Bandlimited Capacitance Driver for Low-Noise Sampling Capacitors
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Solution Overview
Problem
Analog-to-digital converters (ADCs) face limitations in reducing thermal noise associated with sampling capacitors, as existing capacitor drivers like single emitter-follower or source-follower only manage to lower noise to kT/2C or 2 kT/3C, and are not suitable as general-purpose drivers.
Innovation Solution
A capacitance driver circuit is designed to operate over a bandwidth of interest, producing an output voltage that approximates a bandlimited filtered voltage value with a root-mean-square voltage deviation from the target voltage less than sqrt(kT/C), using synthetic admittance or impedance circuits to decouple noise density and equivalent noise bandwidth, thereby reducing thermal noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a single emitter-follower or source-follower is used as capacitor driver, then noise is reduced to kT/2C or 2kT/3C, but the extent of noise reduction is limited and the driver is not suitable as general-purpose
Solution Approach 1:
The driver circuit is segmented into multiple parallel follower circuits (emitter-followers or source-followers) instead of using a single follower. Each follower contributes to driving the capacitive load, and their combined effect provides both noise reduction and improved versatility for general-purpose applications.
Solution Approach 2:
The parallel follower configuration is designed to be universally applicable across different ADC architectures and applications. The circuit topology can serve multiple functions including driving sampling capacitors, providing buffered outputs, and maintaining low noise performance across various operating conditions.
2Object-affected harmful factors
If conventional capacitor drivers are used, then circuit simplicity is maintained, but thermal noise reduction below sqrt(kT/C) is not achieved
Solution Approach 1:
The driver is divided into multiple parallel follower stages, where each stage contributes to the overall performance. This segmentation enables noise reduction below the conventional sqrt(kT/C) limit while distributing the complexity across multiple simple, identical units rather than requiring a single complex circuit.
Solution Approach 2:
Multiple follower circuits are merged in parallel to drive the same capacitive load. The combination of these followers provides cumulative noise reduction benefits while maintaining the simplicity of individual follower units, achieving low noise performance without excessive complexity.
Data Source
AI summary
A circuit having a capacitance driver circuit can allow for reduction of thermal noise to an application circuit. An output of the capacitance driver circuit can drive a capacitor for use by the application circuit coupled to the capacitor at the output of the capacitance driver circuit. The capacitance driver circuit can be structured to operate over a bandwidth of interest. With an input signal, received at the capacitance driver circuit, associated with a target voltage, an output voltage can be provided at the output of the capacitance driver circuit as a bandlimited filtered voltage value of the target voltage, where a root-mean-square voltage deviation of the output voltage from the target voltage, due to thermal noise, is less than a square root of (kT/C). The term k is Boltzmann's constant, T is Kelvin temperature of the capacitance driver circuit, and C is the capacitance of the driven capacitor.