Bootstrapped Switched-Capacitor DAC for Low-Distortion Charge Transfer
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Solution Overview
Problem
Existing switched-capacitor-based sigma-delta converters face challenges with distortion, bandwidth degradation, and temperature sensitivity due to variable charge injection and common-mode voltage issues when using standard CMOS switches.
Innovation Solution
The implementation of bootstrapped switches with a bootstrap circuit to maintain a constant gate-to-source voltage, reducing charge injection effects and improving linearity, and the use of a voltage follower circuit to maintain symmetry and prevent leakage paths, along with reset switches to equalize voltages and define common mode voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If standard CMOS switches are used in switched-capacitor-based sigma-delta converters, then device complexity is reduced, but distortion increases and bandwidth degrades due to variable charge injection and common-mode voltage issues
Solution Approach 1:
A bootstrapped switch structure is introduced as an intermediary between the control signal and the capacitor switching mechanism. The bootstrap circuit generates a gate voltage that dynamically tracks the common-mode voltage, serving as a mediator that eliminates the direct dependence of charge injection on common-mode voltage variations, thereby reducing distortion while maintaining manageable device complexity
Solution Approach 2:
The gate voltage parameter of the switch is dynamically changed to track the common-mode voltage through the bootstrap circuit. This parameter change ensures that the switch operates with a constant gate-to-source voltage differential regardless of common-mode variations, maintaining conversion accuracy without requiring overly complex switch designs
2Ease of manufacture
If standard CMOS switches are used, then ease of manufacture is improved, but bandwidth degrades due to variable charge injection effects
Solution Approach 1:
The bootstrap circuit acts as an intermediary that decouples the switch charging behavior from common-mode voltage variations. By generating a gate voltage that dynamically compensates for common-mode changes, it creates a stable charging condition that maintains consistent charge injection across the bandwidth, improving bandwidth performance while keeping the switch implementation relatively simple
Solution Approach 2:
The bootstrap circuit performs preliminary action by pre-establishing the appropriate gate voltage before the switch operates. This preliminary voltage establishment ensures that the switch is properly biased for each operating condition, maintaining consistent charge injection characteristics across the frequency range and preventing bandwidth degradation
3Device complexity
If standard CMOS switches are used, then device simplicity is maintained, but temperature sensitivity increases due to variable charge injection
Solution Approach 1:
The gate voltage parameter is dynamically adjusted through the bootstrap circuit to compensate for temperature-induced common-mode voltage shifts. This parameter change ensures that the switch operates under consistent conditions across temperature variations, maintaining reliable performance without significantly increasing structural complexity
Solution Approach 2:
The bootstrap circuit serves as a temperature-compensating intermediary that senses common-mode voltage changes (which vary with temperature) and adjusts the gate voltage accordingly. This intermediary action stabilizes the charge injection behavior across temperature ranges while keeping the overall switch structure relatively simple
4Manufacturing precision
If bootstrapped switches are implemented, then linearity improves due to reduced charge injection effects, but device complexity increases
Solution Approach 1:
The switch circuit is segmented into two functional parts: the main switching transistor and the bootstrap circuit. This segmentation allows the complex voltage tracking function to be isolated in the bootstrap portion while keeping the main switch relatively simple, achieving improved linearity without overwhelming increase in overall device complexity
Solution Approach 2:
The bootstrap circuit performs multiple functions simultaneously: it generates the gate voltage, tracks the common-mode voltage, compensates for temperature effects, and maintains consistent charge injection. This multi-functionality reduces the need for additional separate circuits, achieving improved linearity with moderate complexity increase
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 approach enhances settling times, reduces distortion, and maintains performance across a wide temperature range and common-mode voltage range, improving the overall accuracy and stability of the digital-to-analog conversion process.
Implementation Method 1
The shorting switch may be implemented with a bootstrap circuit to maintain a constant common mode voltage of the first side of the capacitors while the shorting switch is activated
Implementation Method 2
the voltage follower circuit may be configured to generate a follow voltage that matches the common mode voltage of the pair of capacitors
Implementation Method 3
a pair of capacitors, and switches including a shorting switch. The switches may be configured to be switched in successive phases to generate a charge transfer through the capacitors
Data Source
AI summary
A charge transfer digital-to-analog converter includes a differential reference voltage, a pair of capacitors, and switches including a shorting switch. The switches are configured to be switched in successive phases to generate a charge transfer through the capacitors to generate an output corresponding to a digital input. The specific switches activated and deactivated in each phase are selected according to the digital input. Each capacitor of the pair of capacitors is connected to a respective pin for the output. The shorting switch is configured to short the pair of capacitors to create a zero-differential charge on a first side of the capacitors. The shorting switch is implemented with a bootstrap circuit to maintain a constant common mode voltage of the first side of the capacitors while the shorting switch is activated.