Approximate ripple carry adders (RCAs) and carry lookahead adders (CLAs) arepresented which are compared with accurate RCAs and CLAs for performing a32-bit addition. The accurate and approximate RCAs and CLAs are implementedusing a 32/28nm CMOS process. Approximations ranging from 4- to 20-bits areconsidered for the less significant adder bit positions. The simulation resultsshow that approximate RCAs report reductions in the power-delay product (PDP)ranging from 19.5% to 82% than the accurate RCA for approximation sizes varyingfrom 4- to 20-bits. Also, approximate CLAs report reductions in PDP rangingfrom 16.7% to 74.2% than the accurate CLA for approximation sizes varying from4- to 20-bits. On average, for the approximation sizes considered, it isobserved that approximate CLAs achieve a 46.5% reduction in PDP compared to theapproximate RCAs. Hence, approximate CLAs are preferable over approximate RCAsfor the low power implementation of approximate computer arithmetic.
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