In a quick attempt, a 10uF bypass capacitor was added across C106 on the SYS_5V supply. This did make a few dB of improvement in the spurs so I thought I was on the right track. Another 10nF shunt capacitor was added on C119 but to no additional help.
Just to get myself calibrated, the first spur is on the order of -93dBFS which translates into 22uV RMS. The ADP150 datasheet shows a power supply rejection ratio (PSSR) of -35dB to -40dB for 2MHz and 4MHz. The amplifier datasheet quotes at least 55dB of PSSR. The ADC does not quote a PSSR but one would expect at least 20dB - 60 dB. The board has separate analog and digital power which are decoupled with a SMT choke. Given all of this I expected to be able to see the source of noise of the board using just a simple scope probe. On poking around on the board both digital, analog, and supply input - no luck, nothing even close.
In an attempt to do better than a simple oscilloscope probe, a white wire from SYS_5V pin was brought out to a 10uF on a spare SMT pad on the board. This was then jumpered to an SMA connect which was then attached to a spectrum analyzer. A picture of that setup is below.
|SYS_5V spurious measurement|
|Low frequency spectrum with BBB powered off and with it on and ADC operating.|
The 2MHz spur at -65dBm above corresponds to roughly 126uV RMS while the -93dBFS from the ADC spectrum corresponds to 22uV. 126/22 = 5.7 which translates to -15dB of attenuation. The bottom line being there is not a whole lot of PSSR compared to what one would expect.
Just to make sure I wasn't doing something silly like pushing the regulator too hard and causing problems I decided to revisit the current consumption budget on the board re-checking the dynamic component. The following table captures that.
|ADC Board Current Consumption|
With the above in hand, I went back to the board to evaluate the spectrum at different points on the supply using the same setup as above. The following graph compares the spectrum at the 5V source, 3.3V on-board regulator output and the 3.3V regulated output after the supply choke into the analog section.
|Low frequency spectrum of supply line at various points on the ADC board.|
At this point it seemed that the noise was more subtle than just supply ripple. With that idea I revisited ground loops (where you won't directly see the currents only the artifacts afterwards and points at which you cannot access for measurement). The ADI MT-031 application note does a good job going through the various aspects of mixed signal grounding in different environments. With this I realized I did exactly the wrong thing by isolating the analog ground plane with a choke. This would have been ok if I had directly tied the two together at the ADC (as one of the examples in the paper shows). Fortunately, this was easily addressed by removing the choke (L101) and jumpering across the exposed ground plane edges near the ADC. The spectrum below was taken with that configuration.
|50 ohm grounded input with jumpered grounds|
So in the end, the noise environment on the ADC ok. Even factoring in recommended PCB layout techniques and best practices with ADCs (at least as best as I could do with the tradeoffs presented in a 2" x 3" two layer PCB) there are still switching related spurs.