both input voltages may be unknown and the output is the digital value of the voltage ratio which is a measure of efficiency fig7 b illustrates example timing for the circuit of fig7 a in fig7 a a switch 702 connects one of two input voltages to an integrating operational amplifier 704 in fig7 a and 7b the two input voltages v out and d v in are assumed to be of opposite polarity assume that during time t 1 switch 702 connects integrating amplifier 704 to v out and assume that v out is negative the integrator is inverting so the output v o of the integrator increases linearly assume that during time t 2 switch 702 connects integrating amplifier 704 to d v in and assume that d v in is positive the output v o of the inverting integrator decreases linearly time t 1 is a fixed predetermined amount of time time t 2 is variable and ends when a comparator not illustrated determines that v o has returned to zero a counter not illustrated counts clock cycles during the time periods t 1 and t 2 the output voltage v o during time t 1 is the output voltage v o during time t 2 is output voltage vo changes by the same magnitude over time periods t 1 and t 2 accordingly the digital efficiency e is the count of clock cycles during time period t 2 time period t 1 is predetermined and known so it.