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Adau1701 noise (tone) issues

Question asked by Bazeman on Jun 23, 2016
Latest reply on Sep 28, 2016 by DaveThib


I have made several prototype boards with the Adau1701 with external ADC/DAC solution (loudspeaker X-over). 1 thing that keeps giving issues, is (too much) noise and more specific a single tone (1~2Khz) is present on the output of the DAC. I know for sure that the issue is related to the ADAU device, as the levitated noise floor (several dB's) and the presence of the sine wave noise tone, appears or dissapears only by reseting the board.


I can say that I'm really sure the issue is not from the PCB design and/or powersupply decoupling or the type of DC-DC converters, ADC/DAC, DSP in Master or slave mode etc, as I have 4 rather different prototype PCB's, with different setup/layout of power supply, ADC/DAC (brand/type), even 1 with one with external Master MCLK device (12.288MHz, 48kHz sample rate). Every pwr supply pin has a decoupling capacitor within 2mm of the device pins, the pwr lines themselves are decoupled and have large enough buffer capacitors. Also adding/removing coils to the power rails, or even completely remove the decoupling caps keeps giving the exact same outcome. At some powerups, the noise floor is low and only the normal hiss is there (noise floor -120 ~ -112dB meassuring at the output terminals of the compression driver amplifier), starting up again, randomly, the complete noise floor is levitated by 2~3dB (also the noise sounds more "spikey", not "soft", or maybe better a description more digital) and the sine wave tone varying between 1~2kHz can be heared (the tone shifts downwards when the pcb heats up)


I'm am rather out of ideas how to solve this issue... Hope someone else has still some hints...