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ADAU1772 - Output Noise level in balanced vs Single-ended

Category: Hardware
Product Number: ADAU1772
Software Version: Sigma studio 4.6

Hi,

I am using the ADAU1772 codec to drive  low impedance headphones (8 Ohms) for ANC and therefore I need an external amplifier between the codec and my headphones.

This additional gain raises the noise floor too much so I try to use differential output to improve my SNR and lower the required gain.

According to the datasheet, if I use the codec in differential mode, I should have an improvement of 3 dB on my SNR (double output voltage minus additional uncorrelated noise from negative side output) but it is not what I am experimenting.

When using differential mode, my noise floor increases by more than 6 dB therefore cancelling the advantage of the double output voltage swing.

You can find attached my sigma studio project with my settings. Note that, since the filter is adaptative, the coefficients are not implemented by default.

Additionally, I experiment a higher noise when using the line mode compared to the HP mode.

Finally, my problem is also the same when I use a ADAU1777 or ADAU1787 whether it is the EVM or my own PCB.

Didi I miss something?

Thank you in advance,

ADAU1772-Project.zip

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