A LOW-NOISE SENSOR INTERFACE FOR OPTICAL PARTICULATE MATTER DETECTORS

Abstract

In this study, I propose a low-noise sensor interface for optical particulate matter (PM) detectors. The particles are classified as particulate matter 10 (PM10) (< 10 μm), PM2.5 (< 2.5 μm), and PM1 (< 1 μm) depending on the diameter of them. The optical PM detector uses a photodiode (PD) to measure the amount of light scattered by particles emitted from a light emitting diode (LED) or a laser diode (LD). The received input current signal is converted to a voltage signal at the transimpedance amplifier (TIA), amplified and filtered after removing the DC offset of the input. It is then applied to an analog-to-digital converter (ADC) for digitization. The conventional correlated double sampling (CDS) technique only can be applied to the discrete-time switched-capacitor (SC) circuit that is generally power-consuming and needs large chip area than continuous-time circuits. Another typical method using a current-steering digital-to-analog converter (DAC) generates large intrinsic device noise or requires large power consumption and chip area to reduce the noise. In this study, I propose a DC offset calibration method using an operational amplifier (OPAMP) with embedded DAC only when the light source is turned on. In addition, an average power control (APC) circuit is used to make the light intensity of the LD less sensitive against the external environmental changes or noise. The APC circuit monitors the light intensity using the reference photodiode and controls the bias current of the LD to get the light intensity be constant when it is changed. This enables small-sized particles to be measured with high resolution. The digital post-processing is performed as the following procedure. First, a PM calculation is performed to calculate the concentration by particle size. Then it is low-pass filtered to remove high-frequency noise. The concentration-based curve fitting algorithm is then carried on to make the concentration profile linear. And next, the offset compensation is performed according to the temperature of which information is generated by on-chip temperature sensor. Finally, the gain-compensated output is sent through a digital interface such as pulse-width modulation (PWM), universal asynchronous transmitter (UART), and serial peripheral interface (SPI). The measurement is conducted in a chamber with two types of optical PM detector that uses LED and LD as a light source, respectively. The result is analyzed by comparing the result of the reference mass profiler located in the same chamber. The experimental results showed high accuracy in PM1, PM2.5 and PM10. The square of the correlation coefficient (R2) showed more than 0.999 in all measurement, and the maximum consistency errors of the LED module and the LD module are +/- 5 % and +/- 1.6 %, respectively, which is much better than other commercial optical PM detectors. All chips were fabricated with a 0.18 um standard CMOS process. The chip areas are 6.49 mm2 and 6.27 mm2, and power consumptions are 5 mW and 10 mW (excluding bias current of the light sources) for the LED PM detector and the LD PM detector, respectively.