Conventional photoplethysmographic sensor, pulse oximeters, and the like that obtain, as photoplethysmographic signals, changes in the intensity of light that passes through a biological body such as a finger or is reflected by the biological body by exploiting the characteristics of blood hemoglobin that absorb visible light to infrared light are known (see Patent Document 1, for example).
Here, the pulse oximeter according to Patent Document 1 includes first and second light-emitting diodes that are driven in an alternating manner by pulse signals outputted from an oscillation circuit so as to irradiate biological tissue with red light and infrared light, and a photodiode that detects a light output after the stated light has been absorbed by the biological tissue. Light reception output from the photodiode is amplified by an amplifier, and is then distributed and inputted to a computing unit in synchronization with the output of the oscillation circuit using a multiplexer. Based on direct current components and pulsation components in respective wave lengths obtained from the light reception output of the photodiode, the computing unit calculates a percentage Φ of the pulsation components of absorbance of an artery blood flow, and then calculates an arterial blood oxygen saturation from the percentage Φ of the pulsation components of the absorbance.
Patent Document 1: Japanese Patent No. 3116252.
Incidentally, external light from sources aside from the light-emitting diodes (a light-emitting element) (sunlight, fluorescent lamp light, or the like, for example) sometimes enters into the photodiode (a light-receiving element). There is a risk that such external light will combine with the light originally to be detected, namely the light that has passed through the biological body or that has been reflected by the biological body, and lead to a drop in the signal to noise ratio of the detection signal.
According to the pulse oximeter of Patent Document 1, when external light has entered the light-receiving element in such a combined manner and the external light component (a DC noise component) has increased, the amplifier output is saturated and the pulsation component (a signal component) can no longer be accurately extracted. However, if the amplification rate of the amplifier is reduced to prevent the output saturation, the amplitude of the pulsation component will also drop, resulting in a risk that the accuracy of detecting the oxygen saturation will drop. In the case where the signal is encoded including the external DC noise component, it will be necessary for the resolution of an A/D converter to be sufficiently high with respect to the pulsation component, which leads to an increase in costs. What is needed, therefore, is a technique that enables an improvement in the signal to noise ratio of a detection signal obtained when a light-receiving element receives light and the resulting signal is amplified by an amplifier.