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Sign inversion ADAU 1772

Question asked by RMSacoustics on Jan 8, 2014
Latest reply on Jan 8, 2014 by BrettG

I designed a feedback system around the ADAU1772 and started with the evaluation board using the balanced output jacks.

Assuming that the tip of the output jack represents the positive phase, the circuit worked well.

Then I implemented the chip in a dedicated PCB and found out that the signals are sign-inverted, even if the path from the input to the output is just a straight connection without filters as shown with the attached file.

The inputs do not use the sign-inverting PGA and the documentation tells explicitly (page 32 "signal polarity") that the ADCs do not invert the sign. Also the DACs are set at non-inverting.

 

This puzzled me quite a bit and then I checked more carefully the circuit diagram of the ADAU1772 evaluation board and guess what? The tip of the balanced output jack is connected to the negative output terminals of the ADAU1772, thereby inverting the signal to the connected balanced input of a power amplifier and thus compensating the internal inversion. This at least explains why I initially had different results in the final application where I connected the positive output terminal of the ADAU1772 to the positive input of the power amplifier.

 

Now my question is: Is the documentation wrong in stating that the ADAU1772 is not inverting the signal sign when not using the PGAs or is somewhere stated that the DSP core inverts the signal sign or do I have a setting wrong that causes sign inversion.

 

I attached the simple configuration to show also the hardware configuration that I use.

 

thanks, Rob

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