Ramp analog-digital converters used in matrix image sensors to provide a digital value representative of a level of illumination of a pixel are provided. Two voltage samples are applied to a comparator, a counter is used to count pulses at a frequency F from a starting instant of the ramp until a toggling of the comparator. Two other voltage samples, one of which is added to a linear voltage ramp having an identical starting instant and slope to the first ramp, are applied to a second comparator, a half counting frequency F/2 is applied to the counter from the toggling of one of the comparators, and the content of the counter at the moment of toggling of the other comparator is stored. Two measurements of samples of the same signal or of two different signals are averaged without undergoing a digital conversion for each signal and a digital addition.

A measurement unit comprising a converter unit and a processing unit and configured to provide a measurement result value, based on a first input signal. The converter unit is configured to provide first digital, quantized values based on the first input signal. The measurement unit is further configured to calculate second values, which represents a reference quantity or a reference value, for a plurality of quantization step sizes associated with different values of the control signal. The measurement unit is configured to change the control signal of the converter unit between determination of different first values and/or a determination of the different second values, such that different first values and/or different second values are provided using different converter quantization step sizes. The processing unit is configured to provide a measurement result value from a predefined number of first values and a predefined number of second values.