Characteristics of Digital ICs

On moving towards the characteristics – they are temperature sensitive in nature and the rest other are Some important characteristics or parameters are given as follows:

• Speed of Operation
• Power dissipation
• Figure of merit
• Fan Out
• Fan In
• Current and voltage parameter
• Noise Immunity
• Operating temperature range
• Discrete Signal Level
• Storage

Speed of Operation

The speed of a digital circuit is expressed in terms of propagation delay. The input and output waveforms are shown in Fig.1(a) and Fig.1(b). The delay times are measured between the 50% voltage levels of input and output.Fig.1(a) shows that when we apply an input to a logic gate, output occurs after some time i.e. propagation delay.Fig.1(b) shows the input and output waveform.

The two propagation delay times are defined as follows:

• t_PLH: It is the propagation delay time in going from logical LOW (0 state) to logical HIGH (1 state)
• t_PHL: It is the propagation delay time in going from logical HIGH (1 state) to logical LOW (0 state)

Power Dissipation

Any digital circuit requires some power for operation. This is the amount of power dissipated in an IC. It is determined by the current an IC draws from the V_CC supply, it is expressed by V_CC ✖ I_CC. This power is specified in milliwatts.

It is shown in Fig.2 that when the gate is in the HIGH output as shown in Fig.2(a), the current drawn by the gate is represented by I_CCH. Similarly, in the LOW output, the current drawn by I_{CCL is} in Fig.2(b).

The power of the circuit is the product of the supply voltage and current drawn by the circuit when the input is applied to the gate as shown in Fig.2, the output changes between logic 1 and logic 0 which means the supply current changes between I_CCH and I_CCL. So the average current drawn by the circuit will be

I_{CC (avg)} = (I_CCH+I_CCL)/2

We take the average of both currents because the power dissipation depends on the duty cycle and is generally specified for a duty cycle of 50%. So, the average power dissipation of the circuit is

P_(avg) = V_CC ✖ I_CC(avg)

P_(avg) = V_CC ✖ [(I_CCH + I_CCL)/2]

If an IC has four gates, then the power dissipated will be four times the power dissipated by each gate.

Figure Of Merit

The figure of merit is defined as the product of speed and power. The speed of the logic circuit is specified in terms of propagation delay, so the figure of merit is specified in milliwatts.

Figure of merit = propagation delay ✖ power

Fan Out

Fanout is the maximum number of similar logic gates that a gate can drive without any degrading in voltage levels. High fanout is advantageous because the need for additional drivers to drive more gates is less. It is used to derive the logical inputs for the working and it means the maximum number of standard logic inputs which an be derive efficiently.

Fan In

It is defined as the number of inputs that can be connected to a gate. For example, 7400 represents two input NAND gates, therefore fan-in of 7400 is 2. So it is basically the number of input it can handle properly for the procedure or working.

Current and Voltage Parameter

Current and voltage parameters are very important in designing a digital system.Fig.3(a) and Fig.3(b) show the current and voltage in the two logic gates.

• V_IH(min) – High-level input voltage: It is the minimum voltage level required for logic 1 as an input. Below this minimum level, it will not be accepted as a HIGH by the logic circuit.
• V_IL(max) – Low-level input voltage: It is the maximum voltage level required for logic 0 at an input. Any voltage above this level will be considered as a HIGH input.
• I_OL(max) – Low-level output current: It is the maximum current level at a logic circuit output in the logical 0 state under the defined load condition.
• I_IH(min) – High-level input current : It is the current that flows into an input when a specified high-level voltage is applied to that input.
• I_IL(max) – Low-level input current

It is the current that flows into an input when a specified low-level voltage is applied to that input.

• I_OL – High-level output level current

It is the current that flows from output in the logic 1 state under specified load conditions.

• I_OL – Low-level output current

It is the current that flows from an output in the logic 0 state under specified load conditions.

Noise Immunity

The input and output levels of a digital circuit are specified by a voltage level.Fig.4 shows the input and output voltage levels. Some unwanted voltage might be induced due to electric and magnetic field, known as noise. This may cause the voltage at the input of a logic circuit to drop below V_IH or rise above V_IL and may produce undesired operation. Noise immunity is the maximum noise voltage that may appear at the input of a logic gate without changing the logical state of its input.

State 1 noise margin – V_OH – V_IH

State 0 noise margin – V_IL – V_OL

Operating Temperature

All IC gates are semiconductor devices. They are temperature-sensitive by nature. The operating temperature ranges of an IC vary from 0℃ to + 70℃ for consumer and industrial applications, temperature ranges from -55℃ to +125℃ for military applications.

Discrete Signal Level

The digital circuits works on this property also which have only two values to represent the low and high voltage level: in binary terms we say it- 0 and 1. This feature helps to process the correct form of data and represent it accurately so we can say that the characteristics of the discrete signal level helps the digital circuit to improve its efficiency.

Storage

In order to store the data, a variety of components have been used by the digital circuits ,these components are based on the short term and long term storage also in which we can include the example of the flip flop, register , RAM , flash memory ,etc. The storage components depend that how much space is needed for the data to set and accordingly the component is used for that , so for both short and long term storage areas are available in digital ICs.

Digital circuits are constructed by using different logic gates, so when we use different ICs, we have to produce different logic gates using different technologies. For the fabrication of ICs, we use semiconductor devices-bipolar and unipolar. Based on the devices, digital ICs are made which are then commercially available. When we use a bipolar device, like a transistor in the IC fabrication technology, it is known as bipolar technology. In unipolar technology, we make use of unipolar devices. e.g., MOSFETs. A group of compatible ICs with the same logic level and supply voltages for performing various logic functions which have been fabricated using specified configuration, is referred to as a logic family.