Pulse Oximetry Front End Offset Cancellation for Low-Noise Gain

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Solution Overview

Problem

Pulse-oximetry systems face challenges in detecting small signal amplitudes due to large ambient offset signals, leading to saturation and increased power consumption, as existing front-end amplifiers require high bandwidth and supply voltage to manage large signal swings.

Innovation Solution

An ambient offset cancellation scheme using digital-to-analog converters (DACs) to estimate and subtract ambient offset signals at the front end, reducing noise integration and allowing for higher gain amplification without saturation, implemented in a two-stage amplifier configuration to achieve low noise and low power operation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If the front end amplifier bandwidth is increased to support pulsed input signals, then the signal response capability is improved, but the equivalent noise bandwidth increases and noise integration increases

Engineering Contradiction:

Improvesignal response capabilityVSAvoidnoise integration

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

The patent divides the amplifier operation into distinct phases: a first mode during the pulse interval with higher gain and lower bandwidth for reduced noise integration, and a second mode during the settling interval with lower gain and higher bandwidth for proper signal settling. This temporal segmentation allows the amplifier to optimize for different requirements at different times.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The amplifier dynamically switches between two operational modes based on the input signal phase. The bandwidth and gain are adjusted in real-time: during the pulse interval, the amplifier operates in a low-noise mode with reduced bandwidth, and during the settling interval, it switches to a high-bandwidth mode to accommodate the pulsed input signal properly.

Inventive Principle:

Principle #15Dynamics

2Reliability

If the front end gain is restricted to avoid saturation due to large offset signal, then the dynamic range is preserved, but the detection capability of small signal amplitudes deteriorates

Engineering Contradiction:

Improvesaturation avoidanceVSAvoiddetection capability of small signals

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The system performs preliminary action by subtracting an estimate of the large ambient offset signal from the total input signal before amplification. This offset cancellation prepares the signal in advance, allowing the amplifier to use higher gain without saturating, thereby improving the detection capability of small signal amplitudes while maintaining saturation avoidance.

Inventive Principle:

Principle #10Preliminary action

3Strength

If the supply voltage is increased to support large signal swings, then the signal handling capability is improved, but the power consumption increases

Engineering Contradiction:

Improvesignal handling capabilityVSAvoidpower consumption

Core Design Contradiction:

StrengthVSUse of energy by moving object

Solution Approach 1:

By subtracting the ambient offset signal in advance, the system reduces the amplitude of the signal that needs to be handled by the amplifier. This preliminary signal preparation allows the amplifier to operate with lower supply voltage while still maintaining the capability to handle the full dynamic range of the actual physiological signal, thereby reducing power consumption.

Inventive Principle:

Principle #10Preliminary action

4Adaptability or versatility

If the front end is operated on higher supply voltage to support large offset signals, then the signal swing capability is improved, but the power consumption increases

Engineering Contradiction:

Improvesignal swing capabilityVSAvoidpower consumption

Core Design Contradiction:

Adaptability or versatilityVSUse of energy by stationary object

Solution Approach 1:

The offset cancellation circuit performs preliminary subtraction of the large ambient offset signal, which reduces the required signal swing capability of the front-end amplifier. This allows the amplifier to operate at lower supply voltage, thereby reducing power consumption while maintaining adequate signal swing capability for the actual physiological signals of interest.

Inventive Principle:

Principle #10Preliminary action

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 significantly reduces ambient noise, enabling high dynamic range and low noise operation, even with large ambient offset signals, thereby improving the detection of small signal amplitudes while minimizing power consumption.

Implementation Method 1

An ambient offset cancellation scheme using digital-to-analog converters (DACs) to estimate and subtract ambient offset signals at the front end

Methodology Applied

Scientific EffectDigital-to-Analog Conversion:

Implementation Method 2

Pulse-oximetry typically utilizes a pair of small light-emitting diodes (LEDs) facing a photodiode through a translucent part of the patient's body

Methodology Applied

Scientific EffectPhotoelectric Effect: Photoelectric Effect

Data Source

PatentUS8847578B2Ambient noise cancellation in pulsed input system

Publication Date: 2014.09.30 TEXAS INSTRUMENTS INC

AI summary

Embodiments of the invention provide a pulseoximetry system with ambient offset cancellation that subtracts an estimated ambient offset to thereby allow a large front end gain while operating the front end on a low supply voltage. This large gain reduces input referred noise of an analog to digital converter in the front end while providing high dynamic range for signals with a large ambient offset.