LOW NOISE MEASUREMENT OF PHOTOCURRENT FOR CONTINUOUS GLUCOSE MONITORING Low Noise Measurement System Enables Continuous Monitoring of Glucose in Subcutaneous Interstitial Fluid

We have developed a data acquisition unit (DAU) for continuous, low noise measurement of glucose concentration in subcutaneous interstitial fluid (ISF). The system is comprised of a glucose sensor (Olesberg, 2006), op-amps for signal conditioning, and delta-sigma (Delta-Sigma) ADCs. The glucose sensor has two IR LEDs which emit light with wavelength of 2.2 to 2.4 mu m where there are peaks in the glucose absorption spectrum. The IR light propagates through a glass fluid chamber containing interstitial fluid and a linearly variable bandpass filter before impinging on a 32 channel photodiode array. The center frequency of the filter varies along one dimension of the filter, so that each photodiode is sensitive to equally spaced portions of the glucose absorption spectrum. Transimpedance amplifiers (TIAs) convert the photocurrents into voltages which are sampled by ADCs. We developed a noise model which predicts the noise characteristics of the system. We use low noise metal film resistors to verify the DAU noise characteristics. Non-ideal characteristics such as limited photocurrent and low photodiode shunt resistance increase difficulty of obtaining low noise measurements. We demonstrate that the DAU provides low noise (41.7 dB SNR) photocurrent measurements and is suitable for use in a continuous glucose monitoring system.