Sigma-Delta Loop Filter Feedback DAC for Lower Integrator Swing
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Solution Overview
Problem
Continuous-time sigma-delta analog-to-digital converters face inefficiencies due to large integrator output swing, which limits signal gain and increases noise, while attempts to reduce swing often compromise signal transfer function or noise performance.
Innovation Solution
The implementation of a capacitively-coupled feedback path using a digital-to-analog converter (DAC) to reduce signal content at the output node of opamp integrators, allowing for increased gain without affecting the signal transfer function, by providing an equivalent voltage to cancel out signal content and reduce current flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If integrator output swing is reduced by scaling, then amplifier distortion and current consumption are reduced, but AC gain is also reduced resulting in large noise contribution from following stages
Solution Approach 1:
A feedback DAC is introduced as an intermediary component that converts the digital output back to analog and injects it at the integrator input. This mediator cancels the signal content at the integrator output through feedback, allowing swing reduction without the need to scale the integrator's AC gain, thereby maintaining noise performance while reducing current consumption
Solution Approach 2:
The patent implements a feedback mechanism where the digital output from the ADC is converted back to analog via a DAC and fed back to the integrator input. This feedback loop enables the system to reduce integrator output swing while maintaining the necessary signal gain, as the feedback path compensates for the gain reduction that would otherwise occur with scaling
2Power
If feedback path is used to reduce integrator output swing, then signal gain can be increased without affecting signal transfer function, but device complexity increases due to additional feedback components
Solution Approach 1:
The feedback DAC serves multiple functions: it reduces integrator output swing, enables increased signal gain, and maintains the signal transfer function. By making this single component multi-functional, the patent achieves swing reduction and gain enhancement without proportionally increasing overall device complexity
Solution Approach 2:
The feedback path creates a copy of the digital output signal and converts it back to analog form to be subtracted from the integrator output. This copying mechanism allows the system to manipulate the output swing independently of the main signal path, enabling gain adjustment without fundamentally altering the signal transfer function
Data Source
AI summary
In one example implementation, the present disclosure provides a loop filter for use in a continuous-time sigma-delta analog-to-digital converter. Specifically, a capacitive feedback digital-to-analog converter path is provided at the input of a first opamp in a series of opamp integrators. The capacitive feedback digital-to-analog converter at the input of the first opamp reduces the signal content at the output of the first opamp, and thereby reduces the output swing of the first opamp. A reduction in output swing provides a more efficient loop filter.