Understanding Filter Designs: Sallen Key Filter vs. 2nd Order RC Filter vs. Active High Pass Filter

Filters are essential components in signal processing, serving to modify or manipulate signals in various electronic systems. They come in diverse types, each with its own characteristics and applications. Three common filter designs—Sallen Key, 2nd Order RC, and Active High Pass—are widely utilized in electronic circuits for distinct purposes. Let's delve into their functionalities, differences, and applications and how to use the online sallen key filter calculator, 2nd Order RC Filter calculator and the Active High Pass Filter calculator.

Sallen Key Filter

The Sallen Key filter is an active filter topology known for its versatility and straightforward design. It’s a type of active filter that employs operational amplifiers (op-amps) along with resistors and capacitors. Its simplicity makes it popular for audio and low-frequency applications. Sallen Key filters can be configured as low-pass, high-pass, band-pass, or band-stop filters by altering the component values.


  • Frequency Response: Offers a steep roll-off and good selectivity.
  • Gain Control: Adjustable gain through the op-amp configuration.
  • Ease of Design: Simple and easy to implement.
  • Versatility: Can be easily configured for various filter types.


  • Audio Processing: Used in equalizers and audio tone controls.
  • Signal Conditioning: Filters noise or unwanted frequencies in sensor applications.
  • Instrumentation: For amplification and filtering in instrumentation circuits.

2nd Order RC Filter

The 2nd Order RC (Resistor-Capacitor) filter is a passive filter that uses only resistors and capacitors to achieve frequency filtering. It's a basic filter design that’s easy to understand and implement but has limitations compared to active filters like the Sallen Key.


  • Frequency Response: Moderate roll-off and less selectivity compared to active filters.
  • Simplicity: Comprises only passive components, making it cost-effective.
  • Limited Filtering Capability: Provides basic filtering functions.


  • Basic Filtering: Used in simple applications where moderate filtering suffices.
  • Tone Control: In audio systems for basic frequency adjustment.
  • Signal Shaping: In simple signal conditioning circuits.

Active High Pass Filter

An active high pass filter is designed to allow higher frequencies to pass through while attenuating lower frequencies. It typically consists of an op-amp and passive components like resistors and capacitors.

2nd order HPF with LM358


  • Frequency Response: Allows higher frequencies to pass while attenuating lower ones.
  • Adjustable Cutoff Frequency: Can be tuned by adjusting component values.
  • Better Signal Integrity: Maintains signal strength in desired frequency ranges.


  • Signal Conditioning: Removes unwanted low-frequency noise from signals.
  • Audio Systems: Blocks low-frequency noise in audio applications.
  • Instrumentation: Filters out low-frequency interference in measurement circuits.

Comparing the Filters:


  • Sallen Key: Moderately complex due to the use of operational amplifiers.
  • 2nd Order RC: Simple due to the use of passive components.
  • Active High Pass: Moderate complexity with op-amp integration.

Filtering Capabilities:

  • Sallen Key: Offers better selectivity and control over the frequency response.
  • 2nd Order RC: Basic filtering with moderate roll-off.
  • Active High Pass: Specialized for attenuating lower frequencies while passing higher ones.


  • Sallen Key: Versatile and suitable for various applications requiring precise filtering.
  • 2nd Order RC: Basic applications where simplicity suffices.
  • Active High Pass: Specific applications needing low-frequency noise elimination.

In conclusion, each filter type, Sallen Key filter, 2nd Order RC filter, and Active High Pass filter, has its unique characteristics, making them suitable for different scenarios. The choice of filter depends on the specific requirements of the application, including frequency response, complexity, and the level of filtering needed. Understanding these distinctions is crucial for engineers and hobbyists alike when designing electronic circuits for various purposes.


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