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Sallen Key Filter Calculator

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Cut-off Frequency:

Quality Factor:

In the world of electronics, designing an effective filter circuit can be both critical and challenging. The Sallen-Key filter, known for its straightforward design and versatility, stands out as a popular choice among engineers. However, calculating the precise values for your circuit can be a daunting task. That's where the Sallen-Key Filter Calculator comes into play, offering a streamlined approach to configuring your filter accurately and efficiently.

The Essence of the Sallen-Key Filter

The Sallen-Key topology is a principal method for creating second-order active filters. These filters are essential in various applications, from audio processing to signal modulation. Unlike many of its counterparts, the Sallen-Key filter is celebrated for its simplicity and ease of design. It can be adapted into low-pass, high-pass, band-pass, or band-reject configurations, with the low-pass setup being the most utilized.

Core Principles and Formulas

At the heart of the Sallen-Key filter is its transfer function, which is determined by the arrangement of resistors and capacitors within the circuit. The formula to find the transfer function for a low-pass filter is:



  • H(s) represents the transfer function,
  • Vout​ and Vin​ are the output and input voltages, respectively,
  • s is the complex frequency variable,
  • 0ω0​ is the cut-off frequency in radians per second,
  • Q denotes the quality factor of the filter.

The cut-off frequency (0ω0​) and quality factor (Q) are pivotal, dictating the filter's response characteristics. These parameters are derived from the resistors and capacitors in the circuit, with common formulas being:



Practical Application: A Step-by-Step Example

To illustrate, let's calculate the cut-off frequency and quality factor for a Sallen-Key low-pass filter using equal component values: ΩR1​=R2​=10kΩ, 2=100 C1​=C2​=100nF. Assuming 3=4=10ΩR3​=R4​=10kΩ for critical damping, the process is as follows:

  1. Calculate the Cut-off Frequency (0ω0​): 0=110,000×100×10−9=1,000 rad/sω0​=10,000×100×10−91​=1,000 rad/s
  2. Determine the Quality Factor (Q): Given the critical damping scenario, =0.5Q=0.5.

This example demonstrates the calculator's utility in streamlining the design process, enabling you to easily adjust component values and immediately understand how these changes affect your filter's performance.

The Calculator in Action

Here's a simplified table showcasing how the calculator processes inputs to provide essential filter parameters:

R1, R210,000Ohms
C1, C2100 x 10^-9Farads

Concluding Thoughts

The Sallen-Key Filter Calculator is not just a tool; it's a bridge between conceptual design and practical application. It demystifies the complexities involved in filter design, allowing both novices and seasoned professionals to experiment and refine their circuits with confidence. This calculator underscores the importance of precision in the electronic design process, making it an indispensable asset in your engineering toolkit.

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