Matched LED Driver Circuit for Pulse Oximeter SNR Gain
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Solution Overview
Problem
LED drivers in pulse oximeters do not operate at maximum signal-to-noise ratio (SNR) due to mismatched transmit and receive shapes, leading to suboptimal power efficiency in high-power applications like heart-rate and blood oxygen measurement.
Innovation Solution
A matched filter, comprising passive RC components, modulates a rectangular pulse to approximate the mirrored impulse response of a decimation filter, ensuring correlation between LED currents and optical sensor outputs, thereby enhancing SNR with adjustable resistors and capacitors for optimal performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a rectangular pulse is used to drive the LED, then the circuit is simple to implement, but the signal-to-noise ratio does not achieve theoretical maximum due to mismatched transmit and receive shapes
Solution Approach 1:
The patent transforms the static rectangular pulse into a dynamic modulated waveform that varies over time. The LED driver applies a time-varying signal shape that matches the impulse response of the photodiode filter, creating a dynamic match between transmit and receive characteristics while maintaining circuit simplicity through programmable control.
Solution Approach 2:
The patent changes the temporal parameters of the LED driving signal from a constant rectangular pulse to a time-varying waveform with specific amplitude and duration profiles. By adjusting the signal shape parameters to match the photodiode's impulse response, the system achieves optimal SNR without complicating the overall circuit architecture.
2Measurement precision
If the LED is driven with high power to improve signal quality, then the signal-to-noise ratio improves, but power consumption increases reducing battery life
Solution Approach 1:
The patent employs periodic pulsed operation instead of continuous high-power driving. By concentrating power delivery into optimized pulse shapes that match the photodiode response, the system achieves high signal quality during measurement intervals while allowing power savings during intervals between measurements, thus extending battery life.
Solution Approach 2:
The patent pre-calculates and stores the optimal signal waveform that matches the photodiode impulse response. This preliminary preparation allows the system to apply the exact right amount of power at the exact right times, avoiding waste and achieving maximum signal quality with minimum 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
The solution achieves high signal-to-noise ratio under limited power, improving power efficiency and extending battery life in wearable electronics and biosensors by using a simple, cost-effective passive filter configuration.
Implementation Method 1
The matched filter is a passive filter which is able to modulate a rectangular pulse into a modulated signal with good approximation to the quadradic impulse response. The matched filter only comprises passive components, such as resistor and capacitor
Implementation Method 2
The matched filter is a high-pass Resistor-Capacitor (RC) filter comprising a first resistor, a second resistor, and a capacitor
Implementation Method 3
a pair of small LEDs to emit red and/or infrared light through a translucent part of a user body
Implementation Method 4
Light emitting diode (LED) has wide applications in various industries
Implementation Method 5
An optical sensor, such as a photodiode, incorporated within the pulse oximeter captures transmitted light (that is not absorbed) for analysis
Data Source
AI summary
An electronic device comprising a matched filter to drive a LED module is disclosed. The matched filter receives an incoming rectangular pulse and outputs a modulated driving signal to drive the LED module. The optical output from the LED is captured by an optical sensor and converted to electrical signal for analysis. A decimation filter couples to the optical sensor in an effort of reducing sampling rate and quantization noise of the converted electrical signal. The modulated driving signal output from the matched filter keeps the LED current, thus LED light, matching the time-mirrored impulse response of the decimation filter to obtain or approach a theoretical maximum signal-to-noise (SNR) at a fixed amount of power.