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.
HTH, Dave > 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>
