AD9249 14-bit mode problem

Good day.

Can you give us any advices in the following situation. We use AD9249 with Xilinx Kintex7 device(xc7k160t-2).

Clock frequency is 62.5 MHz. If we turn on 12 bit mode using SPI, all 64 channels (we use 4 AD9249 chips) works fine.

The problem appears when we try to work in 14 bit mode. We can't see normal frame clock on Xilinx deserealizers.

During normal operation we expect to see continuously repeating 7 logical '1' and 7 logical '0' on parallel output

of deserializer. In our case we see random pattern of '1' and '0' (11001100111000 for example) on roughly half of ADCs (several works normal).

There is no correlation of working ADCs between different pcb boards, so the same ADC works on one board, but doesn't on another.

  We tried to lower frequency by using internal divider (register 0x0B). We see lower frequency of all ADC signals, but the result is the same. So, it's not speed problem, and it's also not PCB mounting problem, as all works fine on all boards in 12 bit mode. We will appreciate any help you can provide for solving this problem.