Digilent Nexys 3 and High Speed Signals - Page 1
Forum section: electronics / general

author: rheb1026
author_rank: Regular Contributor
timestamp: March 22, 2017, 05:14:19 pm
content: I have been wanting to start tinkering with FPGA things again and have a Digilent Nexys 3 board from when I was in school. I really want to play around with high speed ADCs and DACs (~100 MSPS), and I was planning to make my own board and somehow connect it to the Digilent board.I don't know enough about high speed signal routing to know whether or not this Digilent board will be okay. Can I use the PMOD connection points to do this? They also have a 40-pin VHDC connector on the board. Is this a better choice to use? Or if this board is not a good choice, can someone recommend an alternative?

author: mrflibble
author_rank: Super Contributor
timestamp: March 22, 2017, 09:39:26 pm
content: The PMOD connections are not super duper for signal integrity. The VHDCI connector is a lot better in that regard. The main drawback of that connector is availability.The nexys 3 board is a real nice board IMO. Although if I were to buy a new xilinx board now I'd get one with a 7 series fpga, so for example one of these:http://store.digilentinc.com/nexys-4-artix-7-fpga-trainer-board-limited-time-see-nexys4-ddr/http://store.digilentinc.com/arty-artix-7-fpga-development-board-for-makers-and-hobbyists/Not suggesting those for high speed designs per se, more as a generic board to play with a current fpga architecture.

author: rheb1026
author_rank: Regular Contributor
timestamp: March 23, 2017, 03:52:28 pm
content: Thanks, after posting I tried looking around for the VHDCI connectors and couldn't find any 40 pin ones (the Digilent site says it's a 40 pin, but I haven't counted them). Do they go by a different name maybe? Any searches only bring up a 68 pin connector.I do like the Nexys 3 board and will use it as long as I don't need to go to something newer. I know the Xilinx ISE well enough that I only struggle with the HDL design and aren't fighting to learn the software too

author: joelby
author_rank: Frequent Contributor
timestamp: March 25, 2017, 12:18:31 pm
content: It's a 68 pin connector - 40 is the number of I/O signals (the rest are grounds and power). I once made a high speed ADC board with VHDCI for the Digilent Atlys, so it's doable! Getting the right part was a little tricky, and the one I used no longer seems to be manufactured.You can also get some high speed ADC/DACs on FMC evaluation boards - last time I looked they were surprisingly cost effective (compared to the hundreds of hours I probably spent designing and re-designing my ADC board).

author: rheb1026
author_rank: Regular Contributor
timestamp: March 25, 2017, 04:41:37 pm
content: Quote from: joelby on March 25, 2017, 12:18:31 pmOf course, I should have thought of that!I'll definitely have to look at some of the eval boards, but most seem to be over $200 for some of the more recent A/Ds. I found an older Linear eval board (LTC1742) for $100 which looks like it would fit the bill (and the budget).

author: joelby
author_rank: Frequent Contributor
timestamp: March 26, 2017, 05:36:51 am
content: True! The ADC I was using was the LTC2171/2172. The eval board for these is the DC1525A (http://cds.linear.com/docs/en/demo-board-manual/dc1525af.pdf). Considering that the ADC alone is something like $80 (though you can ask for samples), then $200 is not actually too bad for an assembled board. On the other hand you would also need to get a new FPGA dev board with an FMC connector! I'm not sure if they have a combo ADC/DAC board though. I remember seeing some around years ago, but they were not cheap.It all comes down to whether you want to spend your time and money doing PCB design, or doing FPGA stuff. Both are lots of fun, so if you have the time and want to do all of it, then great

author: rheb1026
author_rank: Regular Contributor
timestamp: March 28, 2017, 05:49:02 pm
content: Well in the end I'd like to learn both! But I think it'll be easier and less frustrating to start with a known working board and then roll my own at a later time