Fast-turnoff photodiodes with switched-gain preamplifiers in photoplethysmographic measurement instruments

摘要

An improved photoplethysmographic measurement system is disclosed for use in ambient light. The system has two or more light emitters (20-23) each emitting light at different wavelengths and a detector photodiode (26) for detecting the intensity of light that has passed through a patient's tissue, e.g. a finger (24). The emitters are pulsed sequentially, consecutive pulses being separated by a dark time when no emitter is energised so that the detector measures the ambient light during the dark time. The signal from the detector (26) during the dark time is subtracted from the signal during one of the pulses to give a measure of the tissue transmission at the pulse wavelength. An error is introduced if the emitter does not turn off immediately at the end of a pulse since the light emitted boosts the dark time signal, particularly if the system uses switched-gain preamplifiers. According to the invention, a fast-turn-off photodiode is used in order to minimise the effect of "bleed-through" voltages on the dark time (ambient) light determination. A hardware method and a software method for determining whether a given photodiode has a sufficient decay time are also disclosed.