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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? enter image description here

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?

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  • \$\begingroup\$ How about simulating the circuit? \$\endgroup\$
    – Stefan Wyss
    Commented Jul 12 at 5:29

1 Answer 1

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Plugging those 5 resistor and capacitor values into an MFB opamp circuit calculator shows the frequency is 10.582 kHz. It seems to end up at the same cutoff you calculated.

The comment written on the schematic may be outdated. Or the fg does not mean the cutoff frequency but something else.

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  • \$\begingroup\$ it says critical damping at fg = 6800 Hz, Are they talking about the Q factor of the filter. or poles of the filter?, Am i correct in assuming critical damping is the same as the cutt -off frequency point? \$\endgroup\$
    – AK47
    Commented Jul 12 at 8:24
  • \$\begingroup\$ @AK47 The two poles are at 8.8 kHz and 12.7 kHz. The Q factor is 0.49 so very near being critically damped. But due to tolerances it could end up over- or underdamped but not much. Damping does not mean the cut-off frequency point but will the frequence response have resonance peak. \$\endgroup\$
    – Justme
    Commented Jul 12 at 9:09
  • \$\begingroup\$ Thanks for the response, I have one more clarification , looking at the application note of the board they say the following "The evaluation board contains four discrete digital-to-analogue converters to generate analogue signals from the digital -modulated bit streams. The analogue signals are used for the on-board protection as well as for the external controller. The conversion is done by 2nd order low pass filters with approximately 6.5 kHz bandwidth" -->Where does this 6.5 KHz come from? \$\endgroup\$
    – AK47
    Commented Jul 16 at 3:04

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