Switched-Capacitor Notch Filter With Charge Averaging and Low Offset
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Solution Overview
Problem
Existing notch filters for removing unwanted signal components in chopper stabilized Hall effect sensors require large sampling capacitors and introduce offset, and their accuracy depends on clock phase synchronization, which can be susceptible to sampling non-ideal effects.
Innovation Solution
A switched capacitor notch filter design that alternately charges sampling capacitors and simultaneously averages their charge during an averaging time period, eliminating the need for conventional buffering and allowing for reduced capacitor sizes and improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional notch filters use two sampling stages with resistive averaging or charge redistribution, then unwanted signal components can be removed, but large sampling capacitor sizes (4-10 picofarads) and output buffers are required, increasing circuit area and introducing offset
Solution Approach 1:
The patent merges the sampling and averaging functions into a single stage by using switched capacitor circuits that perform both operations simultaneously. The first and second sampling capacitors are charged during different time periods and then connected in parallel to the operational amplifier input, achieving averaging without requiring separate buffering stages or large capacitor arrays.
Solution Approach 2:
The invention uses periodic clock signals to control the switching of capacitors between charging and averaging phases. The non-overlapping clock phases enable sequential charging of sampling capacitors followed by simultaneous discharge into the feedback network, achieving time-division multiplexing of the sampling and averaging operations.
2Measurement precision
If continuous time averaging approaches are used with two sampling stages, then residual harmonic components can be reduced, but relatively large sampling capacitor sizes and output buffers are required, consuming significant area
Solution Approach 1:
The patent extracts the buffer stage from the conventional architecture by directly connecting the sampling capacitors to the operational amplifier input through the feedback network. The switched capacitor feedback mechanism provides the necessary isolation and averaging function without requiring a dedicated output buffer stage.
Solution Approach 2:
The feedback capacitor and operational amplifier serve multiple functions simultaneously: they provide signal amplification, perform charge averaging from multiple sampling capacitors, and act as the output stage without requiring separate buffering components. This multi-functionality reduces overall circuit complexity.
3Ease of operation
If charge redistribution averaging is used in discrete time, then averaging can be achieved, but relatively large sampling capacitor sizes and output buffers are still required, and offset cancellation is difficult
Solution Approach 1:
The patent uses the operational amplifier with feedback capacitor to dynamically cancel offset contributions. The feedback mechanism continuously adjusts the output to maintain virtual ground at the inverting input, automatically compensating for offset voltages from the sampling capacitors and switches without requiring external offset cancellation circuitry.
4Measurement precision
If signal integration is used synchronized with chopper clock, then unwanted ripple can be integrated to zero, but accuracy highly depends on integrating clock phases relative to chopper clock and is susceptible to sampling non-ideal effects
Solution Approach 1:
The invention uses periodic sampling at twice the chopper frequency with non-overlapping clock phases to naturally reject ripple components. By sampling at specific phases and averaging the results, the circuit achieves ripple rejection without requiring precise phase synchronization between separate clock sources, making the system more robust to timing variations.
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 solution reduces the size and offset contributions of the filter, enhances accuracy by averaging charge simultaneously, and provides low-pass filter functionality, effectively removing unwanted signal components like modulated offset signals.
Implementation Method 1
A first sampling capacitor and a second sampling capacitor are alternately charged during a first time period and a second time period, respectively
Implementation Method 2
an operational amplifier and a feedback capacitor coupled in the feedback arrangement with respect to the operational amplifier
Data Source
Figure 1
Figure 1A
Figure 1B
AI summary
A switched capacitor notch filter (100) for sampling an input signal (111) using multiple sampling capacitors (120a, 120b) and multiple non-overlapping time periods. The charge from the sampling capacitors is averaged and transferred to the filter- output (113) during another non-overlapping time period.