To creat a low cut and high cut filter by ADAU1761

Dear wxzhnly@tom.com,

In times past you would need to work out the fc's and Q's of individual second-order stages to build such a filter, but recent versions of SigmaStudio include the
Nth-Order Filter block which does the math for you. It's available for the ADAU1761, and an example is shown below:

If this isn't sharp enough, just increase the filter's order (even number only; at least in my experience odd numbers haven't worked correctly). It sure beats the old days of running your own Butterworth or Chebyshev functions -- and even worse, dealing with op-amp analog filters and 1% components!

Best regards,

Bob

Hi Bob,

That is very kind of you.While we anlyzed the spectrum for the noise,and used the filter to decrease the noise,it is a little usefull,but there is still some noize.Althroug there is no source attatch the ADAU1761,ther would be still some noise from ADAU1761.If we connect its output to a speak box with 20dB gain,it is very easy for us to hear the noize. Do we have to add an analog low pass filter behind ADAU1761 and is there an example with the parameters for this analog filter ? Or the S/N for ADAU1761 is not good enough.

Clancy.

Hi Bob,

As what suggested,we have to add an analog filter if we use multi NF filter for harmonic?

Thanks.

Hello Clancy,

One way to see where the noise is coming from is to build and load this project:

This removes any chance of noise coming from the analog inputs, internal analog mixer, ADCs and your signal flow -- leaving only the DACs and your analog speaker amplifier. See how much noise is left, and troubleshoot accordingly.

Another SigmaDSP with internal converters, the ADAU1701, absolutely needs a 50 KHz lowpass analog interpolation filter at each DAC output in use. These DACs actually output what appears on an oscilloscope as a noisy mess -- this external filter averages this out into a quiet signal. That's how sigma-delta DACs work -- the alternative would have been a far more costly 20 KHz brick-wall analog filter for each DAC. Looking at both the ADAU1761 data sheet and the schematic for the ADAU1761Z Evaluation Board (see below), I'm surprised that no such lowpass filter is specified or used. Thus the ADAU1761 appears to have it built-in or simply doesn't need it:

However, you could try a simple R-C filter right at your speaker amplifier input to see if it helps. Be sure your 3.3V supplies are properly bypassed and clean (if you're doing your testing on the evaluation board, this is already taken care of for you).

Last but not least, the ADAU1761 has versatile input and output paths capable of bypassing the DSP:

I'm not familiar with their use, but the record (input) and playback (output) paths include analog gains which could be adding noise. Be sure all the switches that insert analog paths are open (I believe that's by default, just not sure).

Best regards,

Bob

Hi Bob,

That is very kind of you.I will try it.

Thanks again.