AMC1306M25: Noise with ~400Hz periodic interval in output

We are using a AMC1306EVM evaluation kit. With the following connections:

• Input J1 is shorted with a wire (1cm)
• VDD2 is 3.3V
• VDD1 driven by onboard transformer
• DCLOCK is driven by a microcontroller SPI peripheral (5MHz, alternatively 20MHz)
• DOUT is connected to SPI peripheral

It is expected that the output generates 010101 patterns (50% 1/0 ratio).  However it appears that the pattern has (after filtering) a noise of +/- 100 bits. SPI communication has been verified with a logic analyzer.

On a 2nd AMC1306EVM board, the output behaves occasionally correct, however most of the time the signal is overlaid with random noise.  The noise seems to have a pattern of ~400Hz.

Any idea what could be wrong? Is the AMC1306M25 sensitive on the DCLOCK input (are there requirements for high signal quality matter)?

Hints would be welcome.