Analog circuit for interfacing silicon optical sensors and driving a dual-slope A/D converter for logarithmic read-out

Abstract

State-of-the-art A/D converter research is usually focussed on either increasing the resolution or the speed of linear A/D converters. However, in many data acquisition systems input transducers, such as LVDT displacement sensors, pressure sensors, photodiode optical sensors or magnetic-field sensors, are used with a dynamic range exceeding four decades and an inaccuracy of around 1%. This common practice favours nonlinear quantizing, which features a constant relative accuracy, rather than the implementation of a linear A/D converter in which the specifications with respect to resolution and accuracy would lead to an undesirable price/performance ratio. The nonlinear A/D converter for the read-out of a photodiode, that is discussed here, is based on the combined operation of a special analog circuit and a moderate-resolution dual-slope linear A/D converter. The analog circuit has been designed to serve as an interface between a photodiode and a commercially-available dual-slope A/D converter and supplies such currents to the dual-slope integrator that a dual-slope counter content is available after conversion proportional to the logarithm of the photocurrent. The combined analog chip with sensor and analog signal conditioning circuits and the dual-slope converter form a dual-chip smart integrated silicon optical sensor with a reading between 00.0 and 99.9 dB.