PURPOSE: To obtain a large dynamic range, through the use of a counter with small capacity, by obtaining a digital output proportional to logarithm of an analog input quantity, through discharging the charge in an integrator exponentially.

CONSTITUTION: When an analog voltage V is applied to an input terminal A, the voltage V is integrated with a resistor 8 and a capacitor 3 only while an integration time setting switch is closed and a voltage at point B linearly increases. When a switch 1 is opened and a pulse signal is applied to a point D, a discharge switch 4 is closed and the charge in the capacitor 3 starts discharge, while a counter 7 starts counting. Since a discharge voltage generator 11 has a discharge characteristic of exponential function, a voltage waveform at point F changes wavenumber functionally, and then a voltage waveform at a point B changes exponentially. When the voltage at the point B reaches a reference voltage Vref, a voltage at a point C goes to low level, the switch 4 is opened and the counter 7 stops counting and a digital output signal proportional to the logarithm of the voltage V.