Bootstrapped Sampling Switch With Jump-Start Turn-On for RF ADCs
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Solution Overview
Problem
Designing analog-to-digital converters (ADCs) that meet both high speed and performance requirements, particularly in applications involving wide bandwidth RF signals, is challenging due to the need for fast sampling rates and precise signal processing, which is complicated by the slow turn-on time of transistors in bootstrapped switching circuits.
Innovation Solution
The implementation of a bootstrapped switching circuit with a positive feedback loop and a jump start circuit to accelerate the turn-on time of the sampling switch, utilizing a bootstrapped gate voltage generator and a jump start circuit to quickly bring the gate voltage to the required level, ensuring fast turn-on of the transistor and reducing the delay in sampling high-frequency input signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a bootstrapped switching circuit is used for sampling high-frequency signals, then the circuit complexity is reduced and power consumption is lowered, but the turn-on time of the sampling switch becomes too slow to handle gigahertz-range input signals
Solution Approach 1:
The jump start circuit activates the output transistor before the positive feedback loop completes, preliminarily establishing the bootstrapped gate voltage condition needed for fast switching. This preliminary action ensures the sampling switch is ready to turn on quickly when the sampling phase begins, resolving the contradiction between circuit simplicity and fast turn-on time.
Solution Approach 2:
The positive feedback loop uses the gate voltage of the sampling switch to reinforce itself, creating a regenerative effect that rapidly charges the boot capacitor and drives the gate voltage to the required level. This feedback mechanism accelerates the turn-on time while maintaining the bootstrapped circuit's simplicity and low power consumption.
2Ease of operation
If the sampling switch turns on slowly, then the circuit operation is simpler, but the signal-to-noise ratio and spurious free dynamic range deteriorate due to distortion in high-frequency signals
Solution Approach 1:
The jump start circuit preliminarily activates the output transistor to establish the necessary voltage conditions before the sampling switch needs to turn on. This ensures the bootstrapped gate voltage is ready in advance, enabling fast switching that preserves signal integrity and maintains high signal-to-noise ratio and SFDR while keeping the circuit operation simple.
Solution Approach 2:
The positive feedback loop rapidly amplifies the gate voltage signal, ensuring the sampling switch turns on quickly enough to accurately capture high-frequency signals without distortion. This maintains measurement precision for signal-to-noise ratio and SFDR while preserving the simplicity of circuit operation through the self-reinforcing feedback mechanism.
3Productivity
If a jump start circuit is added to accelerate turn-on time, then the sampling speed increases, but the device complexity increases
Solution Approach 1:
The jump start circuit adds only a single transistor that activates the output transistor in advance, providing the necessary preliminary action for fast switching. This minimal addition significantly increases sampling speed (productivity) while adding very little to the overall device complexity, as it leverages existing circuit nodes and requires only one additional component.
4Device complexity
If the gate voltage is not brought to the required level quickly, then the circuit requires fewer components, but the delay in sampling high-frequency input signals increases
Solution Approach 1:
The jump start circuit preliminarily activates the output transistor to establish the voltage conditions needed for fast switching. This preliminary action ensures the gate voltage is brought to the required level quickly, minimizing sampling delay and preventing loss of high-frequency signal information, while adding only minimal circuit components.
Solution Approach 2:
The positive feedback loop rapidly amplifies the gate voltage signal, ensuring the required voltage level is achieved quickly to minimize sampling delay. This feedback mechanism accelerates the gate voltage establishment while requiring few additional components, as it uses the existing boot capacitor and transistor gate structure to create a self-reinforcing voltage escalation.
Data Source
AI summary
The trend in wireless communication receivers is to capture more and more bandwidth to support higher throughput, and to directly sample the radio frequency (RF) signal to enable re-configurability and lower cost. Other applications like instrumentation also demand the ability to digitize wide bandwidth RF signals. These applications benefit from input circuitry which can perform well with high speed, wide bandwidth RF signals. An input buffer and bootstrapped switch are designed to service such applications, and can be implemented in 28 nm complementary metal-oxide (CMOS) technology.