AC-Coupled Active Low-Pass Filter for In-Band Noise Suppression
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Solution Overview
Problem
Conventional Sallen-Key low pass filters face challenges with in-band noise due to resistors and amplifiers, limiting resistance values and requiring larger capacitors, which results in area penalties and reduced flexibility.
Innovation Solution
An active low pass filter design with an AC-coupled resistor and amplifier, using a resistor-capacitor network where the noise from resistors and amplifiers is suppressed in the pass band, allowing for increased resistance values and decoupling of output swing, input common mode, and amplifier offset, enabling greater flexibility and area/power savings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If resistance values of R1 and R2 are increased to reduce in-band noise, then in-band noise is reduced, but capacitor values must be increased to maintain cutoff frequency, resulting in increased area
Solution Approach 1:
The patent introduces an AC coupling capacitor (C1) as an intermediary element between the input signal and the resistor R1. This AC coupling configuration allows the resistor R1 to be effectively isolated from DC while still functioning in the AC signal path. By using AC coupling, the design can employ higher resistance values for noise reduction without requiring proportionally larger capacitor values, thus resolving the area penalty issue.
Solution Approach 2:
The patent changes the operating parameters by using AC coupling instead of DC coupling for the resistor R1. This parameter change allows the resistor to operate at higher resistance values (e.g., 100kΩ or higher) without directly affecting the DC bias points, thereby reducing thermal noise while maintaining the ability to use smaller capacitor values compared to traditional DC-coupled designs.
2Object-affected harmful factors
If resistance values are limited to reduce in-band noise, then in-band noise is reduced, but flexibility in circuit design is reduced
Solution Approach 1:
The AC coupling capacitor acts as a mediator that decouples the DC operating point from the AC signal path. This allows independent optimization of resistance values for noise performance without being constrained by DC bias requirements, thereby maintaining design flexibility while achieving noise reduction.
Solution Approach 2:
The patent segments the circuit into AC-coupled and DC-biased portions. By separating the AC signal path (through C1 and R1) from the DC bias network, the design gains flexibility to optimize each portion independently - R1 can be chosen for noise performance while other components handle biasing requirements.
3Speed
If capacitor values are increased to compensate for resistance limitations, then cutoff frequency can be maintained, but area penalty increases significantly
Solution Approach 1:
By changing from DC coupling to AC coupling configuration, the patent enables the use of higher resistance values without the direct proportionality between resistance and capacitance that exists in traditional designs. This parameter change breaks the R-C trade-off constraint, allowing noise reduction through higher R values without requiring proportionally larger C values.
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 in-band noise and achieves area and power savings while maintaining similar cutoff frequency and transient responses compared to traditional filters, with noise contributions primarily from thermal noise rather than amplifier noise.
Implementation Method 1
an active low pass filter (LPF)
Implementation Method 2
noise from the resistor and the amplifier are suppressed in a pass band
Data Source
AI summary
Sallen-Key active low pass filters (LPFs) have been knows for many years; however, these LPFs generally include passive components (i.e., resistors and capacitors) and active components (i.e., amplifiers) that are within the direct signal path that can contribute to the noise at the output of the filter within the pass band. Here, an LPF (which has the same general behavior as a Sallen-Key LPF) has been provided that AC couples passive components and active components to the direct signal path so as to suppress the noise contribution in the pass band.