ADC without Amplifier

In an effort to evaluate the ADC harmonic distortion and spur clusters I decided to create a variant of the ADC board which did not have the amplifier (earlier version).  This allows me to revisit the ground plane, add resistors to the digital lines and re-layout the sample clock signal.  It ends up being as close to the ADC reference design as possible.

BeagleBone ADC board with transformer input.

The first unit anti aliasing filter was not populated to allow evaluation at multiple frequencies.  Two different regulators were tried - the original 150mA and a 300mA version.

Constructed ADC board with transformer input and no amplifier. ADC anti-aliasing filter not populated.  TCXO in upper right, input transformer at lower right, ADC right of upper center, regulator and reset on left.

As in previous measurements, a DDS at 10.640MHz was used as the input.  This was filtered through a 2x 10.7MHz ceramic filter.  Without the amplifer this setup was not able to drive full scale, rather -3dBFS.  That spectrum is shown below.

-3dBFS 10.640MHz input

HD2 is still -79dBc and the spur clusters are present at the same levels and locations as the same version of the board with amplifier ( HD2 compare and spur cluster compare).  The grounded input spectrum looked similar to the previous versions with the amplifier (flat at -107dBc with small clusters at 2MHz and 4MHz in the -100dBFS range).

The continued presence of the spur clusters is disappointing but I am out of ideas on the root cause and decided to put it away for a while and move on to other topics.