Boolean Algebra

Special Characteristics of an IC

A gate IC can be realized using different logic families (explained later) and hence every gate have the following characteristics which may vary with different logic family. The most important parameters are the following

Fan-out: It specifies the maximum number of standard gates that output of a gate can rive without affecting its own working. This normally depends upon the amount of current needed by a gate of same IC family. We generally connect output of a given gate to the inputs of other gates but there is a limit on the number of gates to which the output of a given gate can be inputted and this limit is called fan out of the given gate. If we exceed this limit then this may cause the malfunctioning of the circuit because the given gate may not be able to provide enough power to the gates which are connected to the output of the given gate.

When we have logical 0 at the output current is drawn into the circuit while when we have output as logical 1 then current is supplied to the load. So number of loads that can be connected to the output varies whether we have output as logical 1 or output as logical 0 but we take worst of the two as fan-out e.g. A gate in the TTL family can take load of 10 gates at logical 1 while 5 gates at logical 0. So we take fan-out as 5.

If we have fan out of he given gate as N, then we can connect the output of the given gate to N different gates of same family as shown below.

If we have fan out of he given gate as N, then we can connect the output of the given gate to N different gates of same family as shown below.

Some gates like XOR, BUFFER, INVERTER consume loading factor of 2 i.e. their load is equivalent to 2 usual gates.

If we consider this then a gate with loading factor of 5 can drive 2 XOR gate inputs and 1 OR or AND gate input.

BUFFER has fan-out of 25

Fan-in: It is basically the number to which we can extend the inputs of a gate. Generally this is equal to 8. This means that we can have a gate with maximum of 8 inputs as shown below

Power dissipation:  It is the average of powers dissipated in a gate with all inputs at logic 1 and the gate with all inputs at logic 0 and expressed in mW. If we have a IC with 4 gates then we need power supply which is equal to 4 times the power dissipation of the gate. It is an important factor. Industry is working continuously to decrease this factor further and further.

Propagation delay:  It is the average transition time for a signal to propagate from input to output. If we make a change in the binary signal at the input at time t=0 and we observe the corresponding change in the output at time t sec. Then the propagation delay for gate is taken as t. it is generally expressed in ns. The gate must have the minimum propagation delay. Propagation delay depends on the number of levels we have in a circuit. If we have 3 levels of gates in a circuit and each gate provides a delay of t ns then we have the total delay of the circuit as 3t ns.

Noise margin:  It is maximum noise which can be tolerated at the input of the circuit i.e. the maximum amount of noise which when added to the input does not give an undesirable change in the output. The noise if generally of two types: DC noise-that leads to the shift in the voltage level of the signal and the AC noise which is a random signal that can be created by any other switching signal and this signal s superimposed over the input signal. Noise margin should be as high as possible.

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