Switch Circuit for Reduced Charge-Injection Errors
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Solution Overview
Problem
Existing switched capacitor circuits face issues with charge-injection errors due to clock feed-through and channel charge-injection, particularly in MOSFETs, which are difficult to accurately model and compensate for, leading to erroneous voltage sampling in sensor devices like Ambient Optical Sensors.
Innovation Solution
A switch circuit design featuring two transistors of the same channel-type, connected in a flip-around arrangement to ensure symmetrical charge injection, allowing for effective compensation using 'dummy switches' and reducing the effects of charge injection, thereby mitigating asymmetry and improving accuracy in low-light measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If MOSFETs are used in switched capacitor circuits, then the circuit can be implemented with standard components, but charge-injection errors occur due to clock feed-through and channel charge-injection
Solution Approach 1:
The patent applies asymmetry by using transistors of the same channel-type (e.g., both NMOS or both PMOS) in a symmetric flip-around arrangement, which creates balanced charge injection characteristics. This symmetric configuration ensures that charge injected at one node is mirrored at the other node, enabling effective cancellation through dummy switches while maintaining ease of manufacture with standard MOSFET components.
2Measurement precision
If existing charge-injection compensation techniques are used, then some error reduction is achieved, but the compensation accuracy is limited due to difficulty in modeling source and drain characteristics
Solution Approach 1:
The patent extracts the charge-injection problem by separating the main signal path from the charge compensation function. Dummy switches are introduced specifically to handle charge injection, while the main transistors focus on signal switching. This separation simplifies the modeling requirement by making the charge injection characteristics predictable and symmetric, reducing the need for complex device modeling.
Solution Approach 2:
The patent changes the configuration parameter of the transistor arrangement from asymmetric (conventional source-drain configuration) to symmetric (flip-around arrangement with same channel-type transistors). This parameter change makes the charge injection characteristics predictable and equal at both nodes, significantly improving compensation accuracy without requiring complex modeling of varying source and drain characteristics.
3Measurement precision
If symmetric switch configuration is used, then charge-injection errors are reduced, but the circuit area and component count increase
Solution Approach 1:
The patent merges the function of the main switching transistors with the charge compensation function by using the same transistors for both signal switching and charge injection. The flip-around configuration allows the main transistors to serve dual purposes: switching the signal and providing symmetric charge injection that can be compensated by dummy switches. This merging reduces the need for additional dedicated compensation components, minimizing area overhead.
Data Source
AI summary
A switch circuit for use in a single-ended switched-capacitor circuit for front-end circuitry of a sensor device is disclosed. The switch circuit comprises a first transistor and a second transistor having a same channel-type as the first transistor. A first node is connected to a source of the first transistor and a drain of the second transistor and a second node is connected to a drain of the first transistor and a source of the second transistor. Also disclosed is a sampling circuit comprising the switch circuit and a sampling capacitor, wherein the switch circuit is configurable to electrically couple the sampling capacitor to an integrator circuit or to a voltage reference. An integrated circuit device and a light to frequency converter or light sensor comprising the switch circuit is also disclosed.