Question 4 Part of a 55 Volt digital system contains a 74ACT

Question 4

Part of a 5.5 Volt digital system contains a 74ACT08 integrated circuit (IC). A data sheet for this device is supplied as a separate document.

(a) For this IC, and for an assumed working temperature of 25 °C, determine the following:

(i) The minimum voltage for a logic \'1\' output. (2 marks)

(ii) The worst-case noise margin. (3 marks)

(b) Explain the term ‘propagation delay’ with reference to this integrated circuit (IC). What is the maximum propagation delay for this IC operating under these conditions? (3 marks)

(c) The output of one of the gates is to be used to drive an LED. Design a suitable circuit such that the LED is switched on when both inputs to the gate are set to logic ‘1’. Assume that the LED data is as follows: If = 24 mA, Vf = 1.6 V. Show all calculations. (10 marks)

(d) Explain briefly what is meant by the term ‘open collector’ with respect to a digital output. Draw a diagram showing how such a gate is used and under what circumstances might this type of output be useful. (7 marks)

Solution

a)

i: The min. voltage for any digital circuits is 1.5V to attain the output logic 1. If the input voltage is varying between 0-5V, at temp 25\'C.

ii:Noise margin: is the amount by which a signal exceeds the minimum amount for proper operation.In a digital circuit, the noise margin is the amount by which the signal exceeds the threshold for a proper \'0\' or \'1\'. For example, a digital circuit might be designed to swing between 0.0 and 1.2 volts, with anything below 0.2 volts considered a \'0\', and anything above 1.0 volts considered a \'1\'. Then the noise margin for a \'0\' would be the amount that a signal is below 0.2 volts, and the noise margin for a \'1\' would be the amount by which a signal exceeds 1.0 volt. In this case noise margins are measured as an absolute voltage, not a ratio. Noise margins for CMOS chips are usually much greater than those for TTL because the V_{OH min} is closer to the power supply voltage and V_{OL max} is closer to zero.

b) Propagation delay is a technical term that can have a different meaning depending on the context. It can relate to networking, electronics orphysics. In general it is the length of time taken for the quantity of interest to reach its destination.

Propagation delay (IC) for VCC 5V, at temp 25’C min delay is 1.5, typical delay is 4.5, max delay is 8.0 nano seconds.

c) by connecting the LED to AND gate output. When both inputs are 1 the output is logic 1.