Switched Capacitor Notch Filter for Chopper Amplifier Ripple Reduction
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Solution Overview
Problem
Chopper-stabilized operational amplifiers face a trade-off between low output ripple noise and low offset voltage, with modern designs either having high in-band noise due to aliasing or high output ripple noise, and there is a need for a solution that reduces these issues without increasing quiescent supply current.
Innovation Solution
A chopper-stabilized amplifier incorporating a switched capacitor notch filter that operates synchronously with the chopping frequency to filter ripple voltages, using a first and second operational transconductance amplifier with chopper circuitry and a switched capacitor notch filter to integrate and transfer charge, thereby reducing ripple noise and offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If chopper stabilization is used to reduce offset voltage, then offset voltage is reduced, but output ripple noise increases
Solution Approach 1:
A notch filter is introduced as an intermediary component between the chopper-stabilized amplifier stages. This filter specifically targets and attenuates the ripple noise at the chopping frequency while allowing the DC component and other frequencies to pass through, thus resolving the contradiction by mediating between the offset reduction benefit and the ripple noise penalty
Solution Approach 2:
The invention converts the harmful ripple noise generated by chopper stabilization into a manageable signal by using a synchronous detector that references the same chopping frequency. The detector converts the AC ripple at the chopping frequency back to DC, which can then be filtered out, effectively transforming the harmful high-frequency ripple into a removable low-frequency component
2Object-generated harmful factors
If autozeroing is used to reduce output ripple noise, then output ripple noise is reduced, but in-band noise increases due to aliasing
Solution Approach 1:
The notch filter serves as an intermediary that selectively removes only the ripple frequency components without affecting the in-band signal frequencies. This approach differs from autozeroing which folds broadband noise into the signal band; the notch filter provides targeted attenuation at the chopping frequency while preserving the integrity of the signal bandwidth
Solution Approach 2:
Instead of applying a global reset operation that affects all frequencies (as in autozeroing), the invention applies localized filtering only at the specific chopping frequency. The notch filter provides frequency-selective attenuation, removing ripple noise at the discrete chopping frequency while leaving the continuous signal spectrum unaffected
3Object-generated harmful factors
If chopper frequency is increased to reduce ripple noise, then ripple noise is reduced, but quiescent supply current increases
Solution Approach 1:
The notch filter acts as a frequency-independent mediator that attenuates ripple noise regardless of the chopping frequency. This allows the system to maintain a low chopping frequency (which minimizes quiescent current) while still achieving effective ripple suppression through the filter's frequency-selective attenuation characteristic
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 significantly reduces output ripple noise and offset voltage, achieving a 500× reduction in output ripple voltage compared to conventional chopper-stabilized amplifiers while maintaining low quiescent supply current, making it suitable for micropower applications.
Implementation Method 1
A switched capacitor notch filter (15) filters the chopped output signal by operating synchronously with the chopping frequency of output chopper to filter ripple voltages
Data Source
AI summary
A chopper-stabilized amplifier receiving an input signal includes a first operational transconductance amplifier having an input chopper and an output chopper for chopping an output signal produced by the first operational transconductance amplifier. A switched capacitor notch filter filters the chopped output signal by operating synchronously with the chopping frequency of output chopper to filter ripple voltages that otherwise would be produced by the output chopper. In one embodiment, a second operational transconductance amplifier amplifies the notch filter output. The input signal is fed forward, summed with the output of the second operational transconductance amplifier, and applied to the input of a fourth operational transconductance amplifier. Ripple noise and offset are substantially reduced.