Resolution of a positioning system with signal conditioning, based on a PSD optical sensor. Influence of system noise

This paper presents an analysis of the influence of electronic noise from sensors and electronic conditioning circuits on the resolution of measurement in a system based on a PSD (Position Sensitive Device) optical sensor. In a previous study, the authors proposed the design of signal conditioning circuits for optical sensors, which generally supply currents of tens of nA that must be suitably managed before the analogue-to-digital conversion process (total gain close to 20M). The present work comprises an analysis of resolution influenced by the noise of the proposed circuit, considering different choices for retrieving the desired information and taking into account the equivalent noise bandwidth (ENBW) used in each one. Shot noise, thermal noise and flicker noise have been considered for the different stages, as well as how next stages amplified noise of previous ones. Once the global expression of noise and its influence on resolution have been obtained, sensitivity from different parameters and the result of resolution were calculated and discussed. Some of the main results of this work indicate that the resolution on the sensor surface, depending on the method of information retrieval, is between 3 and 12 μ m . This value translated to resolution in space, at a distance of 3.5 m and with a lens on the sensor of focal f=8 mm is from 1.3 to 5.2 mm. • The study addresses how system noise affects the resolution of a light beam’s impact on a PSD photodetector and its measurement projection. This is crucial for applications requiring precise spatial measurements. • The analysis focuses on obtaining the noise expression at the output of a three-stage conditioning system. This expression is vital for determining resolution, as signal increments smaller than the noise level cannot be detected. • The input voltage noise density of the first stage’s operational amplifier (OA) contributes to over 50% of the output noise. Given the amplification value of 20M, it is essential to use OAs with the lowest possible noise, provided their gain-bandwidth product (GBP) supports the necessary frequencies. • The resistor used for biasing the PSD photodiode significantly influences noise. Its value must be carefully balanced to maintain a linear response while minimizing noise. A small resistor value prevents variation in the polarization point due to the PSD’s continuous current component, but increases noise. Conversely, a large resistor value risks non-linear response. • Sensitivity analysis of different parameters revealed their contributions to the noise power spectral density at the output. This analysis helps identify which parameters most significantly affect resolution, guiding improvements in system design.