ADAU1701 Noise

Vincent Question

Mar 5, 2010 00:00

Hello,

I have created my own ADAU1701 test board and successfully created a simple test program that routes the analog input to the analog output.

The samplerate is set to 48kHz with a 12.288MHz xtal. PLL_MODE0 pulled low and PLL_MODE1 pulled high (256*fs).

After powering up the DSP and pulling Reset low and high there is good quality audio.

Everything seems to be ok by now.

But now, I want the samplerate to be set to 192kHz for future projects. After setting both the DSP core and the analog inputs to 192kHz and changing

the PLL mode to 64*fs @ 12.288MHz, both PLL_MODE0 and PLL_MODE1 pulled low, there is no audio. Just a loud noise (like a radio which is not recieving signal).

All power supply pins are decoupled correctly with 100nF and 10uF. The only thing what could be causing this problem (I think) is the use of a break out board for the ADAU1701 (long leads).

Any sugestions?

Audio Signal Processors

SigmaDSP Audio Processors

adau1701

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ADIApproved Reply

Mar 5, 2010 00:00

Vincent,

Even though you have changed your system's sampling frequency, you do not need to change the PLL clock or settings. In the "Setting Master Clock/PLL Mode" section of the ADAU1701 datasheet, you'll see that if the IC is set to run at quad rate (192 kHz), then the fs scaling factors also change by a factor of 4. So, your 12.288 MHz clock is now 64 x fs, but you should NOT use the 64 x fs setting as shown in Table 12. The multiplication factors in this table assume a nominal 48 kHz fs. At fs=192 kHz, the multiplication factors in the first column would be 16x, 64x, 96x, 128x. So, for your board, you should keep the PLL_MODEx pins set to the same as you had before - PLL_MODE0 = 0 and PLL_MODE1 = 1.

I believe that making this change will give you proper operation on your board.

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Vincent Reply

Mar 5, 2010 00:00

Hi Jerad,

Ok, I wil set the PLL mode back to the default setting and wil let you know the result.

Thnx!

EDIT:

It works!

Now there is only one thing left for me to figure out... I need two oscillators, 19kHz and 38kHz and they need to be in "phase" they actually don`t have to but for example if the DSP starts and the phase angle between 19 and 38kHz is 22 degrees, the phase angle should stay 22 degrees all of the time.

My current setup are two seperated sine wave oscillators (19&38kHz) and the phase angle is shifting all the time. Is there any way to lock the 19kHz with 38kHz?

I already found the divide function (divide 38kHz by 2) but I don`t think I can use this function for dividing sine wave values. ??