CTIA Charge Injection Compensation for Uniform Output Swing
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Solution Overview
Problem
Capacitive Trans-Impedance Amplifier (CTIA) circuits face challenges in compensating for charge injected by the MOS reset transistor, leading to voltage errors and reduced output voltage swing, especially at low infrared radiation levels, requiring a solution for adjustable and uniform charge compensation across pixel arrays.
Innovation Solution
The implementation of a CTIA circuit with compensation capacitors, such as metal-oxide-metal capacitors, and digital-to-analog converters (DACs) to control and vary the compensation charge, allowing for adjustable and uniform charge compensation, minimizing the impact of charge injection and enabling a minimum-sized reset transistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the CTIA feedback capacitance is made small to achieve high conversion gain, then the input-referred noise is reduced, but the charge injected by the MOS reset transistor causes large voltage error at the CTIA output
Solution Approach 1:
The patent applies preliminary anti-action by injecting a compensation charge of opposite polarity to the reset charge before the reset transistor turns off. This is achieved by using a compensation capacitor connected to the inverting input port that is charged during the reset phase and then discharged to cancel the charge injection effect, thereby preventing the voltage error before it occurs
Solution Approach 2:
The patent introduces a compensation capacitor as an intermediary element between the reset transistor and the amplifier input. This capacitor serves as a mediator that stores the compensation charge and delivers it to counteract the charge injection, isolating the amplifier from the direct impact of reset charge while maintaining the high conversion gain
2Ease of operation
If a MOS reset transistor is used for reset function, then the reset operation is simple, but charge injection leads to reduced output voltage swing
Solution Approach 1:
The patent converts the harmful charge injection effect into a beneficial compensation mechanism. By intentionally injecting a controlled compensation charge of opposite polarity through the compensation capacitor, the harmful reset charge is neutralized, and the previously harmful effect becomes part of the solution to maintain output voltage swing
3Object-affected harmful factors
If the injected charge is compensated, then the voltage error is reduced, but the injected charge varies due to slope of reset clock signal and impedance variations
Solution Approach 1:
The patent implements dynamics by making the compensation charge adjustable and adaptable to varying conditions. The compensation capacitor can be configured with different capacitance values, and the reset clock signal slope can be optimized to ensure consistent charge injection characteristics across different operating conditions and pixel locations
Solution Approach 2:
The patent applies parameter changes by adjusting the compensation capacitor value and the reset clock signal characteristics to maintain uniform charge compensation. By optimizing these parameters, the system achieves consistent compensation performance across the pixel array despite variations in impedance and operating conditions
4Object-affected harmful factors
If compensation elements are added to the CTIA circuit, then charge injection is compensated, but the layout area increases
Solution Approach 1:
The patent applies the nested doll principle by integrating the compensation capacitor into the existing CTIA circuit structure. The compensation capacitor is positioned and connected in a way that utilizes the available space efficiently, nesting the compensation function within the existing circuit footprint rather than adding separate external compensation elements
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 injection, maintains high conversion gain, and ensures uniform output voltage response across the pixel array, maximizing the output voltage swing and allowing for testing and screening of defective pixels.
Implementation Method 1
A photodiode is connected to the inverting input port of the amplifier. The photodiode is biased to generate a photocurrent upon receiving photons.
Implementation Method 2
One or more compensation capacitors are provided, wherein each compensation capacitor has a first port connected to a compensation signal and has a second port connected to the inverting input port of the amplifier.
Data Source
AI summary
A capacitive trans-impedance amplifier circuit with charge injection compensation is provided. A feedback capacitor is connected between an inverting input port and an output port of an amplifier. A MOS reset switch has source and drain terminals connected between the inverting input and output ports of the amplifier, and a gate terminal controlled by a reset signal. The reset switch is open or inactive during an integration phase, and closed or active to electrically connect the inverting input port and output port of the amplifier during a reset phase. One or more compensation capacitors are provided that are not implemented as gate oxide or MOS capacitors. Each compensation capacitor has a first port connected to a compensation signal that is a static signal or a toggling compensation signal that toggles between two compensation voltage values, and a second port connected to the inverting input port of the amplifier.