Photodetector integrator circuit having a window comparator

Abstract

An improved photodetector integrator circuit is provided having a photodetector, such as a photodiode, which produces photocurrent responsive to incident illumination. The photodetector is coupled to an integrator stage which converts the photocurrent into voltage and integrates the voltage over an integration period to provide an output signal. A window comparator in the circuit receives the output signal from the integrator stage and compares the output signal to a first threshold and a second threshold to provide, as a measurement signal, a pulse having a width which corresponds to the time interval over which the output signal increases from the first threshold to the second threshold. In the window comparator, the second threshold is greater than the first threshold. The integrator stage has two inputs coupled across the photodetector which are biased by a bias voltage below the first threshold, and a switch, preferably a mechanical relay, coupled to the integrator stage which when enabled resets the integrator stage to provide its output signal substantially equalling the bias voltage. A programmed microcontroller may also be provided for controlling the switch to reset the integrator stage, determining the time interval corresponding to the width of the pulse, and measuring the photocurrent produced by the photodetector responsive to the determined time interval. The circuit can be used in an optical analyzing instrument, such as spectrophotometer, to increase accuracy for measuring photocurrents.