This section the aMG SIGCON-A is used to op-amp active filters, cases Butterworth low pass filter Sallen-Key topology, the schematic is shown below.
The terminology, Butterworth filter is a type of signal processing filter designed to have as flat a frequency response as possible in the pass-band  and the Sallen-Key topology is one of the most widely-known and popular second-order topologies . It is low cost, requiring only a single op amp and four passive components to accomplish the tuning.
The cut-off frequency (fc) of this circuit can calculate from the equation:
From the circuit diagram above yield the fc:
= 20 kHz
At the given value of passive components as seen on the circuit yield the cut-off frequency at 20 kHz.
Configuration of aMG SIGCON-A: Soldering to Jump J4 and X8. Place 100 kΩ resistor to X2 and X3, 56pF capacitor to X5 and 112 pF to X1 .
Butterworth filter is one of many signal processing algorithms and Sallen-Key topology is one of many electronics active filter topology. For more details, you can study from the REFERENCES [1, 5, 6].