A Simple Front-End for Pulse Oximeters With a Direct Light-to-Time-to-Digital Interface Circuit

Pulse oximeters are electronic devices that provide the values of two physiological parameters: heart rate (HR) and functional O2 saturation, SpO2. Acquiring this information requires a photodiode and a set of red and infrared light-emitting diodes (LEDs) to generate two plethysmograms, which are digitally processed to find the HR and SpO2 values. The circuits currently used to generate these plethysmograms require multiple analog modules, such as voltage references, transimpedance amplifiers (TIAs), voltage amplifiers, and comparators. Analog-to-digital converters (ADCs) or switched integrators (SIs) are also needed to digitalize the signals that provide the HR and SpO2 values when introduced in a digital processor (DP). This article proposes a new circuit that eliminates the need for all these analog modules or converters, replacing them with a single capacitor and two resistors. The circuit is based on a light-to-time-to-digital conversion performed using the DP, which does not require any special characteristics to carry out this task. As a proof of concept, the new design has been implemented using a field-programmable gate array (FPGA) as the DP. The results show plethysmograms with good detail regarding amplitude and time, allowing the device to be used for clinical purposes. A comparison with two conventional commercial pulse oximeters shows that the new circuit provides similar HR and SpO2 values.