Bridged RC Low-Pass Filter for First-Alias Suppression
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Solution Overview
Problem
Conventional on-chip anti-alias filters consume excessive power and have limited alias suppression, particularly for the first alias, due to their smooth roll-off characteristics and the inability to achieve complete phase cancellation in bridged-T filters.
Innovation Solution
A low-pass filter circuit with a bridged RC ladder configuration, incorporating a bridging capacitor and resistor in parallel with series resistors, and tunable capacitors and resistors, to improve alias suppression and tunability, while minimizing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional on-chip anti-alias filters are used, then power consumption is reduced, but alias suppression capability deteriorates
Solution Approach 1:
The filter is divided into multiple RC sections (first RC section, second RC section, third RC section) connected in series, with each section contributing to the overall filtering function. This segmentation allows the filter to achieve better alias suppression through cumulative effect while maintaining low power consumption by using passive components in each segment.
Solution Approach 2:
The bridging RC network (comprising bridging resistor and bridging capacitor) is nested within the overall filter structure, connecting between intermediate nodes of the cascaded RC sections. This nested configuration enables the filter to achieve asymptotic roll-off characteristics and improve first alias suppression without requiring additional power-consuming active components.
2Reliability
If bridged-T filters are used, then alias suppression is improved, but power consumption increases
Solution Approach 1:
The filter employs tunable capacitors (first tunable capacitor, second tunable capacitor) that can dynamically adjust the notch frequency to match the sampling frequency of the ADC. This dynamic tuning capability allows the filter to maintain optimal alias suppression performance across different operating conditions without requiring multiple fixed filters, thereby avoiding increased power consumption.
Solution Approach 2:
The filter uses tunable capacitors whose capacitance values can be changed to adjust the notch frequency. By changing the capacitance parameters of the tunable capacitors, the filter adapts to different sampling frequencies and maintains effective alias suppression. This parameter adjustment approach is more power-efficient than using multiple fixed filters or active tuning circuits.
3Device complexity
If fixed filter parameters are used, then device complexity is reduced, but adaptability deteriorates
Solution Approach 1:
The filter incorporates tunable capacitors that can be controlled by digital signals to adjust their capacitance values. This dynamic control mechanism allows the filter to adapt to different notch frequencies corresponding to different ADC sampling rates, providing versatility without significantly increasing device complexity. The tuning is achieved through simple switchable capacitor banks or variable capacitor structures.
Data Source
AI summary
A low-pass filter circuit comprising: a low-pass filter input terminal; a low-pass filter output terminal; a reference terminal; at least three filter resistors connected in series with each other between the low-pass filter input terminal and the low-pass filter output terminal, such that there is a resistor-connecting-node between each adjacent pair of filter resistors; a plurality of filter capacitors, one for each of the resistor-connecting-nodes, wherein each of the filter capacitors is connected between an associated resistor-connecting-node and the reference terminal; and a branch connected in parallel with the at least three filter resistors, wherein the branch comprises a bridging capacitor and a bridging resistor in series with each other.