Bootstrap Sampling Switch Circuit for Low-Distortion RF Sampling
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Solution Overview
Problem
Existing sampling switch circuits face challenges with high input loading and harmonic distortion due to input-dependent on-resistance variations, particularly at higher speeds and in direct RF applications, limiting their performance as voltage mode samplers for ADCs.
Innovation Solution
A sampling switch circuit design that includes a current source and switching circuitry to alternate between precharge and output configurations, using reference voltage signals to maintain a constant gate-source voltage independent of the input signal, thereby reducing input loading and harmonic distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a sampling transistor is used as a simple switch in voltage mode sampling, then the circuit structure is simple, but input-dependent on-resistance variations cause harmonic distortion
Solution Approach 1:
A bootstrapping circuit is introduced as an intermediary mechanism to control the gate voltage of the sampling transistor. This circuit uses a bootstrap capacitor to generate a gate voltage that tracks the input signal, thereby maintaining a constant gate-source voltage and minimizing on-resistance variations without requiring a completely different switch topology.
Solution Approach 2:
The gate voltage parameter is dynamically adjusted to track the input signal voltage. By changing the gate voltage in proportion to the input signal, the gate-source voltage difference remains constant, which stabilizes the on-resistance of the sampling transistor across different input levels and reduces harmonic distortion.
2Object-generated harmful factors
If a bootstrapping circuit is used to maintain constant gate-source voltage, then harmonic distortion is reduced, but input loading increases
Solution Approach 1:
The bootstrapping circuit operates in periodic phases synchronized with the sampling clock. During the track phase, the bootstrap capacitor charges to maintain the constant gate-source voltage. During the hold phase, the capacitor maintains the voltage without drawing additional input current. This periodic operation reduces average input loading while maintaining low distortion during the critical sampling interval.
3Productivity
If the sampling switch operates at higher speeds, then productivity increases, but input loading and distortion become more significant
Solution Approach 1:
The bootstrapping circuit performs preliminary action by pre-charging the bootstrap capacitor during the hold phase before the track phase begins. This ensures that when high-speed sampling occurs, the gate voltage is already properly established, allowing the sampling transistor to operate with minimal on-resistance variation from the start of the track phase, thereby reducing distortion at high speeds.
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 design achieves reduced input loading and improved performance at higher speeds, enabling effective voltage-mode sampling with increased impedance and reduced distortion, suitable for direct RF applications and large arrays.
Implementation Method 1
a capacitor, a current source configured to cause a defined current to flow therethrough
Implementation Method 2
switching circuitry configured to switch between a precharge configuration and an output configuration in dependence upon a clock signal
Data Source
AI summary
A sampling switch circuit, including an input node, which receives an input voltage signal to be sampled, a sampling transistor having gate, source and drain terminals, the source terminal connected to the input node, a capacitor, a current source configured to cause a defined current to flow therethrough and switching circuitry configured to alternate between a precharge configuration and an output configuration depending upon a clock signal. In the precharge configuration, the switching circuitry connects the capacitor into a current path between said current source and a first voltage reference node to form a potential difference across the capacitor which is dependent on the defined current. In the output configuration, the switching circuitry connects the capacitor between a second voltage reference node and the gate terminal of the sampling transistor so that a voltage level applied at the gate terminal of the sampling transistor is dependent on the defined current.