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TAS5760M idle noise

Other Parts Discussed in Thread: TAS5760M, PCM1808, TAS5756M

I built a prototype board for audio output with 2 TAS5760M chips and a PCM1808 for input.

The board is attached to a Papilio One 500k FPGA board.

I designed the board based on "Figure 52. Stereo BTL using Software Control, 32 Pin DAP Package Option" on page 51 of the data sheet, but I added additional 33uH 1uF RC filters on each power supply rail ( PVDD 5V and VDD 3.3V ).

For the output filters I'm using 33uH Lfilt and 1uF Cfilt as per page 3 in sloa119b.pdf

If I pull /SPK_SD low on both chips there is no output as expected.

If I take either chip out of shutdown and output all zero audio data to the other I have audible noise on the output of the chip receiving zeros.

I've tried changing the digital gain, analog gain, volume control, and PWM rate and nothing seems to affect the noise volume or frequency.

I'm trying to figure out what to try next.

  • Hi, Joseph,

    Thanks for posting to e2e, and thanks for your interest in our devices!

    Sorry that you are having difficulties with our devices.

    I would check the i2c bus and make sure it's communicating properly.

    I have also seen problems due to the phase of I2S clocks. Verify your clock source matches the clock diagrams in the d/s.

    Let me know if these solve the issue.

    -d2

  • Hi Don,

    I double checked the i2c functionality by powering the chip off and on, then reading severall registers and comparing them to the defaults from the datasheet.

    I also wrote various values to the registers and read back the values. Communications look solid.

    Also if I send a signal (such as a sine wave ) over i2s I can modify the volume by changing digital or analog gain but the noise is still superimposed on it.

    So, I guess I'll investigate i2s phase a little more closely.

  • The datasheet for the TAS5760M didn't have i2s waveforms.

    It provides a short text description that the waves on my scope appear follow properly.

    I also looked at the datasheets for some similar chips such as the TAS5756M which has diagrams of i2s waveforms, which also seem to correspond well with the scope output.

    I tried various drive levels on the FPGA outputs. Although it did reduce ringing on the data lines a bit, the noise still does not appear to be affected.