In　this　paper,　an　amplitude-to-phase　converter　for　optical　incremental　encoders　is　presented.　The　proposed　converter　is　based　on　a　linearization　technique　that　coverts　the　sinusoidal　signals　into　a　nearly　perfectly　linear　output　signal,　from　which　displacement　can　be　determined　precisely　using　a　simple　linear　equation.　The　theoretical　error　of　the　converter　is　within　+/-　0.0196　mu　m　for　an　optical　encoder　with　a　period　of　20　mu　m.　Simulation　results　indicate　that　the　proposed　converter　is　significantly　more　robust　to　the　signal　imperfections　than　the　commonly　used　arctangent　algorithm.　A　signal　pre-processing　circuit　is　also　developed　to　further　reduce　the　positioning　error　caused　by　the　signal　phase-shift　error.　The　proposed　converter　was　successfully　implemented　with　a　Field　Programmable　Gate　Array　(FPGA),　and　applied　to　an　optical　encoder.　The　robustness　and　effectiveness　of　the　converter　have　also　been　confirmed　from　experimental　results.　(C)　2015　Elsevier　B.V.　All　rights　reserved.