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ADAU1772 EQ filter cannot shift 180 degree?

Question asked by Felix.Lee on Dec 20, 2016

Dear ADI team:

I was develop ADAU1772 project, and we have meet some issue need your check.

There was two confusion:

① In ADAU1772 Sigmastudio system, the General Filter integrated several EQ filter, but there was a different from ADAU176x/1701, 1772 EQ cannot shift 180 degree, it is a important factor to implement noise cancellation.

So pls help to point out how can I implement the 180 degree shift in all pass filter, or have other way to do it.

                                                                     figure 1





② the other question is also about the filter, in ADAU1772 Sigmastudio system, the all pole filter is a low pass filter with 180 degree shift, but the shift feature seems not work after  AP test, and below is the test figure,

remark: red curve is bypass channel, blue curve is enable chennel, yellow curve is 180 shift channel.

figure 2 show frequency response is expectation,

figure 3 show phase curve, the blue and yellow curve coincide, means the 180 degree shift did not work.


figure 3

                                                    figure 2



figure 4

                                                   figure 3


So pls help to check the reason, and forward your feedback.