Ambient Light Compensation in Optical Sensors Using Current Mirror

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Solution Overview

Problem

Existing methods for ambient light compensation in optical sensors, particularly those using modulated or pulsed useful light, face challenges in independently dimensioning signal amplifiers and ambient light compensation due to the placement of compensation circuits downstream of the signal amplifier.

Innovation Solution

The use of a current mirror circuit with a lowpass filter between two photodiodes, where the photocurrent of one photodiode is mirrored and ambient light is suppressed, allowing the useful signal to pass through while damping ambient light, and an integrator can be added for further compensation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If ambient light compensation is performed downstream of the signal amplifier, then the compensation circuit can be implemented, but the signal amplifier and ambient light compensation cannot be dimensioned independently

Engineering Contradiction:

Improvecompensation circuit implementationVSAvoidindependent dimensioning of signal amplifier and compensation circuit

Core Design Contradiction:

Ease of manufactureVSAdaptability or versatility

Solution Approach 1:

The optical sensor is divided into multiple photodiodes (first photodiode for signal, second photodiode for ambient light compensation) that operate independently. Each photodiode has its own current path, allowing the signal amplifier and compensation circuit to be dimensioned independently without mutual constraint.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A current mirror circuit is introduced as an intermediary between the two photodiodes. The current mirror circuit copies the current from the first photodiode and adjusts it to compensate for ambient light effects on the second photodiode, enabling independent design of the amplification and compensation stages.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Object-affected harmful factors

If a current mirror circuit with lowpass filter is used to suppress ambient light, then ambient light compensation is improved, but the amplitude of the useful signal is reduced

Engineering Contradiction:

Improveambient light suppressionVSAvoiduseful signal amplitude

Core Design Contradiction:

Object-affected harmful factorsVSMeasurement precision

Solution Approach 1:

The lowpass filter is applied selectively in the current mirror circuit path for ambient light compensation, while the main signal path retains higher frequency content. This localized filtering allows ambient light suppression without excessive attenuation of the useful signal, as the signal amplifier can compensate for the mild damping.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The current mirror circuit creates a copy of the first photodiode's current and adjusts it to match the ambient light conditions of the second photodiode. This copying mechanism allows precise ambient light compensation while maintaining the original signal amplitude characteristics through the differential measurement.

Inventive Principle:

Principle #26Copying

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 effectively suppresses ambient light, allowing for independent dimensioning of signal amplifiers and improving ambient light compensation, albeit with a reduced amplitude of the useful signal, which is tolerated for better light suppression.

Implementation Method 1

at least one first photodiode, at least one second photodiode, said at least one first photodiode being adapted to be exposed to substantially the same useful light and ambient light as said at least one second photodiode

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Implementation Method 2

the current mirror circuit comprising a lowpass filter connected between the input and the output of the current mirror circuit

Methodology Applied

Scientific EffectLowpass filtering: Filter (electronic)

Data Source

PatentUS8822901B2Device for ambient light compensation for optical sensors exposed to both useful light and ambient light

Publication Date: 2014.09.02 ELMOS SEMICON AG

AI summary

The device for ambient light compensation for use in optical sensors exposed to both useful light and ambient light comprises at least one first photodiode and at least one second photodiode, the at least one first photodiode being adapted to be exposed to substantially the same useful light and ambient light as the at least one second photodiode. The device further comprises a current mirror circuit having an input and an output, the at least one first photodiode being connected to the input of the current mirror circuit, and the at least one second photodiode being connected to the output of the current mirror circuit. The current mirror circuit comprises a lowpass filter connected between the input and the output of the current mirror circuit. The output of the current mirror circuit provides an output signal representing the useful signal compensated for ambient light.