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TAS5805M: High distortion at 10 kHz

Johannes Nadler
Prodigy 10 points
Part Number: TAS5805M
Other Parts Discussed in Thread: TAS5805

Hello,

we are using the TAS5805M to output sine waves at frequencies up to 20 kHz. 
At 1 kHz the Amplifiers output looks very smooth without much distortion.


Distortions start to appear from around 3 kHz. This is the output signal at 10 kHz:

If I run the FFT, I see lots of harmonics.



Is this an expected behavior? I am using a 10 Ohms resistor on the output. The filter has a 10 uH inductor and 2x330nF = 660 nF capacitor. 
I2S sends the sine values at 48 kHz sample rate. Amplifier is supplied by 12 V. 



My I2C init sequence is very simple. I do not use any signal processing.
Switching frequency is set to 768 kHz.

uint8_t initSequence[][2] = {
{ 0x00, 0x00 }, // Change page to page 0
{ 0x7f, 0x00 }, // Change book to book 0
{ 0x03, 0x02 }, // HIZ
{ 0x01, 0x11 }, // Reset registers
{ 0x03, 0x02 }, // HIZ
{ CFG_META_DELAY, 5 },
{ 0x03, 0x00 }, // Sleep
{ 0x46, 0x11 }, 
{ 0x03, 0x02 }, // HIZ
{ 0x61, 0x0b }, // ADR as FaultZ Output
{ 0x60, 0x01 }, // ADR pin as output
{ 0x7d, 0x11 },
{ 0x7e, 0xff },
{ 0x00, 0x01 }, // Change page to page 1
{ 0x51, 0x05 },
//Register Tuning
{ 0x00, 0x00 }, // Change page to page 0
{ 0x7f, 0x00 }, // Change book to book 0
{ 0x02, 0x00 }, // fsw = 768 kHz
{ 0x30, 0x00 }, // SDOUT outputs DSP output
{ 0x4c, 0x30 }, // Digital Volume: 0 dB (default)
{ 0x53, 0x03 }, // ANA bandwidth = 175 kHz
{ 0x54, 0x00 }, // Ana gain = 0 dB
{ 0x03, 0x03 }, // PLAY
{ 0x78, 0x80 }, // Clear analog faults

};

Best regards,

Johannes Nadler

  • 0
    TI__Expert 3540 points

    Hi Johannes

    It's less likely caused by TAS5805 itself, there are some advice to you

    1.check your signal source is stable, check the FFT with no signal input

    2. Do you have AP, it's recommended that you can use AP to measure the THD+N again, I'm not sure if it caused by your oscilloscope setting.  I have test at TAS5805 EVM, input 10kHz, load 10ohm, PVDD=12V, and the THD+N is less than 0.01%

    3. I set a default configuration which verified on EVM, you can try to use this attachment file.

    3463.TAS5805.h

  • 0 in reply to Feeney Xiao
    Prodigy 10 points

    Thank you for your advices. I tried your default configuration file. Unfortunately, it did not help.
    I have found an important information in the datasheet of the TAS5805M:


    The THD+N measurements were all taken using a 20 kHz brickwall filter.
    We plan on using the TAS5805M for a different application, that is not audio related. Harmonics, Inharmonics and noise outside the audio bandwidth of 20 kHz cannot be neglected in our project. 

    Have you tested the TAS5805M without the brickwall filter and it showed THD+N of less than 0.01%? 

    Best regards,
    Johannes

  • 0 in reply to Johannes Nadler
    TI__Expert 3540 points

    Hi Johannes

    We are concerned about audio application so we only have the THD+N result with 20kHz filter. As your screenshot shows, the THD+N start drop from about 6.5kHz, it caused by 20kHz filter the harmonics. If you don't add 20kHz filter, the THD+N will increase and don't drop from 6.5kHz.