PHOTODIODE SENSING IN PULSE OXIMETERS

Pulse oximeters monitor blood oxygen levels noninvasively by placing a peripheral probe (sensor) on a thin part of a person’s anatomy such as a fingertip, toe, or earlobe. The probe typically includes two light emitting diodes (LEDs): one in the visible red spectrum (660 nm), and the other in the infrared spectrum (940 nm), although some designs may employ more than two diodes. The percentage of oxygen in the body is determined by calculating the ratio between these two intensities of light after they have transmitted through the body.

Photodiodes are used for light detection, conversion, and measurement. The light shining on the photodiode produces a small current that is converted to a voltage by an amplifier in a classical resistor-feedback transimpedance configuration (I/V). Usually a large resistor is used in the amplifier’s feedback loop, so the circuit is very sensitive to small changes of light. The resulting voltage goes through filtering and conversion before values are shown on the LCD of the instrument. Maintaining fast, accurate measurement on a battery-powered device is challenging.

The AD8663/AD8667/AD8669 16 V CMOS operational amplifier family offers supply current consumption of 210 µA typical per amplifier. The devices’ rail-to-rail output swing helps increase measurement dynamic range, and low bias current over the temperature range will mitigate calibration errors caused by IB. Low input capacitance relative to the capacitance of the photodiode supports stable, fast measurement. This family of products combines voltage noise density of 23 nV/√Hz typical @ 1 kHz, current noise density of 0.05 pA/√Hz typical @ 1 kHz, offset voltage of 175 µV typical., input bias current of 0.3 pA typical, and gain bandwidth of 520 kHz. In addition, these amplifiers can be powered by single or dual supplies ranging from 5 V to 16 V or ±2.5 V to ±8 V, respectively. The AD8663 is available in 8-lead LFCSP and SOIC; the AD8667 is available in 8-lead MSOP and SOIC; and the AD8669 is available in 14-lead SOIC and TSSOP.