Below is the filter schematic and spice simulation. I used the Elsie software package and selected L/C values I had on hand for a 50 ohm termination and notch response at HD2 for a 10.7MHz signal.
|Notch Filter for Approximately 21.4Hz (HD2 of 10.7MHz signal)|
|Constructed HD2 Notch Filter|
|Measured Response of Filter|
Using the above notch filter and a 10.7MHz BPF and ADC Unit#1 the following spectra was obtained with a synthesizer/mixer source (non-DDS).
|ADC measurement of 10.640 MHz input through notch filter and BPF.|
My first estimate was that the distortion was occuring in the fully differential amplifier (FDA). The datasheet shows -80dBc to -90dBc for HD2 and HD3 under various feedback configurations and loads at 10MHz using a 2Vpp output and a single ended input. There is a figure showing 10dB of variation in this with different common mode voltages (near the range I am operating using the ADC common mode voltage output).
In an effort to probe the FDA contribution, I changed the gain of the FDA down and up (ensuring all resistors were 1%). In no case did this change HD2, HD3 or the spur clusters levels.
The last aspect of this which I can think of is that my input resistor is small compared to general applications referenced in the datasheet. The exact impacts of this on distortion are not clear to me (if any).
In the update of the board, a BPF filter was added between the ADC and FDA just for this concern. The next step is to leverage the filter and monitor response changes in HD2/HD3 and the spur clusters with the idea being if they are attenuated the FDA is the source, if not, the ADC is the prime contributor.