Other Parts Discussed in Thread: , NE5532
I am working on a design utilizing the TAS5548. My Dev PCB for software has op-amp based low pass filters at the output of CH1-8 PWM outputs for line level output. The layout and schematic for CH1-8 was performed using Altium's Multichannel design, so i am confident that all channels are identical in layout and schematic. The layout for CH1-2 is literally copied and pasted to equal CH3-4, CH5-6, and CH7-8. With that said I am experiencing a noise that is only persistent on CH1.
This noise sounds like a repeating tearing sound that is down around our noise floor. It is repeating at a rate of around 2Hz.
What is unique about this noise is that it does not track my input signal. If I connect the APx to I2S input of the TAS5548 and turn off the generator the noise is still there. If I then use the input or output mixers of the TAS5548 to route, say input 2 to output 1 the noise is still there. Or conversely if I route input 1 to output 2 the noise is not present.
If I set the shared volume 0xD9 to 0x02 0x45 (minimum) the noise on CH1 is still present.
If I set CH1 volume 0xD1 to 0x02 0x45 (minimum) the noise is still present.
If I enable DAP automute register 0x04 bit D4 = 0, then the noise is muted when signal disappears. However, if i keep the DAP unmuted by sending signal to CH2 and removing it from CH1 the noise persists on CH1.
If I route the output of CH1-2 to the I2S output and feed that back into my APx the noise is not detected.
This leads me to conclude that there is some block adding noise to the output of CH1 between the output mixer and the DAP automute block. Unfortunately I cannot find any information in the data sheet about anything that exists between those blocks.
I have the TAS5548EVM, and am unable to make a measurement of this noise, as the noise floor of this EVM is greater than the noise floor on my PCB. It kinda sounds like it is there, but it is hard to say definitively.
I am using the following register writes to perform a minimal interaction with the TAS5548:
0x03 = 0xA0
0x04 = 0x13
0xD9 = 0x00 0x00 0x00 0x48
My application cannot have an automute function, and I am at a loss as to why this issue is only present on CH1. I have also gone through and verified the power on sequence, and manually toggled all of the control lines with delays of seconds, manually written the I2C registers with an aardvark and still see the issue.
CH1 = Blue
CH2 = Red
Image 1: Noise Floor With Auto-mute Enabled.
Image 2: Noise floor with automute disabled and shared volume = 0dB (0xD9 = 0x00 0x00 0x00 0x48)
Image 3: Noise floor with automute disabled and shared volume 0xD9 = 0x00 0x00 0x02 0x45
Image 4: CH1-2 routed to I2S output. Shared volume = 0dB. 0xC5 = 0x07 0x00 0x01 0x00 0x03