Electronic Circuit for the Signal Conditioning of PSD Optical Sensors

Abstract

This article presents the methodological process to be followed in the design of signal conditioning circuits for optical sensors (position sensitive detector (PSD) or photodiodes), which generally supply currents of nanoamperes that must be suitably managed before the analog-to-digital conversion process. The first step is to define the requirements for the signal supplied by the sensors and the signal to be sent to the analog-to-digital converter (ADC), taking into account the operating conditions and the configuration of the sensors. The various aspects to be taken into account in the design stages are presented below. In the first stage of the conditioning circuit, the output of the sensors is adapted with a high-gain transimpedance stage and low-pass filtering. In the next stages, the gain is increased to the values required to make maximum use of the dynamic range of the ADC (total gain close to 20M). Two alternatives are studied, one of first order and the other of second order. In both cases, high-pass filtering is performed to eliminate low-frequency optical signal noise, the noise bandwidth is reduced to obtain a flat response in the passband, and the output offset is limited. In the third stage, a gain adjustment is introduced and the appropriate bias is added to the signal to be delivered to the ADC. The design methodology is analyzed using SPICE to verify that the necessary requirements are met. The circuits are then implemented to verify that they behave as required.