Decoupling In Logic Circuits

EE JULY/AUGUST 1987 Failing to heed the importance of adequately decoupled supply rails is one of the most serious mistakes a constructor of digital circuits can make. Two important facts necessitate a reappraisal of the effec- tiveness of decoupling: the introduction of the fast HC and HCT series of CMOS chips, and the general availability of ever larger dynamic RAM (DRAM) devices. The 41256 256Kbit DRAM and 6264 CMOS SRAM, for instance, have become com- monly used integrated circuits, available at relatively low cost. The fast spreading use of the new CMOS series of logic cir- cuits has created the widely heard misunderstanding that these devices can be used without paying the least atten- tion to decoupling of the supply lines. However, a reduced cur- rent consumption relative to TTL devices is by no means a carte blanche for designers to skimp on decoupling pro- visions, as will be seen below. Why does a logic circuit draw current? The current consump- tion of TTL chips goes m...