Effect of Automatic Gain Control on Non-Contact Capacitive Measurement of Respiratory Movements during Sleep ^†

An unobtrusive device capable of measuring electrocardiogram (ECG) and respiratory movements (RM) of the chest and abdomen is under development by using non-contact electrodes under the bedsheet. However, when the coupling between the body and the electrode changes from capacitive to resistive by sweat during sleep, amplitude of the RMs tends to decrease. Consequently, detection accuracy (DA) of RMs becomes unstable. In this study, we introduced an automatic gain control (AGC) circuit to improve DA. The AGC circuit was composed of a variable gain amplifier, a RMS-to-DC converter, a standard voltage circuit, and a comparator. We examined the efficacy of the AGC circuit by comparing the DAs before and after introduction of AGC circuit during sleep. Analysis for the initial one hour sleep in three subjects revealed that DAs in the chest improved for all subjects and increased by 15.4% on average. In the abdomen, the number of undetected RM decreased considerably in subject #1, whereas that of the false detection increased modestly in subject #3. Advanced analysis implied that the introduced AGC circuit was useful for the reduction of the number of undetected RM due to amplitude attenuation, but tended to increase that of false detection caused by the deformative waveform of RM.