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We present a theory for metastability error power in Successive Approximation A/D converters. The traditional measure, BER, does not account for the error influence on signal quality, only the error rate. The metastability error is instead compared with noise, and a Signal-to-Metastability-error-Ratio, SMR, is suggested as a new measure. Suppressing SMR below SNR imposes a gain requirement on the comparator.