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Take the long way home: Building a signal breakout box for the FPGA-based Digilent Analog Discovery 2 double DSO and Waveform Generator

Xilinx Employee
Xilinx Employee
0 0 30.7K


Digilent sent me an Analog Discovery 2 early last month. (See “Just arrived: Digilent’s $279 Analog Discovery 2 multi-instrument based on a Spartan-6 FPGA and a ton of ADI chips.”) The Analog Discovery 2 lists a two-channel USB digital oscilloscope (1MΩ, ±25V, differential, 14-bit, 100MS/s, 30MHz+ bandwidth) and a two-channel arbitrary function generator (±5V, 14-bit, 100MS/s, 12MHz+ bandwidth) among its many integrated instruments (all made possible by the reprogrammable capabilities of a Xilinx Spartan-6 FPGA) and I wanted to break these particular instruments out so I could use them more easily with conventional BNC-terminated probes and cables. Now Digilent sells an Analog Discovery BNC Adapter Board with four BNC connectors for $19.99 so the most reasonable thing to do would be to order one.



Digilent BNC Adapter.jpg



Digilent’s $19.99 BNC Adapter Board for the Analog Discovery 2




I built my own instead. Why not? How hard could it be?


My first inclination was to get four BNC connectors and an extruded aluminum box and drill out the box for the connectors over at TechShop San Jose where I am qualified to use one of the Jet end mills. (Cue the “Tim ‘the Toolman’ Taylor” alpha male grunting.) I did get as far as stopping by the most excellent Excess Solutions surplus electronics store in San Jose where I picked up four panel-mount BNCs, but then an even better solution occurred to me. (More grunting.)


I got a 4-channel OdiySurveil Cat 5 Passive HD Transceiver balun box from Amazon for $12.59. These balun boxes are designed to take as many as four channels of HD video from coax cables and route them over long lengths (1500m !!!) of twisted-pair cabling. The balun box’s beefy steel enclosure is pre-punched for four BNCs and comes complete with four BNC connectors connected to an RJ45, Cat5 Ethernet connector and an 8-pin Phoenix Contact screw-terminal block through a pcb securely mounted inside of the box. It’s twelve bucks and change, nicely finished with no drilling or milling required, delivered to your door in two days by Amazon Prime.



Original Balun Box.jpg 



OdiySurveil Cat 5 Passive HD Transceiver




Here’s what it looks like inside:



Original Balun Box Interior.jpg 



OdiySurveil Cat 5 Passive HD Transceiver Unmodified Interior




Sure, there are some pesky balun transformers and a few other unneeded passive components on the board but I quickly popped those off the pcb with a screwdriver (more grunting), cleaned up the pads, and shorted out the appropriate connections on the board with eight short jumper wires. I still know which end of the soldering iron to hold, apparently.




Modified Balun Box Interior.jpg



OdiySurveil Cat 5 Passive HD Transceiver Modified Interior




Then, I needed to decide whether to use the box’s Phoenix Contact terminal block or the RJ45 connector to connect to the Analog Discovery 2’s double-row, 0.1-inch header strip. At first, I thought I’d use the terminal block but the more I considered that RJ45 connector, the better it looked. A quick trip over to IT here at Xilinx secured a Cat6 cable in a huge box of orphaned Ethernet cables.


I snipped one end off of the cable and then cut the outer insulation off of the cable end. There were the four unshielded, twisted-wire pairs that I needed for the connection to the Analog Discovery 2. I also ordered a reasonably large kit of crimpable “Dupont” pins with plastic headers from eBay for about $9 (including shipping).


Note: Somehow in the last 30 years, I missed the transition point where “Berg pins” became “Dupont pins.”



When everything arrived, I stripped the twisted pairs, crimped on the Dupont pins, and inserted the pins into a 2x5-pin plastic header.


A big piece of heat-shrink tubing cleaned up everything and there you go, one bulletproof breakout box. Finally, I made a printable label using Visio and attached it to the top of the box, which now looks like this:




Finished Breakout Box.jpg



Finished Analog Discovery 2 Breakout Box



Works great.



(Note: This isn't a great example of a wise make-versus-buy decision. If I weren't blogging this project, I'd probably just buy Digilent's adapter board.)




For more information about the Analog Discovery 2, please contact Digilent directly and see these previous Xcell Daily posts:






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