The wrist worn device is intended to be used for unobtrusive and continuous monitoring of atrial fibrillation arhytmia. It uses biooptical signal, i.e., photoplethysmogram for this purpose. Biopotential signal, i.e., electrocardiogram is acquired synchroneously and is used as a reference. The reference channel can be detached from the device in unobtrusive usage scenario. This is in contrast with other commercial devices (e.g., Fitbit Surge, MioAlpha) that are not able to record the reference data. Morover, there is no commercial biooptical signal based wrist worn device for arrhythmia monitoring currently on the market. There are two scenarios in which CARRE Wrist worn device can be used: real-time atrial fibrillation arrhythmia monitoring, e.g., during hemodialysis procedure, and long-term, offline atrial fibrillation arrhythmia monitoring, e.g., in rehabilitation settings or at user home.