Common-Mode Feedback Filter for Linear SE-to-Differential Conversion
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Solution Overview
Problem
Existing single-ended to differential-ended (SE-to-DE) conversion methods in filters either compromise on linearity due to common mode signal swings or increase area and energy consumption with the use of additional operational amplifiers.
Innovation Solution
Incorporating a common mode feedback circuit that compares voltages from both paths and modifies them based on a reference voltage, utilizing transconductors and an amplifier to maintain high impedance and reduce complexity, thereby achieving SE-to-DE conversion with minimal area and energy consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If common mode feedback is used to force differential output in a single-ended-to-differential filter, then area and power consumption are minimized by reusing existing hardware, but linearity deteriorates due to common mode signal swing at the virtual ground
Solution Approach 1:
The filter is divided into two separate paths (first path with VI+ input and second path with VI- input) that are processed independently through separate buffers and RC circuits, with each path having its own feedback mechanism. This segmentation allows differential processing without common mode signal interaction, resolving the linearity issue while maintaining area efficiency.
Solution Approach 2:
An intermediary circuit is introduced between the RC circuits and the differential amplifier that converts the single-ended outputs to differential outputs without requiring the virtual ground to swing. This intermediary structure eliminates the common mode signal swing problem while maintaining the area-efficient topology.
2Manufacturing precision
If additional operational amplifiers are added to provide SE-to-DE conversion before the filter, then linearity and differential processing advantages are improved, but area and energy consumption increase
Solution Approach 1:
The existing buffers and RC circuits in the differential filter topology are made to serve dual purposes: they both filter the input signals and perform the SE-to-DE conversion function. This multi-functionality eliminates the need for separate conversion amplifiers, achieving both linearity and area efficiency simultaneously.
Solution Approach 2:
The SE-to-DE conversion function is merged with the existing filter structure by using the same buffers and RC circuits for both conversion and filtering operations. This consolidation achieves differential processing benefits without adding separate conversion stages, thus avoiding increased area and power consumption.
3Device complexity
If virtual ground is used in the differential amplifier to reconstruct differential format, then area is minimized by hardware reuse, but common mode signal swing causes poor linearity
Solution Approach 1:
Instead of using a virtual ground that swings with common mode signals, the invention inverts the approach by using two separate real grounds for the two paths. This inversion eliminates the common mode signal swing at the reference point, maintaining linearity while still achieving area-efficient hardware reuse through the shared differential amplifier structure.
Data Source
AI summary
A filter including common mode feedback can provide single-ended to differential-ended conversion with minimum loss of performance.