Low noise measurement of photocurrent from low impedance photodiodes

We have developed a data acquisition unit (DAU) for continuous, low noise measurement of photocurrent from low shunt resistance photodiodes. The system is intended for use with a glucose sensor as part of a continuous glucose monitoring system. This paper describes only the design of the photocurrent measurement system and biocompatibility issues are beyond the scope of this paper. The DAU is comprised of a glucose sensor, analog circuitry for signal conditioning, and delta-sigma analog to digital converters (Δ-Σ ADCs). The glucose sensor operates in the near infrared (IR) region where glucose has absorption features that allow concentration measurement. The sensor is comprised of two IR LEDs, a glass fluid chamber, a linearly variable bandpass filter, and a 32 element photodiode array. IR light propagates through the fluid chamber containing analyte and the filter, impinging on the photodiode array. Transimpedance amplifiers (TIAs) convert the photodiode photocurrents into voltages that are sampled by the ADCs. We developed a noise model that predicts the noise characteristics of the system. In this paper we use low noise metal film resistors to verify the DAU noise characteristics in preparation for future use with glucose sensors. A high signal-to-noise ratio (SNR) is needed to accurately quantify the concentration of glucose. The SNR available is limited by the low luminosity available at the photodiodes and the low impedance of the photodiodes (Rpd).