The DC bias comes from the ADC itself, which is biased to be symmetrical about -0.5 (not 0).
Jason On 03 Mar 2011, at 01:42, Dan Werthimer wrote: > > > hi andrew, mandana, > > i think all the casper adc boards > have balun transformers or ac coupling > capacitors, so a signal's DC bias > can't get to the ADC. > > dan > > On 03/01/2011 10:44 PM, Andrew Martens wrote: >> Hi Mandana >> >> >> fft_shift is set to 0xffffffff by default in tutorial 4 poco >> design. Any other suggestions? >> When I monitor my ADC input levels, they are about 4.6 bit used. >> >> >> Ok, well that should rule out overflows from the FFT. >> >> I assume that there is no clipping with terminated inputs? >> At what power level does overflow _not_ happen? >> >> Is there any DC bias in your input signal? A DC blocking filter is >> good here as this may be causing overflow in the FFT. >> >> The DC channel will often cause the equaliser overrange bit to be set >> as the value in this channel is much larger than the others. This is not >> a problem as long as there is no overflow in the FFT. >> >>> >>> My next question is what is the dip that I see in the middle >>> of the auto- and cross-correlation plots, regardless of >>> signals applied or just terminated inputs. >>> >>> Not so sure about this, possibly include a plot if you can. >> Please see the attached plot. I have a Roach board and 2 x 1GS/s ADC >> boards. In this plot, a 500MHz sine is connected to the first input >> (channel a), while the other 3 inputs are terminated (channels b, c, >> d) and an 800MHz clock is supplied. What is the small dip in the >> centre (@channel 500). This is also present in the plot posted on >> page 15 of casper_workshop_tut4.pdf. >> >> I have no idea to be honest. There are some artifacts but there >> has not been a study on what these are and what causes them >> yet. They are generally very small, stable and can be catered >> for in any final system. >> >> Sorry I have not been more useful. >> >> Regards >> Andrew > > -- > > Dan Werthimer > Space Sciences Lab and Berkeley Wireless Research Center > University of Calfornia, Berkeley > > >
