A Technique for Measuring the Equivalent RMS Input Noise of A/D Converters

A simple accurate technique is described for measuring the equivalent rms input noise of A/D converters. Noise can typically be measured with 10-percent accuracy in 1 s, and the method has successfully been applied to converters with up to 16 bits of resolution. The measurements are made at input voltages corresponding to the test converter´s decision levels, where the effects of noise are most pronounced. A feedback loop incorporating the unit under test locates and locks onto these levels. The method utilizes a theoretical relationship between the input noise and an expected number of counts derived digitally from the feedback loop response. A low-noise wide-band operational amplifier is the only critical component required.