Comparative Assessment of PPG-Derived HRV Using MAX30102 Sensor and Analog Circuitry with ADS1115 ADC

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study presents a comparative evaluation of basic HRV metrics derived from PPG signals using two different acquisition pathways: a MAX30102 optical sensor and a custom analog front-end based on the ADS1115 ADC. The authors carefully aligned the analog and digital filtering chains to preserve HRV-relevant frequency components. The manuscript addresses a relevant topic in wearable biosensing; however, the topic is highly relevant but limited in originality. This manuscript needs major revision before acceptance. The writing needs to be updated

 

1. The authors state the limitation of existing integrated sensors (e.g., fixed gain stages, limited ADC resolution, inability to modify analog filtering) that their custom design overcomes to better highlight the gap.
2. While the methodology appears sound in principle, several critical details are missing or require improvement to ensure rigor. The study compares two PPG-based methods (MAX30102 vs. Custom Analog + ADS1115). There is no mention of a reference standard.
3. The Intraclass Correlation Coefficient (ICC) is mentioned but not reported in the results. Bland-Altman analysis is presented only for heart rate (BPM), not for the core HRV metrics (SDNN, RMSSD, pNN50).

 

3. The abstract and conclusions repeatedly state that the ADS1115 system achieves improved signal-to-noise ratio due to high-resolution ADC and low-noise analog amplification. However, the abstract and conclusions repeatedly state that the ADS1115 system achieves improved signal-to-noise ratio due to high-resolution ADC and low-noise analog amplification. However, no quantitative SNR measurements (e.g., dB values, noise floor analysis) are provided anywhere in the manuscript. The conclusion is therefore speculative and not supported by the data.no quantitative SNR measurements (e.g., dB values, noise floor analysis) are provided anywhere in the manuscript. The conclusion is therefore speculative and not supported by the data.

 

In addition to the above improvements in English clarity, structural organization, and proper definition of abbreviations are recommended prior to final acceptance.

 

1. The English writing requires revision. Several paragraphs and sections are difficult to follow, please improve the overall clarity and readability.
2. Please restructure and reorganize the section from Processing pipeline for Max sensor (line 108) through the notes of the summary (line 251) to enhance logical flow and clarity.
3. Please correct the spelling errors throughout the manuscript.
4. All abbreviations, including those for systems and key terms, should be clearly defined at their first occurrence.

Comments for author File: Comments.pdf

Author Response

We thank Reviewer 1 for your careful and constructive comments. The reviewer’s comments were highly pertinent and highlighted several issues that the authors had not identified during manuscript preparation. We have considered and addressed each suggestion and revised the manuscript accordingly; these changes have substantially improved the clarity, rigor, and presentation of the work. We appreciate the reviewer’s valuable input, which has materially strengthened the manuscript.

 

We have added 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a comparative study between the MAX30102 integrated digital sensor and a custom analog circuit employing the ADS1115 converter, with the aim of measuring heart rate variability (HRV). Overall, the paper is clearly written, formally sound, and supported by recent and accessible references (even if only 12). The topic is relevant, and the comparison between an integrated digital solution and a custom analog front-end is potentially of interest.

However, in its present form, I believe the manuscript requires major revision before it can be considered for publication.

The main concern is that the contribution remains largely engineering-oriented and offers limited methodological novelty. While the hardware comparison is interesting, the scientific contribution would need to be better substantiated.

A major limitation is the absence of an ECG reference signal. Without a gold-standard measurement, it is not possible to distinguish differences attributable to the sensors themselves from errors introduced by the peak-detection procedure. This issue is particularly important because it directly affects the reliability of the derived HRV metrics. In the current form, this limits the strength of the conclusions that can be drawn from the comparison.

Moreover, the analog front-end, although interesting, is not characterized with sufficient rigor. The discussion of the frequency-dependent positive feedback remains mostly qualitative, and the manuscript does not provide a quantitative analysis of key aspects such as noise, linearity, and the overall transfer function. Similarly, the claims of improved signal-to-noise ratio (SNR) should be supported by explicit measurements, ideally including spectral analysis or other objective performance indicators.

The experimental validation is also rather limited. The dataset includes only a small number of subjects, recordings are short, and no controlled assessment of finger pressure is reported. Since contact pressure can significantly affect PPG signal quality, this factor should either be controlled experimentally or explicitly discussed as a limitation.

In summary, although the manuscript is well presented and addresses a relevant topic, substantial revisions are needed to strengthen the methodological rigor and the experimental validation. In particular, the inclusion of a reference ECG, a more complete quantitative characterization of the analog front-end, and a more robust experimental protocol would significantly improve the paper.

 

Author Response

We thank Reviewer 2 for your careful and constructive comments. The reviewer’s comments were highly pertinent and highlighted several issues that the authors had not identified during manuscript preparation. We have considered and addressed each suggestion and revised the manuscript accordingly; these changes have substantially improved the clarity, rigor, and presentation of the work. We appreciate the reviewer’s valuable input, which has materially strengthened the manuscript.

We have added a PDF file with responses.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors, your paper is a nice description how to design and implement present to day sophisticated electronic circuits for wearable applications, especially photoplethymography.

Make it also clear that due to the differences in the ADCs resolution range (18 versus 16 bit) the in built programmable amplifier in the ADS1115  apparently compensates the lower bit-range - correct?

In the attached paper you will find a series of minor and major items to be revised  and or corrected.  Several spelling errors are also indicated.

Comments for author File: Comments.pdf

Author Response

We thank Reviewer 3 for your careful and constructive comments. The reviewer’s comments were highly pertinent and highlighted several issues that the authors had not identified during manuscript preparation. We have considered and addressed each suggestion and revised the manuscript accordingly; these changes have substantially improved the clarity, rigor, and presentation of the work. We appreciate the reviewer’s valuable input, which has materially strengthened the manuscript.

 

We have included a pdf file with responses.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Paper is acceptable in this form

Author Response

Comment 1: Paper is acceptable in this form

Response 1 : Thank you very much for your time and effort in reviewing this paper in detail.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors, your revisions improved your paper considerably. Nevertheless there are a few  paragraphs which need clarification and/ or additional comments. For instance,  the different signal pick up for the MAX  and the ADS must be adressed in detail. The data in the tables presented with 2 decimals suggest accuracy not achievable with a sample rate of 200 or 250 sps. IR LEDs which differ in wavelength penetrate the tissue to various depth, thus signal quality may differ even though it is nice that you could demonstrate good agreement. Comment necessary.

You find these comments in more detail in the revised version (file named sensors-4233634-peer-review-v2.pdf).

Comments for author File: Comments.pdf

Author Response

We thank Reviewer 3 for your quality as a reviewer careful and constructive comments. The reviewer’s comments were highly pertinent and highlighted several issues that the authors had not identified during manuscript preparation. We have considered and addressed each suggestion and revised the manuscript accordingly; these changes have substantially improved the clarity, rigor, and presentation of the work. We appreciate the reviewer’s valuable input, which has materially strengthened the manuscript.

 

We have attached a PDF containing comments and responses to the reviewer, the revised manuscript with changes highlighted in color, and an additional PDF with minor annotations and their corresponding responses.

Author Response File: Author Response.pdf