PPG DC Offset Cancellation Filter for Faster Signal Settling
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Photoplethysmography (PPG) devices face challenges in accurately acquiring cardiac waveforms due to high power consumption, large circuit area requirements, and motion artifacts from wrist-based wearable devices, with significant DC offset components overwhelming the signal of interest, leading to saturation and slow signal acquisition.
Innovation Solution
The implementation of a feedforward cancellation signal generated from the DC component to an amplifier, combined with a low-pole frequency filter calibration to accelerate settling time and improve signal tracking, utilizing a transfer function mapping for efficient DC offset cancellation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low gain, high bandwidth amplifier chain with oversampling ADC is used to avoid clipping, then signal saturation is prevented, but power consumption increases and signal acquisition speed decreases
Solution Approach 1:
The patent applies preliminary action by performing DC offset cancellation before the signal enters the amplifier chain. A DC cancellation filter removes the DC component from the photodiode current prior to amplification, allowing the subsequent amplifier to operate with higher gain without saturation. This preprocessing step enables the use of lower power consumption components while maintaining signal integrity and acquisition speed.
2Measurement precision
If amplifier gain is increased to detect the weak blood vessel signal, then signal detection sensitivity improves, but amplifier saturation occurs due to the strong DC component
Solution Approach 1:
The patent segments the signal processing into two distinct paths: a DC cancellation path that extracts and removes the DC offset component, and an AC signal path that amplifies only the blood vessel signal. The DC cancellation filter separates the DC component from the AC component, allowing independent processing. This segmentation enables high gain amplification of the AC signal without saturation from the DC component, improving both detection sensitivity and reliability.
3Measurement precision
If a brute-force approach with extra averaging is used to handle motion artifacts, then signal tracking accuracy improves, but processing speed decreases and power consumption increases
Solution Approach 1:
The patent implements feedback by continuously monitoring the output of the DC cancellation filter and using this information to dynamically adjust the cancellation process. The system measures the actual DC offset present in the signal and applies compensating adjustments in real-time. This feedback mechanism enables accurate signal tracking without requiring extensive averaging, thereby maintaining both high precision and fast acquisition speed while reducing power consumption.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the speed and accuracy of PPG signal acquisition by effectively canceling DC offsets, reducing power consumption, and improving the signal-to-noise ratio, enabling robust and efficient heart rate detection in wearable devices.
Implementation Method 1
The light is generated by a pulsed Light Emitting Diode (LED) which is placed against the skin (often a wrist) and detected by a photodiode also placed against the skin in near vicinity to the LED
Implementation Method 2
The light is generated by a pulsed Light Emitting Diode (LED)
Data Source
AI summary
Described is an apparatus which comprises: an amplifier to receive a reference voltage; and calibration logic which is operable to receive a first voltage and to provide the reference voltage to the amplifier, wherein the calibration logic is operable to generate a look-up table (LUT) that maps the first voltage to a drive current.