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GATE 2014 EC (EC4) – Question 51

Digital Circuits · Combinatorial Circuits · 2 marks · Numerical answer

A 16-bit ripple-carry adder uses 16 identical full adders as shown. Each has carry-propagation delay 12 ns and sum-propagation delay 15 ns. Find worst-case delay (ns).

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Correct answer: 194.9 to 195.1

Explanation

A carry traverses the first15full adders before the most-significant sum is produced. This path takes15×12+15=**195ns**. The final carry path takes16×12=192ns, so the sum path determines the worst case. Using16sum delays would not represent the ripple path.