Capacitive Feedback DAC for Low-Noise Linear Voltage Ramps
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Solution Overview
Problem
Existing voltage ramp generators for analog-to-digital converters in image sensors face issues with noise, non-linearity, and high power consumption, particularly due to switching current sources of varying sizes, leading to differential non-linearity and increased power consumption.
Innovation Solution
A digital-to-analog converter design utilizing capacitive feedback circuits and controlled switches to generate voltage ramps and jumps, operating under a 1.8-V nominal voltage, with capacitors connected to an amplifier stage and controlled by a controller to manage charge accumulation, reducing noise and power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If switching current sources of various sizes are used in existing digital-to-analog converters, then the converter can generate voltage ramps, but glitches of variable amplitudes are created resulting in differential non-linearity that degrades sensor performance
Solution Approach 1:
The patent changes the fundamental operating parameter from current-based switching to voltage-based switching. By using a voltage ramp generator that switches voltage levels rather than current sources, the invention eliminates the variable amplitude glitches that cause differential non-linearity. The voltage ramp is generated by charging a capacitor through a constant current source and then switching the voltage level, which maintains consistent switching behavior and improves linearity.
Solution Approach 2:
The patent replaces the mechanical/electrical switching of current sources with an electronic voltage switching mechanism. Instead of physically switching between multiple current sources of different sizes, the system uses a single constant current source to charge a capacitor and then electronically switches the resulting voltage levels, reducing mechanical wear and electrical noise from frequent switching.
2Reliability
If existing digital-to-analog converters are designed to meet noise and linearity specifications, then performance requirements are met, but power consumption is high especially for image sensors
Solution Approach 1:
The patent employs periodic charging and discharging of the capacitor to generate the voltage ramp. The constant current source charges the capacitor in a controlled periodic manner, and the switching occurs only when needed to change voltage levels. This periodic action reduces power consumption compared to continuous switching of multiple current sources, while maintaining the required noise performance through controlled charging cycles.
Solution Approach 2:
The patent extracts and eliminates the need for multiple switching current sources by using a single constant current source combined with capacitor charging. This extraction of the redundant current sources significantly reduces power consumption while maintaining the voltage ramp generation function and noise performance requirements.
3Power
If high currents and high voltage are used in digital-to-analog converters, then the converter can drive the ramp generator, but the circuit complexity increases requiring boost converters
Solution Approach 1:
The patent introduces a capacitor as an intermediary energy storage element between the constant current source and the voltage output. The capacitor converts the constant current input into a linearly increasing voltage output, eliminating the need for complex boost converters. This intermediary component simplifies the circuit while achieving the required voltage levels through the relationship V = (I/C) * t, where voltage increases linearly with time.
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
The solution achieves low noise density, improved linearity, and reduced power consumption, enhancing sensor performance by minimizing switching noise and decorrelating current levels from voltage ramps.
Implementation Method 1
a capacitive feedback circuit, connected between the inverting input and the output of the first amplifier stage, and comprising at least one capacitor
Implementation Method 2
modify the charge accumulated in said feedback circuit and thus generate a sequence comprising one or a plurality of voltage ramps
Data Source
AI summary
In an embodiment a digital-to-analog converter includes a plurality of first capacitors, each having a first electrode and a second electrode, wherein the second electrodes are connected together and are connected to an inverting input of a first amplifier stage having its non-inverting input coupled to ground, a plurality of first switches, each of the first capacitors having its first electrode connected to a corresponding one of the first switches, wherein each of the first switches is configured to occupy a first state where the first electrode of a corresponding first capacitor is coupled to a first reference voltage and occupy a second state where the first electrode of the corresponding first capacitor is coupled to a second reference voltage different from the first reference voltage, a capacitive feedback circuit connected between the inverting input and an output of the first amplifier stage, the capacitive feedback circuit including at least one second capacitor and a controller.