propagation delay when the output changes from logic 0 to 1 and TLH is the delay when the output changes from logic 1 to 0. The maximum value of THL and TLH is considered as the propagation delay for that logic gate.
Fan in and Fan out
Fan-in refers to the number of inputs in a digital logic gate family. For the example given in the figure below, the EX-OR gate has three inputs. So fan-in for the given EX-OR gate is 3.
Fan-out refers to the number of inputs that is driven by the output of another logic gates. For example, the following circuit has an EX-OR gate, which drives 4 NOT gates. So fan-out of EX-OR gate is 4.
Both fan-in and fan-out values are given by the manufacturer at the time of designing and the data is specified in the datasheet. When the number of inputs or outputs are changed, it may cause some malfunction to the device.
It is the amount of power that the digital circuit dissipates. The power dissipated is determined by the average current, that is drawn from the supply voltage.
The average current is the average value of the current at LOW gate output(logic ‘o’) and the current at HIGH gate output(logic ‘1’).
Noise Immunity and Noise margin
What is Noise?
It is an unwanted signal that is superimposed on the normal operating signal. Noise may be due to various factors like operating environment, radiations, stray electrical and magnetic fields.
In digital logic circuits, the binary values 0 and 1 represent the LOW and HIGH voltage levels. Due to the interference of the noises, the voltage levels may increase or decrease. This may lead to the wrong operation of the device.
The noise immunity is the ability of the logic device to tolerate the noise without causing spurious change to the output voltage. Noise margin allows the logic device to function properly within the specified limits.
Figure of merit
For an efficient operation of any device, whether it may be digital or analog, the power dissipation and the speed are notable characteristics. Achieving a higher speed with less power dissipation is a highly challenging task.
In the digital logic circuit, a trade-off exists between these two characteristics. That is, for higher speed, the power dissipation will be more.
The figure of merit or Speed Power Product is a common means of measuring the performance of circuits in the digital logic family.