Switched-Capacitor Current-to-Frequency Conversion Without Reset Noise
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Solution Overview
Problem
Traditional in-pixel ADC imagers face challenges with reset pulse widths and noise as semiconductor technology advances, leading to instability and reduced accuracy due to increased gate speeds, making it difficult to fully discharge integration capacitors and maintain accurate photo-current integration.
Innovation Solution
A self-referencing current to frequency converter using a switched capacitor with a Master-Slave Flip Flop and Double-Pole, Double-Throw switch network automatically reverses the integration capacitor's polarity at reference charge levels, eliminating the need for standard reset mechanisms and associated noise, while improving performance with increased gate speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If standard reset mechanisms are used to discharge integration capacitors, then the capacitors can be fully discharged, but reset noise and discharge current are introduced that reduce measurement precision
Solution Approach 1:
The patent extracts and eliminates the reset mechanism entirely from the system. Instead of using traditional reset switches and discharge paths that generate noise, the invention uses a self-referencing current-to-frequency converter that continuously integrates photo-current without requiring periodic discharge events, thereby removing the source of reset noise and discharge current.
Solution Approach 2:
The integration process continues uninterrupted without reset events. The current-to-frequency converter maintains continuous integration of photo-current, converting it to a frequency signal that can be read out without stopping the integration process, thus eliminating gaps and noise associated with periodic resetting.
2Speed
If gate speeds are increased to improve converter performance, then conversion speed improves, but stability and accuracy deteriorate due to reset pulse width control issues
Solution Approach 1:
The patent implements a self-referencing feedback mechanism where the integration capacitor's voltage is continuously compared to a reference voltage. When the capacitor voltage reaches the reference level, the comparator triggers a reset of the integration process. This feedback loop automatically adjusts the integration timing based on the actual capacitor voltage, ensuring stable and accurate operation regardless of gate speed variations.
Solution Approach 2:
The invention changes the operating parameters by using a continuous current-to-frequency conversion process instead of periodic voltage integration and resetting. The frequency output naturally scales with the photo-current magnitude, allowing the system to operate stably at higher gate speeds without the timing-critical reset pulse width problems that plague traditional architectures.
3Area of moving object
If pixel size is reduced to increase resolution, then imaging resolution improves, but photo-charge storage capacity diminishes
Solution Approach 1:
The patent replaces the physical photo-charge storage mechanism (capacitor voltage storage) with a frequency-based digital representation. Instead of storing charge as voltage on a capacitor that is limited by pixel area, the system converts photo-current to a frequency signal that can be represented digitally in a register, effectively substituting physical storage with digital storage that is not constrained by capacitor size.
Solution Approach 2:
The invention changes the storage parameter from voltage (analog, capacitor-limited) to frequency (digital, register-limited). The current-to-frequency converter transforms the photo-current into a frequency-modulated signal that is then captured in a digital register, allowing the storage capacity to be determined by the register's bit depth rather than the physical capacitor area, thus enabling small pixels to maintain high effective storage capacity.
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 enhances the performance of the converter by eliminating reset noise and discharge current, allowing continuous integration during switching events and improving flux-to-frequency bandwidth, even as gate speeds increase, without the stability issues associated with traditional reset mechanisms.
Implementation Method 1
a circuit or imager within a pixel to accumulate charge from a photo-diode, the charge corresponding to the flux of light of various wavelengths incident on the photo-diode
Implementation Method 2
the charge is accumulated on a capacitive element which effectively integrates charge, producing a voltage that corresponds to the intensity of the flux over a given time interval
Data Source
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AI summary
According to one aspect, embodiments herein provide a current to frequency converter comprising a node configured to be coupled to a photodetector and to receive a photo-current from the photodetector, a capacitor having a first terminal and a second terminal and configured to accumulate electrical charge derived from the photo-current on the first terminal and the second terminal, a switch network configured to selectively couple one of the first terminal and the second terminal to the node, and a Master-Slave (MS) Flip Flop (FF) coupled to the switch network and configured to operate the switch network to toggle which of the first terminal and the second terminal is coupled to the node based on a voltage at the node.