Home > Circuits > Low Pass Filter
Modified:21:15, 22 October 2013
CIRCUIT DESCRIPTION
A very useful circuit to remove high frequency signals from an audio source. This can be used for subwoofer design, active filters and demodulating signals. The circuit is based upon the standard Sallen Key design for 12dB filters. This Low pass filter will allow a range of frequencies to pass up to its break frequency, at which point the gain of the signal will drop rapidly.

The supply can be anywhere from +/- 5v to +/-15v depending on the IC you use. The common 741 op-amp works well but there are better quality versions available. LF351 or CA3140 are recommended.

The break frequency (fo = the point at which it starts to cut the signal) is calculated using the following formula:

fo = 0.707 / (2 Pi R1 C2)

therefore the above circuit has a break frequency of

fo = 0.707 / (2 x 3.141 x 22000 x 0.000000047)

=108Hz

Of course you can change the fo of this filter. To calculate components for a certain frequency:

  • Choose C2, use approx 10-100nF below 1KHz, 10 - 1nF above 1KHz
  • Find R1 using the formula R1 = 0.707 / (2 Pi fo C2)
  • R2 = R1
  • C1 = 2 x C2

Notes: Keep R1/R2 above 1K and below 100K, keep C2 below 1uF
TEST REULTS

The following results were produced by the filter shown left using a 5v AC signal as an input at various frequencies.

Red trace = Input signal
Blue trace = Output signal


Test frequency = 100Hz


Test frequency = 200Hz


Test frequency = 500Hz

RESPONSE CHARACTERISTICS
The response of this filter is plotted in red. Note the slight hump at the break frequency. By increasing R3 you can increase the overall gain of the filter which currently is set to 1.1, but this will also increase the hump.

The blue trace is the result if you cascade 2 filters in series. This gives a much sharper cut-off slope and may be preferable for some applications.

PCB's
Below is the PCB for a single stage of the filter. The values shown are for fo=108Hz. De-coupling capacitors have been added to the input, output and the supply rails. The power connection is at the bottom and uses a split power supply.
This version was used to drive a an amplifier and subwoofer. It has 2 stages cascaded and provides a much sharper fo cut-off slope. Again, decoupling capacitors have been added. The copper ground plane around the tracks on the underside is not connected. It may be advisable to connect this to 0v or Ground to improve circuit stability and reduce noise.

DOWNLOADS

Single stage filter simulation, save to file first
Dual stage filter simulation, save to file first
 PCB Layouts of single and dual versions

The filter design can also be used for a High Pass filter by swapping the capacitors and resistors around.

For more details go to High Pass SK Filter page

LINKS:

Wikipedia - Sallen Key Low Pass Filters
Written by Phil Townshend 2008
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