Differential audio output filter

Thread Starter

AlexX 52

Joined Mar 15, 2018
Hello everyone,

I'm using the CS4272 (Datasheet) to convert I2S data to analog audio signals. The only thing I need to do is designing the output stage. CS recommends an differential output filter on page 33 of the datasheet. However, I've had a hard time figuring out what the filter actually does. I recreated the given circuit to make component numbering easier (see attachment).
The input stage of the codec's output look like lowpass filters (R1&C3 and R3&C4). Their output in turn is fed to a differential amplifier stage (R2, R4, C1, C2), although the feedback and ground resistors are capacitors (some kind of integrators?). C6 is definitely an AC coupling capacitor. R7 and R8 seem to be attenuating resistors. I have no idea what R5, R6 and C5 do. R6 and C5 may probably contribute to the lowpass filter's performance.
I don't have a lot experience with more complex filter circuits yet.

Am I on the right track? And where do all these values come from?

Thanks in advance.

Hey AlexX:

This topology is a two-pole (i.e. 12dB/octave) differential-in / single-ended out active low-pass filter. C1, C2, C3 and C4 plus R1, R2, R3, R4 and R6 determine the -3dB frequency. C5 AC couples R6, because the outputs of the CS4272 are DC biased at about 1/2 Vcc. These outputs can't drive significant DC current - thus a big coupling cap in C5. The CS4272 is a delta-sigma DAC, and I believe it uses a 5th order modulator. What this means is that it's noise preformance below 1/2 the sampling frequency is quite excellent, but that noise is pushed out of the audio band above 1/2 the sampling frequency. It's quite a bit of noise and can sometimes upset analog or ADCs that it drives - thus the lowpass filter.