I found this circuit which filters an incoming pwm pulse train from a Delta-Sigma ADC to an analog value. The PWM pulse has an amplitude from 0 to 5 V and duty cycle varies from 10% to 88%. And the frequency of the PWM pulse is 20 MHz. If you look at the comment, it says fg is 6800 Hz and gain is -1.
How are these values derived from this filter?
When I calculate the cut off frequency for a low pass filter which is 1/(2pi * sqrt(R1 * R2 * C1 * C2)).
Plugging in these values 1/(2pi * sqrt(27k * 56k * 680pF * 220 pF)), I get the value to be around 10.5 kHz? Am I missing something here?