hi adam,

the effect you are seeing is visible on many adc's - some digital noise
gets into the analog input,
and is often easy to see when there is no signal input, especially when the
adc's DC offset
happens to lie near the boundry of two ADC steps (then any noise will cause
the ADC to toggle).

if you test the adc is a more realistic situation, by injecting wideband
noise with an RMS about
about 20 ADC units, the spikes will likely still be there, but they will be
very small
and you'll have to integrate for a long time to see them.

best wishes,

dan




On Tue, 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 at 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
>
> ----------------------------------------------------
>
>
>

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