# Flexible Wearable Heart Rate Monitoring System and Low-Power Design: A Review
## 4 Design of the Flexible Wearable Heart Rate Signal Preprocessing Module
### Low-Power Hardware Design of Flexible Wearable Heart Rate Signal Preprocessing Module
Meanwhile, the circuit structure combines a traditional second-order RC low-pass filter with a high-pass filter, whereas the circuit structure of traditional band-pass filter designs is often more complex, leading to higher power consumption. Specifically, by using adjustable pseudo-resistors and specific capacitors, they precisely adjusted the cutoff frequency, which not only removed high-frequency noise in heart rate signals but also alleviated low-frequency interference, thereby reducing additional power consumption. The total system power consumption is approximately 460 uW. With the same noise suppression effect, this design reduces power consumption by about 30% compared with unoptimized band-pass filters. In practical flexible wearable systems, the effectiveness of hardware filtering strategies is highly contingent on scenario-specific constraints. A comprehensive evaluation integrating the noise characteristics, resource limitations, and performance requirements of the intended usage scenario is essential, rather than simply promoting a single method. The following is an assessment of the applicable scenarios for the three aforementioned flexible filter modules:  
Optimized low-pass filter modules excel in suppressing high-frequency noise, such as electromagnetic interference. They offer distinct advantages in heart rate monitoring scenarios under static conditions, where signals are stable and low-frequency drifts are negligible. However, their limitations become prominent in motion scenarios: due to their inability to handle low-frequency interferences like baseline drift, the signal quality in dynamic environments will significantly decline. Consequently, they are unsuitable for long-term dynamic heart rate monitoring or high-intensity exercise scenarios [89]. Figure 4: (**a**) Active low-pass filter for PPG signal conditioning. Reproduced with permission [62], copyright 2020, IEEE. (**b**) Circuit with a Band\(-\)pass Filter. Reproduced with permission [44], copyright 2021, IEEE. Tunable high-pass filter modules achieve adaptive suppression of low-frequency baseline drift through dynamic parameter adjustment. This makes them particularly practical for heart rate signal preprocessing in moderate-intensity exercise scenarios. It should be noted, however, that the introduction of tunable components increases circuit integration complexity. Additionally, some tunable components are sensitive to ambient temperature, which may lead to performance degradation in extreme temperature scenarios. Therefore, their applicability is limited in resource-constrained microdevices or scenarios involving operation in special temperature zones [90]. Low-power band-pass filter modules integrate the advantages of both low-pass and high-pass filters, being capable of simultaneously suppressing high-frequency electromagnetic interference and low-frequency baseline drift. They perform optimally during intense exercise in complex environments. However, their power consumption is significantly higher than that of single low-pass or high-pass filters, and their complex circuit structure increases the difficulty of integration onto flexible substrates. This means they are better suited for scenarios with high precision requirements but relatively relaxed power budgets, such as medical-grade exercise monitoring, rather than ultra-low-cost systems or miniature flexible wearable systems [91]. In summary, the aforementioned flexible filtering circuit modules have fully considered low-power characteristics in their design. By selecting low-power electronic components, such as adjustable pseudo-resistors, optimizing the parameter configuration of components like resistors and capacitors, and adopting optimized circuit structures, these circuits significantly reduce energy consumption while ensuring the functionality of the module compared with traditional unoptimized filter designs. This not only extends the battery life of flexible wearable systems but also helps ensure the long-term stable operation of these systems.