Signal distortion ADC/DAC on Cyclone III dev kit

[Altera_Forum]

Hi All, 

I'm new to FPGA's, and have been having difficulty doing basic signal I/O. I modified the factory example that came with the Cyclone III software (see attachment for block diagrams). This should allow me to pass a signal from the ADC to the DAC. However, the signal I receive from the DAC (which is 50 Ohm terminated) is incredibly noisy and distorted.   The only conclusion I can come to after many weeks of documentation-reading is that the clock settings are incorrectly configured. Does anyone have any advice?  ** I apologize for any lack of clarity, I've only just started FPGA's and need to use them in a research setting.   System Specs: Boad: Cyclone III EP3C120F780  Software Version: Quartus II, 13.1 64-bit   Cheers, Chris

[Altera_Forum]

It would be useful if you posted images of the ADC output (captured using a scope) and the DAC data plotted using say MATLAB's stair() function (so that it looks like sampled data). 

 Can you post a link to the specific board(s) you are using, so that we can see what ADC/DAC is used.  What frequency are you trying to generate? Many ADC/DAC kits have balun/transformer inputs/outputs which limit the range of frequencies you can test with.  There are some FPGA signal processing resources on this page you might like to read;  https://www.ovro.caltech.edu/~dwh/correlator/cobra_docs.html  Take a look at the EELive! slides and the ESC 2013 paper, slides, and code.  Cheers, Dave

[Altera_Forum]

Here's the DAC output connected to an oscilloscope (see attachment). The blue signal is output from a function generator, 2 V p-p. The yellow is output from the ADC, 50mV p-p. As for the ADC into the board do you recommend using the SignalTap II logic analyzer to acquire data?  

  I'm using the Cyclone III DSP Development Kit with an HSMC card: https://www.altera.com/products/boards_and_kits/dev-kits/altera/kit-cyc3-dsp.html  Here's info for the DAC: (TI DAC 5672) http://www.ti.com/lit/ug/slau139b/slau139b.pdf  and ADC: (Analog Device AD9254) www.analog.com/media/en/technical-documentation/data-sheets/ad9254.pdf   I'm working from 1 MHz to 75MHz (since the ADC sampling rate is 150Msps). The DAC/ADC are transformer-coupled, and looking physically on the HSMC card, these are the transformers with a frequency bandwidth of .004 to 300 MHz, which is plenty.  http://www.mcl-yokohama.co.jp/webdata/tt1-6-w38.pdf   I'll start reading through the EELive! slides and ESC paper.   Thanks!  Chris

[Altera_Forum]

"It would be useful if you posted images of the ADC output (captured using a scope) and the DAC data plotted using say MATLAB's stair() function (so that it looks like sampled data)." 

 Attached is a picture taken from an oscilloscope. The blue signal is 2V p-p and is the sinusiodal output from a function generator at 1MHz. The yellow is the corresponding output from the FPGA DAC with ~50mV p-p. As for the ADC, do you recommend using the SignalTap II logic analyzer for analog aquisition?  "Can you post a link to the specific board(s) you are using, so that we can see what ADC/DAC is used."  I'm using the Cyclone III DSP Development kit with an HSMC data conversion card: https://www.altera.com/products/boards_and_kits/dev-kits/altera/kit-cyc3-dsp.html The ADC is the Analog Devices AD9254, and the DAC is the TI DAC5672: http://www.analog.com/media/en/technical-documentation/data-sheets/ad9254.pdf http://www.ti.com/lit/ug/slau139b/slau139b.pdf  "What frequency are you trying to generate? Many ADC/DAC kits have balun/transformer inputs/outputs which limit the range of frequencies you can test with."  The goal is to use a frequency comb filter on an analog-->digital signal and then delay that signal by some amount before converting it back digital--> analog. The frequencies should be from 1 - 75MHz (nyquist frequency of the ADC) The HSMC card features transformers, so that shouldn't be the issue. , but their frequency range is .004 - 300MHz http://www.minicircuits.com/pdfs/tt1-6-kk81.pdf   I will definitely check out those documents you mentioned!  Thanks!  Chris