If you ever plan to measure the amplitude of a sine wave with a MCU by using the analog to digital converter (ADC), you will realize that the sampling frequency of the ADC and the timing for when to perform the measurement is crucial. This circuit allows for a simple measurement of a DC voltage for determining the sine wave amplitude by cutting the sine wave in halv and only keeping the positive half wave of the sine and then low-pass filtering it. The low-pass filter must have its knee near the sine wave frequency in order to achieve a shorter settling time. The output of the measurement must then be divided by 0.3197 in order to the the amplitude. (the value may be slightly incorrect. I will fix it later.)
