PLL losing lock intermittently
I have a mature design with 8 x adau1701 in one PCB piece of insturmentation. All APs are synced to one common MCLK clock running at 12.288 MHz (I think). The design has been released for 3 years and we have produced approx 100pc without issue. Most recently across a batch manufactured we have some APs are breaking out of lock unpredictably.
The condition is very very peculiar in my experience of PLLs. The VCO in the chip is held (by the PLL loop) to within a few 100Hz of 12.288MHz * divider (this is about 10-20ppm of locked) by the PLL but drifts at a frequency every so slightly higher than reference. If I slightly bias the PLL_FL pin (the VCO line I assume) with a resistor of 100K to 3v3 the device locks. Biasing it high causes the osc to approach lock from the low F side. I have tried many many combinations of loop filters and the only thing that seems to have any effect is the resistor. Many APs will actually lock with no filter caps suggesting that the lock mechanism is normally robust. Anything from 1M to 47K seems to help a lot. <10Kohm knocks it out completely as I would expect. My general experience is that PLLs will lock or jitter but I have never seen on run continuously out of F and phase BUT by such a TINY and fairly consistent amount (+20ppm)
The occurrence is intermittent and in some cases gets better after 10 mins or so running and in others only starts after 10 mins. I have looked very closely at the loop filter pin and the MCLK signal. The MCLK it is right on 3V pk-pk with risetimes of about 5-10ns. Looks v clean. I can measure no difference in impedance between PLL_LP and adjacent components or rails on good and bad devices. I am pretty frustrated by now and would appreciate any pointers at to root causing this issue.
about 5% of AP devices are affected. Because my app requires synchronous behaviour between the APs I cannot live with this.
I note from Ezone that there have been similar issues in the past that sound rather similar but were taken off line.
HELP
Dan Linehan
