I am designing a NMR Signal Acquisition Board. What it does is receiving the analog signal modulated at 63.8MHz, and then converted into digital signal using AD9653. And then the digital signal enters the FPGA for downsampling, downconversion, Cascaded Integrator–Comb (CIC) decimation filtering, etc. After processed by the host computer, the image in the first picture can be got.
However, the question is the final image has a 180 degree flip shadow.
The second picture is the input of AD9653 when AD9653 is not working.
The third picture is the input of AD9653 when AD9653 is working.
The sampling frequency used is 100Mhz.
It seems that crosstalk appears between AD9653 and the pre-stage analog circuit.
So, could you offer me some suggestions for solving this problem?