TAS3204 noise

Hi, Thomas,

The TAS3204 is supported by another team. I am going to move your post to the Audio Forum where they will see it.

-d2

My self build TAS3204 PCB has a high noise level on the DAC Outputs too and I can't figure out why. What I have done so far.

Supplying the Analog- and Digital-Frontend with seperate Voltagereferences. No improvement.

Short Circuit of the Analog Inputs, so the TAS measures a diffential Zero. No improvement.

Streaming the digital Input to the DAC section with no signal present (digital 0). No improvement.

So for me it seems, like the DAC itself is generating the noise, since the Input Signals and Voltages are all clean.

My PPS Program is as simple as it could be. Only the Init and Framwork Block for the TAS4302 are present and the ADC is linked one on one to the DAC.

Maybe you Thomas could check on a similar basis if you DAC is also generating the noise independently from the source settings, than we would be facing the same problem.

Yours faithfully,

Christopher

Yes Christopher

the input was shorted to Vmid and the TAS3204 was streaming from the input to the output.

What amp are you using to listen to the noise?

Thomas

Hi Thomas,

I plan to have a TAS5630 behind the DSP but up to now, I don't need it. The noise can be clearly seen with an Oscilloscope. Are you useing the EVM Board or is your PCB a selfbuild version too? Unfortunately I don't have a EVM Board, so I have no clue if the Eval Board is making the same noise, which I doubt. I'am really lost in this case. I tried most of the poor documented registers but the noise stayes the same. The audio volume also has no impact on the noiselevel . The Hardware seems fine, all Voltages are nominal in the ranges they are supposed to. The Quarz is working stable and still there is this noise problem.

Hey Thomas,

I think I got a clue to solve my problem. When I took a closer look at the TAS3204EVM Module Users Guide, which can be found online on the TI Page, it seems that the Op-Amp at the analog outputs is configured as LP-Filter. You can see the symetrical setup of the output Op-Amp in a picture, together with some caps. This would mean, that the DAC is supposed to have noise on it, and you need to filter the output of the TAS. Unfortunately I don't have a EVM Board, so I can't make measurments on it. Maybe TI or a person with a EVM Board could help us here with simple test. The AOut testpoints are in front of the supposed LP-Filter opamp. This would mean, that you can see the noise at the testpoints, but it should be filtered out at the analog outputs.

Yours faithfully,

Christopher

Hey Christopher,

maybe your on the right way.

Did you check the x-over frequence of the LP filter?

What frequencys are dominant in the noisepattern that you measure?

Thanks,

Thomas

Hello Thomas, Since I don't have a EVM Board or the schematics for it I can't say if the EVM Board has a Post-LP-Filter or with witch frequency it is working. I just made an educated guess based on a picture of the EVM Module. I would expect something lika a sallen-key structure for a possible Post-LP-Filter. What I can tell you, is that the noise I have measured had a main frequency somewhere arround 730 kHz. What would be pretty easy to Filter out. So if you Thomas do have an EVM Module could you make the measurements if have mentioned above? Best regards, Christopher

Hello again,

I now had some time to think a little bit more about the noise problem of the TAS3204. With an additional Lowpass-Filter behind the DAC I was able to reduce the noise too an almost not anymore hearable amount. Now I have to figure out, how to filter out the rest of the noise. What I don`t understand is why something so important as a needed post LP-Filter is not described in the data sheet. There is not even a hint that a additional filter may be needed. But since this solution is working while every thing else failes it seems to be the only way to get rid of the noise of the TAS3204 analog outputs.

So please TI give me some specifications for the post LP-Filter you are using on your TAS3204 EVM Boards, so I can create a noise free DSP Board.

Yours faithfully,

Christopher

In this post,

http://e2e.ti.com/support/applications/audio/f/22/t/98583.aspx

you got the schematic of our EVM board, would that help at all?

Susan

Hello Susan,

You're right I do have the schematics for the EVM Board. There you can find the post low pass filter in form of a MFB Filter Type with an additional analog type filter in front. Unfortunaltely these MFB Filters are hard to calculate and the additional analog filter doesn't make this task easier. Since I would like to have a different filter type with an additional signal amplification integrated it would realy help if I know what kind of signal exactly must be filtered out. So if you would be so kind and try to look up these informations for me?

Yours faithfully,

Christopher

Christopher,

I asked the HW engineer who designed the board, this is what he told me:

The cut off frequency I use is ~29-32 kHz to keep the audio band flat 10Hz-22 kHz.

Hope this helps.

Susan

Hello Susan,

thank you for your efforts. The values are in the range that I would have expected for the lp filter. I will try to figure out how to eleminate the rest of the noise I got on the outputs. Still a definition for the noise pattern on the DACs would be nice.

Yours faithfully,

Christopher