The invention relates to electronic circuits for measuring, by synchronous detection, weak signals whose reference level is not well known and is subject to large fluctuations. A first correlated double sampling is performed between a time T1 situated just before the start of the measurement pulse and a time T2 situated just before the end of the measurement pulse; subsequently, a second correlated double sampling is performed between time T2 and a time T3 situated after the end of the measurement pulse. Finally, the difference between signal levels coming from the two measurements is taken, this difference being a representation of the signal value Vm considered with respect to a reference level that is intermediate between the reference levels at times T1 and T3.