A Simple front-end for pulse oximeters with a direct light-to-time-to-iigital interface circuit

Abstract

Pulse oximeters are electronic devices that pro- vide the values of two physiological parameters: heart rate (HR) and functional O2 saturation, SpO2. Acquiring this infor- mation requires a photodiode and a set of red and infrared light-emitting diodes (LEDs) to generate two plethysmo- grams, which are digitally processed to find the HR and SpO2 values. The circuits currently used to generate these plethys- mograms require multiple analog modules, such as voltage references, transimpedance amplifiers (TIAs), voltage ampli- fiers, 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.