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The frequency dependence of the power supply interference rejection of four commercial digital sensors with PWM output has been experimentally analyzed because data sheets do not fully specify it, neither do they describe the measurement method in detail. In some sensors, the mean and standard deviation of the output depend on the amplitude and frequency of sine wave interference added to their supply voltage. Bias errors arise from locking effects when the interfering frequency is close to the output frequency or an integer multiple of it, and increase with the interference amplitude. For a 100 mV amplitude sine wave interference, the standard deviation can be from 3 to 100 times that obtained when the supply voltage is clean.