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1701 Oscillator start problem

Question asked by GCook on Apr 11, 2012
Latest reply on Dec 31, 2015 by GCook

We are using 7 ADAU 1701's on the same board. One is the master with a 12.288 MHz clock and is programmed to send a 3.07 Mhz clock to the other 6 via MP11. The master DSP oscillator has had a history of not starting reliably over several hundred PCA's. Some boards are OK, some are not. Several modifications have been tried, including changing the layout to one you see here:

 

DSP Osc.jpg

C38 and C39 are 22 PF, 27 PF has been tried. The crystal has been changed to a different manufacturer with no effect. The resistor R256 is 100 ohms. If the crystal is tapped it will start. Changing R256 to 0 ohms will result in reliable starting. Bypassing is extensive with a 10 UF ceramic cap and 2 .01UF caps on both 1.8v and 3.3V close to the DSP. C416 is a 680PF cap bypass. This is the schematic of this section of the board:

DSP Circuit.jpg

There is a ground layer under the top copper shown in red. The PLL line runs under the traces to the crystal on the signal layer under the ground layer.

Any thoughts would be very much appreciated.

G Cook

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