Approximate Ripple Carry and Carry Lookahead Adders - A Comparative Analysis

TL;DR

This paper compares approximate ripple carry and carry lookahead adders in 32-bit addition, showing significant power-delay product reductions and highlighting the advantages of approximate CLAs for low-power arithmetic circuits.

Contribution

It provides a comparative analysis of approximate RCAs and CLAs using 32/28nm CMOS technology, demonstrating their power efficiency improvements with various approximation sizes.

Findings

Approximate RCAs reduce PDP by up to 82%.

Approximate CLAs reduce PDP by up to 74.2%.

Approximate CLAs outperform RCAs in power efficiency.

Abstract

Approximate ripple carry adders (RCAs) and carry lookahead adders (CLAs) are presented which are compared with accurate RCAs and CLAs for performing a 32-bit addition. The accurate and approximate RCAs and CLAs are implemented using a 32/28nm CMOS process. Approximations ranging from 4- to 20-bits are considered for the less significant adder bit positions. The simulation results show that approximate RCAs report reductions in the power-delay product (PDP) ranging from 19.5% to 82% than the accurate RCA for approximation sizes varying from 4- to 20-bits. Also, approximate CLAs report reductions in PDP ranging from 16.7% to 74.2% than the accurate CLA for approximation sizes varying from 4- to 20-bits. On average, for the approximation sizes considered, it is observed that approximate CLAs achieve a 46.5% reduction in PDP compared to the approximate RCAs. Hence, approximate CLAs are preferable over approximate RCAs for the low power implementation of approximate computer arithmetic.