PCM2906C – frequency response

Today I did some measurement on the HifiBerry USB. The first measurement was the frequency response. Some interesting things happened.

For this basic test I used ARTA. Input and output of the card were directly connected. Therefore the results of these measurements represent the input and the output stage together. If there are any influence between both (e.g. crosstalk), this will have an impact on the measurements.

The frequency response was not very flat. The high frequency starts to roll of at 15kHz and the -3db limit was at about 17kHz – too low for 48kHz sample rate. There are also some glitches at 3kHz and 6kHz. See the frequency response below:


Is the chip really that bad? Or is there a problem with my hardware design? I did the same measurement with a Behringer UCA202. The frequency response was exactly the same. Then I wanted to see what happened to the roll-off if I decrease the sample rate. It should look even worse. But check out, what happens if you use 44.1kHz sample rate:


Much better! The roll-off starts at 19kHz, -3db is above 20kHz. No glitches anymore. What happened here? I guess, that the chip is resampling everything to 44.1kHz. I will contact Texas instruments to find out more about this behavior.

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