Ramp Generator Power Supply Rejection in Image Sensors
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Solution Overview
Problem
Conventional CMOS image sensors suffer from unsatisfactory power supply rejection ratios, leading to noise coupling from power supplies that result in unwanted horizontal ripple in captured images due to limitations in the design and layout of source follower circuits.
Innovation Solution
A ramp generator with a power supply compensation circuit is implemented in the readout circuitry of image sensors, which includes a delay circuit with a variable resistor and filter capacitor to match the delay ripple from the power supply to the bitline output, and a capacitive voltage divider to match the coupling ratio, ensuring that power supply variations are canceled out at the analog to digital converter input.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If source follower circuits are used in pixel cells to amplify signals, then signal amplification is achieved, but power supply rejection ratio deteriorates to unsatisfactory levels (e.g., -20 dB)
Solution Approach 1:
A ripple generator circuit is introduced as an intermediary component that actively generates compensation signals matching the power supply ripple characteristics. This intermediary circuit receives power supply voltage as input and produces a ripple signal that is then subtracted from the pixel output signal, effectively mediating the rejection of power supply interference while preserving signal amplification functionality.
Solution Approach 2:
The ripple generator is configured to receive power supply voltage as input and generate a ripple signal that feeds back into the signal processing path. This feedback mechanism continuously tracks and compensates for power supply variations, creating a closed-loop system that maintains high power supply rejection ratio while preserving the amplification function of the source follower circuits.
2Device complexity
If conventional readout circuitry is used, then circuit simplicity is maintained, but noise from power supplies couples into the output signal path causing horizontal ripple in images
Solution Approach 1:
The ripple generator acts as an intermediary that introduces controlled ripple signals to cancel out harmful noise. By generating a compensation signal that mirrors the power supply ripple characteristics and subtracting it from the output path, the system eliminates horizontal ripple artifacts while maintaining relatively simple circuit architecture.
Solution Approach 2:
The invention converts the harmful power supply ripple into a beneficial compensation signal. The ripple generator deliberately generates signals that match the harmful ripple characteristics, then uses these signals to cancel out the noise through differential subtraction, effectively transforming the harmful power supply variations into a useful noise-cancellation mechanism.
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 significantly improves the power supply rejection ratio, reducing noise and ripple in the image data, resulting in higher quality image capture by effectively isolating power supply variations from the signal path.
Implementation Method 1
A ramp generator with a power supply compensation circuit is implemented in the readout circuitry of image sensors, which includes a delay circuit with a variable resistor and filter capacitor to match the delay ripple from the power supply to the bitline output
Implementation Method 2
A capacitive voltage divider is coupled to the delay circuit to generate the ramp generator input reference signal
Data Source
AI summary
A ramp generator for use in readout circuitry includes an integrator coupled to receive a ramp generator input reference signal to generate a reference ramp signal coupled to be received by an analog to digital converter. A power supply compensation circuit that is coupled to generate the ramp generator input reference signal includes a delay circuit including a variable resistor and a filter capacitor coupled to receive a power supply signal. The variable resistor is tuned to match a delay ripple from the power supply to a bitline output. A capacitive voltage divider is coupled to the delay circuit to generate the ramp generator input reference signal. The capacitive voltage divider includes a first variable capacitor coupled to a second variable capacitor that are tuned to provide a capacitance ratio that matches a coupling ratio from the power supply to the bitline output.