Analog to digital conversion with noise reduction

An analog-to-digital converter comprising:

• an integration circuit including an integrator for integrating an input signal;
• means for discharging the integrator at a predetermined rate to obtain a measure of an amplitude of the input signal, said integrating occurring during a first interval of time, said discharging occurring during a second interval of time immediately following said first interval of time, an elapsed time of said discharging having a duration which is longer than said first interval and serving as a measure of the amplitude of said input signal, a sum of said first and said second intervals being a composite interval;
• and
• wherein said integration circuit is operative in response to electric power obtained from A.C. excitation at any one of a plurality of frequencies commensurate in terms of a common measurement interval equal to an integral number of periods of the excitation at each of said plurality of frequencies, one of said excitation frequencies being 60 Hertz and a second of said excitation frequencies being 50 Hertz, said common measurement interval being equal to six periods of the 60 Hertz excitation, to five periods of the 50 Hertz excitation, and to four of said composite intervals to remove noise associated with the 50 Hertz and the 60 Hertz excitation;
• and
• said converter further comprises timing means for operating said integration circuit periodically to provide an even integral number of said composite intervals during said common measurement interval, said timing means including means for strobing said integration circuit to perform said integrating during said first interval of time in each of said composite intervals;
• and
• means for measuring the elapsed time of a discharging during each of said composite intervals, said measuring means outputting an average value of elapsed time measured during a succession of composite intervals occurring within the single common measurement interval, the average value of elapsed time being proportional to an amplitude of the input signal substantially free of interference from the A.C. excitation.