Column-Parallel Sigma-Delta ADC With Reference-Regulated Noise Mitigation
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Solution Overview
Problem
Sigma-delta modulation sensing circuits and analog-to-digital converters in CMOS imagers face noise issues due to the ratio-based operation, which affects the accuracy of pixel and reset signal voltage conversions.
Innovation Solution
A sigma-delta modulation sensing circuit with a regulation branch using a common reference voltage, modulating resistance to generate an adjustment current that mitigates noise by converting the difference between reset and pixel signal voltage levels, and provides gain control for the imager.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ratio-based operation is used in sigma-delta modulation sensing circuits, then the conversion of pixel and reset signal voltage levels can be achieved, but noise issues affect the accuracy of the conversion
Solution Approach 1:
A regulation branch is introduced as an intermediary element between the pixel signal branch and the reset signal branch. This regulation branch includes a control transistor that adjusts its conductance to equalize the total current in both branches, thereby mediating the noise issue by creating a balanced current distribution that reduces noise impact on the differential signal
Solution Approach 2:
The invention dynamically changes the conductance parameter of the control transistor in the regulation branch to achieve current equalization. By adjusting the control voltage applied to the gate of the control transistor, the system adapts to varying signal conditions and maintains optimal noise performance across different operating ranges
2Reliability
If conventional sigma-delta sensing circuits are used, then analog-to-digital conversion can be performed, but noise reduction and gain control are insufficient
Solution Approach 1:
The regulation branch serves multiple functions simultaneously: it acts as a current balancing element, a noise reduction mechanism, and a gain control pathway. The control transistor in this branch can be adjusted to provide both noise mitigation through current equalization and gain control by modulating the overall circuit conductance, thereby achieving multiple objectives with a single structural addition
3Productivity
If floating diffusion region is used for charge storage, then charge accumulation and transfer can be achieved, but kT/C noise on reset signal becomes excessive due to low capacitance
Solution Approach 1:
The sensing circuit implements a feedback mechanism where the control transistor continuously adjusts its conductance based on the differential voltage between the pixel signal and reset signal branches. This feedback control equalizes the currents in both branches, effectively compensating for the kT/C noise generated by the low-capacitance floating diffusion region and improving the signal-to-noise ratio
Data Source
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Figure 2A~2B
Figure 3A~3B
AI summary
A sigma-delta modulation sensing circuit and an analog-to- digital converter for an imager that do not rely on the ratio of the reset and pixel voltage levels being sensed. The sensing circuit includes a regulation branch (273) based on a reference voltage common across multiple columns of the imager. The regulation branch has an adjustable resistance switched capacitor circuit) that is modulated during the sensing operation, which creates an adjustment current that is applied during the sensing operation to a current associated with one of the reset and pixel signals . The sensing circuit and analog-to-digital converter can generate a digital code based on the difference between the reset and pixel signal voltage levels, which substantially mitigates noise associated with the pixel and reset signal voltages. The reference voltage can also be used as a gain control for the imager as well.