Current Mirror Circuit for Optical Sensor Dark Current Cancellation
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Solution Overview
Problem
Current cancellation techniques in optical sensors are limited by leakage current, such as dark current, which degrades signal resolution and prevents detection of the entire range of ambient light conditions due to variations in photo sensor diodes.
Innovation Solution
A circuit with a first current source and a current mirror coupled through a switch, controlled by a sigma delta modulator, to mirror and subtract currents accurately, ensuring that the average current flowing into a node is equal to the current source current, effectively canceling unwanted current at the node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If photo sensor diodes are used to detect ambient light conditions, then light detection capability is provided, but dark current leakage degrades signal resolution and limits detection range
Solution Approach 1:
The patent generates a controlled dark current in a compensation photodiode that matches the dark current in the signal photodiode. This harmful dark current is converted into a beneficial cancellation signal by mirroring it through current mirror circuits and subtracting it from the signal path, thereby eliminating its degrading effect while preserving the useful light detection capability
Solution Approach 2:
The patent introduces a compensation photodiode and current mirror circuit as intermediary elements between the signal photodiode and the output. These intermediaries capture and replicate the dark current, then use it to cancel the harmful leakage current in the signal path through controlled current subtraction, improving measurement precision without directly modifying the signal photodiode
2Measurement precision
If current cancellation techniques are applied to cancel dark current, then signal resolution is improved, but circuit complexity increases due to additional components
Solution Approach 1:
The patent divides the current cancellation function into separate modular components: a compensation photodiode for generating dark current, current mirror circuits for replicating and transferring the current, and switch circuits for controlled connection. This segmentation allows the complex cancellation function to be implemented through simpler, independent modules that can be designed and analyzed separately
Solution Approach 2:
The patent uses current mirror circuits to create accurate copies of the dark current from the compensation photodiode. By replicating the dark current signal through matched transistor pairs, the circuit generates an identical current profile that can be subtracted from the signal path, achieving precise cancellation without requiring direct measurement or complex processing of the original dark current
3Adaptability or versatility
If dark current varies with process variations, temperature, and junction depth, then manufacturing flexibility is maintained, but current cancellation accuracy is degraded
Solution Approach 1:
The patent implements a dynamic current cancellation system where the compensation photodiode and current mirror circuits continuously track and adapt to changes in dark current characteristics. By using matched transistor pairs and controlled current mirroring, the system dynamically adjusts the cancellation current to match the actual dark current level, maintaining high cancellation accuracy despite variations in process, temperature, or junction depth
Solution Approach 2:
The patent compensates for dark current variations by changing the operational parameters of the current mirror circuits. Through controlled adjustment of mirror ratios and switching sequences, the system adapts the cancellation current magnitude to match the actual dark current level under different operating conditions, maintaining cancellation accuracy across varying temperature and process parameters
Data Source
AI summary
Techniques are described to mirror currents and subtract currents accurately. In an implementation, a circuit includes a first current source coupled to a first node to provide a current IPD1 and a current mirror coupled to the first node through a first switch T1 to provide a current IREF1. In a closed configuration, the current IREF1 flows from the current mirror into the first node. A sigma delta modulator controls the switch T1 such that over a period of time an average current flowing from the current mirror into the first node is equal to the current IPD1 flowing out of the first node. The sigma delta modulator generates a digital output to control switch T2 to allow a current IREF2 into a second node, thus subtracting a portion of a current IPD2 at the second node over a period of time.