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ADAU1701 BCLK jitter

Question asked by rpress on Mar 13, 2012
Latest reply on Apr 26, 2012 by BrettG

I'm working on a custom design using the ADAU1701.  I have some problems with the external I2S interface.

 

The internal ADC and DAC are working fine if no I2S is connected.  I am using the 1701 in I2S master model, with BCLK out tied to BCLK in and the same for LRCLK.  When I try to drive my ASRC (SRC4192)  in slave mode with the I2S from the 1701, the ASRC has trouble locking to the signal, and the data is not in sync with the LRCLK.

 

When I look at the BCLK signal with my scope the jitter is huge!  There is jitter even if everything is disconnected except for the scope probe.  It's about 40 ns(p-p).  It's not my MCLK, as that is very clean, and running the ASRC as I2S master generates a jitter free BCLK.  What could cause this jitter on BCLK?  I checked the PLL filter and it's as the datasheet specifies.  I replaced the ADAU1701 IC and the problem remains.

 

Another very strange problem is that adding capacitive load to the BCLK signal messes up the internal DAC.  Are these somehow tied together inside the 1701?  Even adding something like 30 pF to the BCLK causes dropouts from the internal DAC.  I suspect this is related to the first problem somehow.

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