Approximate adders : an exploration on logical and physical synthesis and image quality impact

Abstract

The ripple carry adder (RCA) is the simplestadder, with the lowest circuit area and power consumption.Its main drawback is the high propagation delay in the carrypropagation path. In error-tolerant applications, the use of ap-proximate adders can bring significant benefits, including re-duced propagation delay, circuit area, and power consumption.This work aims to analyze the advantages and disadvantages ofapproximate adder topologies compared to the RCA. Differenttopologies are described and subjected to both logic and phys-ical synthesis. The comparison is based on metrics such as thecircuit area, power consumption, propagation delay, and errorrate of each approach. Additionally, the approximate addersare embedded in an image filtering process, applied to imagesfrom the Berkeley Segmentation Dataset. The results show thatLower-Part XOR and OR Gating Adder (LXOA) and Lower-Part OR Gating Adder (LOA) exhibit reduced error while theLower-Part Truncation Adder (LTA) presents lowest circuitarea and propagation delay. Application-level evaluation con-firms that approximation-induced errors remain below percep-tually relevant thresholds.