Improved sensitivity of portable bio-sensor systems using lock-in detection method

Recently, a number of portable biosensors have been developed to check the health condition of human beings regardless of time and place. Though those sensors have a merit of a handy one, the signals are inevitably measured with intensive background noises. In order to detect a weak signal concealed in intensive background noises such as Johnson noise, 1/f noise, and shot noise, a special technique is essentially required. As of a special technique, the lock-in detection method is able to minimize the effects of these noises using modulated signal and reference signals. Previously, we applied the lock-in detection method to infrared detection system. The sensitivity of this system was decided by the 1/f noise at the photodetector. In this work, we have demonstrated the portable bio-sensor system using high frequency infrared laser diode and photodetector. Therefore, the main signal of interest may keep away from the effect of the 1/f noise with hybrid-type highly sensitive lock-in detection module. This technique provides the minimum detection range of 3 mV and the dynamic range of 25.8 dB at the noise level of 120 mV.