Hi, Adam,

I haven't looked at the spectral content of a terminated input before so I 
don’t have any comparative results, but I think the spikes you are seeing are 
caused by mismatched gains and/or offsets of the ADC’s interleaved cores (I 
think there are a total of 8 cores, also called "branches" in the datasheet).  
Have you tried sampling band limited noise (e.g. low pass at Fs/20)?  I think 
the severity of the spikes will be less in the presence of a signal.  In the 
worst case you will have to ignore the 8 frequency channels with these spikes, 
but you might be able to improve things by tweaking the coarse gain and/or fine 
gain registers in the ADC.


> On Sep 20, 2016, at 11:46 AM, Schoenwald, Adam (GSFC-564.0)[AS and D, Inc.] 
> <adam.schoenw...@nasa.gov> wrote:
> Hi All,
> I have a question regarding the ADC16x250-8 coax rev 2 running in demux by 4 
> mode. When I terminate the inputs with a 50 ohm connector and collect data 
> using a basic snapshot block design, I get a little bit of noise on the LSB. 
> I export the data to a csv file and find that it is a collection of 0’s and 
> -1’s, indicating that the level being converted by the adc is sitting between 
> these two values and there is some sort of internal noise causing us to jump 
> between values. What is alarming is that when I collected 2^18 samples and 
> run spectral analysis I see large spikes at intervals of FS/16 (See Attached, 
> pwelch(x,1024,256,[],800)).
> I am running the ADC at 800MHZ and the FPGA at 200MHZ. I am using one of the 
> more recent casper builds after the bitslipping commit on Mar 25th. I also 
> tried to use the ruby script located 
> athttps://github.com/jack-h/casper_adc16/tree/master/ruby/lib 
> <https://github.com/jack-h/casper_adc16/tree/master/ruby/lib>.
> I run the bof file with “adc16_init.rb --reg=0x3a=0x0202,0x3b=0x0202 -d 4 
> roach_ip boffile”, where I change the ic inputs. We only connected 8 of the 
> 16 coax cables to the board and I had to switch off the default.
> Has anyone else had a similar experience or ideas? 
> ----------------------------------------------------
> Adam Schoenwald
> ----------------------------------------------------
> <adc16_terminated.png>

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