SLOA049D July   2000  – February 2023

 

  1.   Abstract
  2.   Trademarks
  3. 1 Introduction
  4. 2 Filter Characteristics
  5. 3 Second-Order Low-Pass Filter Standard Form
  6. 4 Math Review
  7. 5 Examples
    1. 5.1 Second-Order Low-Pass Butterworth Filter
    2. 5.2 Second-Order Low-Pass Bessel Filter
    3. 5.3 Second-Order Low-Pass Chebyshev Filter with 3-dB Ripple
  8. 6 Low-Pass Sallen-Key Architecture
  9. 7 Low-Pass Multiple Feedback (MFB) Architecture
  10. 8 Cascading Filter Stages
  11. 9 Filter Tables
  12. 10Example Circuit Simulated Results
  13. 11Non-ideal Circuit Operation
    1. 11.1 Non-ideal Circuit Operation: Sallen-Key
    2. 11.2 Non-ideal Circuit Operation: MFB
  14. 12Comments About Component Selection
  15. 13Conclusion
  16.   A Filter Design Specifications
    1.     A.1 Sallen-Key Design Simplifications
      1.      A.1.1 Sallen-Key Simplification 1: Set Filter Components as Ratios
      2.      A.1.2 Sallen-Key Simplification 2: Set Filter Components as Ratios and Gain = 1
      3.      A.1.3 Sallen-Key Simplification 3: Set Resistors as Ratios and Capacitors Equal
      4.      A.1.4 Sallen-Key Simplification 4: Set Filter Components Equal
    2.     A.2 MFB Design Simplifications
      1.      A.2.1 MFB Simplification 1: Set Filter Components as Ratios
      2.      A.2.2 MFB Simplification 2: Set Filter Components as Ratios and Gain = –1
  17.   B Higher-Order Filters
    1.     B.1 Fifth-Order Low-Pass Butterworth Filter
    2.     B.2 Sixth-Order Low-Pass Bessel Filter
  18.   C Revision History

12 Comments About Component Selection

Theoretically, any values of R and C which satisfy the equations can be used, but practical considerations call for certain guidelines to be followed.

Given a specific corner frequency, the values of R and C are inversely proportional to each other. By making C larger, R becomes smaller, and vice versa.

Making R large can make C so small that parasitic capacitances cause errors. However, smaller values of R makes the current in the circuit larger, which consumes more power as a trade-off.

The best choice of component values depends on the particular circuit and the tradeoffs the designer is willing to make. Adhering to the following general recommendations helps reduce errors:

  • Capacitors
    • Avoid values less than 10 pF
    • Use C0G (NP0) dielectrics
    • Use 1%-tolerance components
    • Surface mount is preferred
  • Resistors
    • Values in the range of a few hundred ohms to a few thousand ohms are best
    • Use metal film with low temperature coefficients
    • Use 1% tolerance (or better)
    • Surface mount is preferred