Differential Current-to-Voltage Circuit for Ambient Light Rejection
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Solution Overview
Problem
Existing technologies face challenges in detecting small AC current signals in the presence of larger ambient light DC current signals, making it difficult to accurately measure small current signals in applications like optical proximity sensing.
Innovation Solution
A differential current-to-voltage conversion circuit comprising an input sampling stage, a differential integration and DC signal cancellation stage, and an amplification and accumulator stage, which samples and integrates current signals differently during distinct periods to isolate and amplify the AC signal, reducing charge-induced errors and improving signal-to-noise ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional current-to-voltage conversion is used, then the circuit structure is simple, but small AC current signals cannot be accurately detected in the presence of large DC current signals from ambient light
Solution Approach 1:
The circuit segments the current-to-voltage conversion process into two distinct phases: a first phase where the switch connects the photodiode to the first node for integration, and a second phase where the switch connects the photodiode to the second node for differential measurement. This temporal segmentation allows separate handling of DC and AC components, enabling accurate detection of small AC signals despite large DC ambient light interference.
Solution Approach 2:
The circuit changes the operational parameters by switching between two different conversion gains: a first conversion gain during the first phase and a second conversion gain during the second phase. This parameter change enables the system to optimize for different signal conditions - integrating DC components during one phase and measuring AC differential signals during another phase, thereby achieving high precision in both regimes.
2Measurement precision
If separate ambient light cancellation circuitry is added, then the detection accuracy improves, but the device complexity increases
Solution Approach 1:
The circuit merges ambient light rejection functionality directly into the current-to-voltage conversion process itself. By using a single switch to alternately connect the photodiode to two different nodes with different conversion gains, the circuit simultaneously performs integration, differential measurement, and common-mode rejection in one unified structure, eliminating the need for separate ambient light cancellation circuitry.
Solution Approach 2:
The differential current-to-voltage conversion circuit serves multiple functions: it converts current to voltage, integrates signals over time, performs differential measurement, and rejects common-mode ambient light interference - all within a single circuit architecture. This multi-functionality achieves high detection accuracy without requiring additional dedicated circuit blocks for each function.
3Device complexity
If single-ended proximity architecture is used, then the circuit is simpler, but charge-induced errors such as charge sharing and clock feedthrough are present
Solution Approach 1:
Instead of using a single-ended architecture where the photodiode connects to one node, the invention inverts the approach by using a differential architecture where the photodiode alternately connects to two different nodes. This inversion allows the circuit to measure the difference between two paths, thereby canceling out charge-induced errors like charge sharing and clock feedthrough that affect both paths equally.
Solution Approach 2:
The circuit employs feedback through the differential measurement process, where the output from the first conversion gain path and the second conversion gain path are compared. This feedback mechanism allows the system to identify and cancel charge-induced errors by measuring their effect on both paths and subtracting the common-mode error components from the differential signal.
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 solution effectively reduces charge-induced errors and enhances the power supply rejection ratio and common mode rejection ratio, allowing for accurate detection of small AC current signals even in high ambient light conditions, without requiring separate ambient light cancellation circuitry.
Implementation Method 1
a light detector (e.g., a photodiode) operable to generate current signals in response to detected light signals
Data Source
AI summary
An apparatus includes a differential current-to-voltage conversion circuit that includes an input sampling stage circuit, a differential integration and DC signal cancellation stage circuit, and an amplification and accumulator stage circuit. An input common mode voltage of the differential current-to-voltage conversion circuit is independent of an output common mode voltage of the differential current-to-voltage conversion circuit.