ADAU1372 anti-aliasing filter

Hi,

i am designing a system where i have to pass some strict frequency mask tests on the analog audio inputs/outputs. These tests include unwanted signals that are not within the first nyquist zone of an 192 kS/s ADC and therefore need to be attenuated somewhere. To get an understanding of what anti-aliasing/reconstruction filter i have to provide external to the ADAU1372 i have some questions:

- What is the sample rate of the delta-sigma ADC/DAC? (not the sample rate i get on the serial ports) From the following datasheet plot and from the fact that is is a delta-sigma converter i assume that the sampling frequency is (much) higher than 192 kS/s because otherwise the decimation filter could not provide attenuation at frequencies above 96 kHz.

- Can you provide the decimation filter-response up to the nyquist-frequency of the ADC/DAC?

- Are there any analog anti-aliasing or reconstruction-filters inside the ADAU1372?

Best Regards

Michael

  • 0
    •  Analog Employees 
    on Jan 27, 2018 12:32 AM

    Hello Michael,

    I think I am understanding your question so let me give you some answers.

    Both the ADC and the DACs rates are  6.144MHz.

    For the ADC:

    Internally it will take this 1-bit 6.144MHz rate and decimate it down to the internal sample rate that is set by the register setting. This would be 96kHz or the default 192kHz.

    Then this is sent to the ASRC to convert this rate to whatever the serial port sampling rate is set to, or what is coming into the clock inputs if it is a slave.

    For the DAC:

    The serial input port will go through an ASRC to convert the serial port rate to the internal 192kHz sampling rate. Then this is sent to the DAC modulator to interpolate up to the 6.144MHz rate.

    So the external filter needs are not extensive. I guess it depends on how much stop band rejection you require. I am not certain if I can find the actual response curves of the internal filters. This was usually done by the designers early in the design and we do not publish all of the internal details for obvious reasons. So I am not certain if I can provide specifications. The ASRC specs are published.

    Dave T

  • Hello Dave,

    thanks for your helpful answer. The information you gave me is enough to continue with the design. Nevertheless it would be great if you could provide the filter responses with an extended frequency range

    Michael

  • 0
    •  Analog Employees 
    on Jan 31, 2018 6:15 AM

    Hello Michael,

    Unfortunately, all I have is what is in the datasheet.

    Dave T