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A compact chip identification (ID) circuit with improved reliability is presented. Ring oscillators are used to measure the spatial process variation and the ID is based on their relative speeds. A novel averaging and postprocessing scheme is employed to accurately determine the faster of two similar-frequency ring oscillators in the presence of noise. Using this scheme, the average number of unstable bits i.e. bits which can change in value between readings, measured on an FPGA is shown to be reduced from 5.3% to 0.9% at 20degC. Within the range 20-60degC, the percentage of unstable bits is within 2.8%. An analysis of the effectiveness of the scheme and the distribution of the errors is given over different temperature ranges and FPGA chips.