Magic sampling—a digital sampling strategy that discriminates against mains interference (noise)

Abstract Mains interference is a problem because its fundamental frequency (50 Hz in Australia and the U.K., 60 Hz in the U.S.A.) lies in the centre of the frequency range within which electrochemical phenomena are usually perturbed (10−3Hz to 105Hz), and, therefore, once it has entered a measurement it is virtually impossible to remove it without distorting the true signal in some way. In the present work we first review the background to this problem, and then propose a digital sampling strategy that discriminates against mains interference even when the interference is harmonically distorted. The strategy consists of three parts: first, a digital presampling filter to attenuate high frequency harmonics in the interference; second, a ‘magic’ sampling rate to manoeuvre all the surviving harmonics to the Nyquist frequency; and third, a postsampling digital filter to block everything at the Nyquist frequency whilst allowing the true signal to pass. Since the only restrictions on this strategy are minor (the signal to be measured should be continuous and have no frequency components greater than fmains) it should find widespread application in the digital acquisition of electrochemical data.