AD9257 waveform jump

Hello,

I‘ve had a problem using AD9257. I used it to acquire a sine wave, but the waveform will jump if the input exceed a value, as shown below.

The problem does not occur in all of the 8 channels. 

I made a few test boards, and each board has different error channels.

I use AD8138 as the differential Driver. I watched the differential input of AD9257 with an oscilloscope, and it is a standard sine wave.

I used the test pattern function of AD9257 to test my driver program, and I can receive the test pattern correctly.

What is the possible reason for this problem?

Appreciate all your efforts and help.

 

Thanks

Zhao