# ADC is detecting a sine wave as several clusters of points

I'm using an ADC (ADS4129) running at 125MSPS. I'm applying a 100kHz sine wave to a low pass filter (fc=15MHz). The ADC is being driven differentially, as intended. When probing the input signal with a scope I see a decent sinusoid.

My problem is that looking at the ADC output I get this: The frequency is right, but the points appear in clusters in specific points. At the moment I'm lacking several of the 10nF decoupling capacitors needed (I'm waiting for them). Could these capacitors be the cause of this problem? It seems strange to me that this appears at specific points.

On the other hand, analysing the input in the frequency domain seems to just indicate poor performance of the ADC, which would make sense without the decoupling needed. Edit:

The ADC is in a Mezzanine board. I'm using a FPGA to interact with it. The FPGA is expecting a LVDS input signal, and it is converting the the data from the ADC format (DDR, alternating even and odd bits). This was tested in a testbench. The FPGA inputs were contrained according to the ADC setup and hold time, with a margin to account for possible trace length mismatch.

Edit 2:

transition:

0b111000011 451,

0b111111111 511,

0b111000001 449,

0b111111111 511,

0b111111101 509,

0b111111100 508 • How exactly are you "looking at the ADC output"? Please provide a link to the manufacturer's datasheet for the ADC. Nov 14, 2020 at 15:01
• Exactly, between the ADC output and the picture in your answer is "A THING" that is undefined by your question. Nov 14, 2020 at 15:04
• It's difficult to be sure just from the graph, but it looks to me that those large jumps are suspiciously close to a value of 192 each time. Why do I think 192? well 192 is 128 + 64 and it's the sort of thing I've seen when adjacent bits get 'stuck together'. I suggest you check your connections on the D6_D7 lines. Nov 14, 2020 at 15:13
• Looks like 2 bits missing altogether, probably bits 8 and 9. Not enough codes to tell (and they aren't consistent : 448 = 511?) so I can't really tell. Nov 14, 2020 at 17:38
• Don't keep tacking on "EDIT" "EDIT EDIT" "NEW EDIT" etc. Just edit the post so it makes sense from the first time. You shouldn't answer the question in the actual question either. If an answer is correct, mark it as accepted and possibly add a short comment there.
– pipe
Nov 16, 2020 at 0:33

From looking at the last picture, we see that multiple votlages are mapped to the same discrete value, this means that some bits will not get transmitted/are lost. It seems we always have four repetitions that could be "pulled apart" again to form a nice continuous curve: This tells us that there must be two consecutive bits that are lost. After a little bit of trial and error I noticed that you cannot just zero them out due to the even distribution of those large "steps" across zero: If we instead implant the first two significant bits at the bits that are lost, we get exactly what we are lookign for:

So basically I'd say the DAC outputs are

[d1, d2, d3, d4, d5, d6,...]


(where dn represents the the n-th most significant bit), but your program somehow reads

[d1, d2, d1, d2, d5, d6,...]


Actually it might also be d(n),d(n+1) that get overwritten by d1, d2 for some n, as I don't know what resolution I'm looking at in your plot.

I made a quick plot to simulate this behaviour, and we can see pretty much the exact same outcome: Here is the MATLAB code used written for this plot:

bits = 9;
x = 0:1e-2:2*pi;
y = round((sin(x)*0.85 + 1.15)/2 * (2^bits-1));
yd = nan(size(y));
perm = [1,2,1,2,5,6,7,8,9];
for i = 1:numel(x);
bin = dec2bin(y(i), bits);
bin = bin(perm);
yd(i) = bin2dec(bin);
end
plot(x, yd,'.-b');


I suspect that you have somehow reconstructed the order of the bits incorrectly. This may be simply a wiring error between the ADC and the FPGA, or you may have ordered the DDR data backwards, or the FPGA code has some kind of ordering error for the bits.

• Could also be a short circuit between bit lines.
– jpa
Nov 15, 2020 at 9:47