TAS3251: Qusetion of analog and digital ground of TAS3251

Tsai Anson

Prodigy 120 points

Part Number: TAS3251

Hello~

I made a TAS3251 PCBA.

The I2S input of TAS3251 is connected from another analog to I2S board.

But the output sound of TAS3251 is noisy.

When I power off, it will be normal before voltage drop to zero (about a second)

I use switch power supply, the Vin is 27V

Does TAS5251 need to separate analog and digital ground ?

thanks.

over 3 years ago

0 Sam(MPAA) Zhang over 3 years ago

TI__Mastermind 19080 points

Hi,

TAS3251 doesn't need to split the AGND and DGND.

Regarding the noisy issue, do you have any test equipment to isolate the audio source problem? For example, using an other digital input demo to play the I2S audio input, and check if the noise is still there.

Furthermore, you may test the DAC_OUT+/- pins of the TAS3251, this is for isolating the problem coming from digital domain or analog domain.

Thanks!

Regards,

Sam

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

Thanks for your help.

I have one more question.

Is it normal for my speaker output to have a frequency of 530~ 600KHz ?

This problem also occurs with the TI TAS3251 EVM board

This problem occurs when using a switching power supply or a AC to DC adapter.

thanks again.

0 Sam(MPAA) Zhang over 3 years ago in reply to Tsai Anson

TI__Mastermind 19080 points

Hi,

Looks like your probe is not grounded?

Regards,

Sam

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

I am sure the probe is grounded(The photo does not show the ground wire), I measured that the 3.3V,12V voltages did not have this problem,but the speaker terminal will have this frequence.

Thanks

+1 Sam(MPAA) Zhang over 3 years ago in reply to Tsai Anson

TI__Mastermind 19080 points

Hi Anson,

OK, I see. It should be the ripple after the L-C filter. The frequency is the switching frequency of class-d amp. This ripple is well above the audible range, so won't cause any artifact on hearing.

Regards,

Sam

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

Thank you very much for your help before, but I have one more question:

May I know why does the sine wave of 600KHz on the speaker terminal (OUT A and B) jitter up and down after LC filtering.

The sine wave is not be stable. Is this a feature of TAS3251

thanks again.

0 Sam(MPAA) Zhang over 3 years ago in reply to Tsai Anson

TI__Mastermind 19080 points

Hi Anson,

It's more likely caused by the oscilloscope probe setup. You minimize the length GND wire of the probe and try again, just like measuring the output ripple of a power IC.

Thanks!

Regards,

Sam

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

thanks, I will try again.

But, the currently, my PCBA is broken, why my OUT A/B have half of the PVDD voltage?

After I changed a new TAS3251, it is still the same, the other parts are not broken

thanks.

0 Sam(MPAA) Zhang over 3 years ago in reply to Tsai Anson

TI__Mastermind 19080 points

Hi Anson,

This is the way how class-d amp works. In idle status, each of the output legs of TAS3251 switches at 50% duty cycle, thus maintaining a 50%*PVDD common mode voltage on OUTA/B.

Regards,

Sam

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

Thank you for your explanation.

I understood why the output A/B has 50% PVDD.

I found that when I playing a single tone in no EQ mode, for example, frequency is 1KHz.

There will be a 600KHz carrier (switching frequency)always on 1KHz sine wave, the amplitude about 1.2V.

How can I filter this carrier wave ? Or I don't need to filter 600KHz, this a feature of TAS3251?

1KHz output from OUT A/B with 600KHz carrier

Standard 1KHz sine wave

Thank you again.

0 Sam(MPAA) Zhang over 3 years ago in reply to Tsai Anson

TI__Mastermind 19080 points

Hi Anson,

You don't need and actually can't filter it out. This is the way class-d amp operates(just like a SMPS).

The 600kHz is the switching frequency. Human ear is not able to respond to such a high frequency. Audible frequency range is 20-20kHz.

Thanks!

Regards,

Sam

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

Understood.

So this output waveform ( with 600KHz carrier) is a feature of CLASS D, I can't filter it out.

Thank you.

0 Tsai Anson over 3 years ago in reply to Sam(MPAA) Zhang

Prodigy 120 points

Hello Sam,

May I know, does the SNR and THD test values in the TAS3251 data sheet include a 600KHz carrier?

Thank you.

+1 Sam(MPAA) Zhang over 3 years ago in reply to Tsai Anson

TI__Mastermind 19080 points

Hi Anson,

For class-d audio amplifiers, the well accepted test setup includes a AES-17 filter(or some other filters), switching frequencies component are filtered out as they are not in the audio range. You may find the statement in the test condition on datasheet..

Thanks!

Regards,

Sam

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TAS3251: Qusetion of analog and digital ground of TAS3251