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How To Decouple A Logic Circuit?

written by: Swagatam • edited by: KennethSleight • updated: 8/13/2010

Simply by adding a 100n decoupling capacitor as close as feasible to the supply terminals of logic ICs, a sound and foolproof functioning of the circuit can be ensured.

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    Introduction

    We have been studying about logic gates and logic circuits. You also learnt how to make different useful circuits by yourself, such as the DC mobile charger, ghost detector and so forth. Most electronic circuits are not complete without an IC or Integrated Circuit chip. In the upcoming few articles we will be getting involved in a series on logic circuit lectures that will tell you some very useful tips, advice and knowledge

    Over a period of time, with the advent new and sophisticated ICs, the overall efficiency of logic circuits has improved radically. Due to the rugged features and specifications of these modern ICs, we often take them for granted and seldom pay any heed to their many safety parameters, which may be not so critical but nevertheless does effect the performance of the IC. One of the things that most constructors are careless about, or forget to add to the logic ICs, is a decoupling capacitor. We will discuss this in detail, but first let's take a look at the evolution of logic ICs.

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    How have Logic ICs Improved?

    Not too long ago TTL devices (74 and 74LS series) were very common in the logic circuits. Then came the CMOS versions (40 series), which had a distinct advantage over their TTL counterparts. TTL ICs though faster, require a fixed 5 volts supply and the current consumption is also quite high compared to the CMOS ones. When more sophisticated CMOS ICs emerged into the scene in the form of HC and HCT the TTL devices became less popular and obsolete.

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    What is Decoupling?

    Decoupling in a CMOS IC, Image   The HC and HCT series of CMOS ICs are really good as these can be safely operated with a supply voltage ranging from 3-15 volts. The current requirement is also very low, and they are as fast as TTL ICs. But still there is a common factor between all these ICs that has to be taken care of- since the peak switching current in both TTL and CMOS ICs are equal, at the instant when switching in these ICs take place, the charge of their output capacitance must be immediately reversed (their internal switching current is too small and can be neglected). This may give rise to an instantaneous drop in supply voltage to a dangerous level. Therefore it becomes essential to add a capacitor as close as possible to the supply rail of these ICs. This is known as decoupling. The proximity of this capacitor to the IC supply terminals becomes necessary to negate any stray inductance due to useless track extensions. Also a voltage spike may then cause a rebounding effect in the circuit. This can be a real nuisance especially if the circuit is a tuned one.

    The value of a decoupling capacitor may particularly depend on the number of logic outputs activated simultaneously. Certain manufacturers advice the use of a 330n capacitor to decouple a CMOS IC, but in practice a 100n acts well in most cases in most cases.

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