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Hello, I am trying to interface my FPGA board to the DAC board. The DAC needs four lines from the FPGA to work properly . I optocially isolated the four lines using four HCPL 2400. I powered up the FPGA board and DAC board separately using two different DC Lead acid battery (+5volts) inorder to preserve isolation. Now, I probed the power pins of the DAC and the optocoupler with oscilloscope to check the power supply and found sine waves of 60 Hz. The output of the optocoupler also producing sine wave of 60 Hz. I then connect the earth ( ground) to the ground of the floating DC power supply of the DAC board, checked with the oscilloscope and the sine wave went away. The signals look a little crapy, the power pins look good. But this solution shorted the return paths of both the lead acid battery so eliminating the optoisolation. The FPGA gets programmed by DB25 connector connected to the computer. I removed the cable and get my optoisolation back but the problem is that I can not program the FPGA any more. Can anybody advice me what to do? Regards, John
john wrote: [snip] > The FPGA gets programmed by DB25 connector connected to the computer. > I removed the cable and get my optoisolation back but the problem is > that I can not program the FPGA any more. Can anybody advice me what > to do? Have you ruled out the possibility that the scope probe is *introducing* the 60Hz by acting as an antenna? If so, maybe a good-sized nonpolarized capacitor from DAC ground to earth ground?
On Wed, 19 Mar 2008 10:38:57 -0700 (PDT), john <c...@hotmail.com> wrote: >Hello, > >I am trying to interface my FPGA board to the DAC board. The DAC >needs >four lines from the FPGA to work properly . I optocially isolated the >four lines using four HCPL 2400. I powered up the FPGA board and DAC >board separately using two different DC Lead acid battery (+5volts) >inorder to preserve isolation. > > >Now, I probed the power pins of the DAC and the optocoupler with >oscilloscope to check the power supply and found sine waves of 60 Hz. >The output of the optocoupler also producing sine wave of 60 Hz. > > >I then connect the earth ( ground) to the ground of the floating DC >power supply of the DAC board, checked with the oscilloscope and the >sine wave went away. The signals look a little crapy, the power pins >look good. But this solution shorted the return paths of both the >lead >acid battery so eliminating the optoisolation. To get any sensible 'scope readings from the DAC board, the 'scope ground _must_ be connected to the DAC board ground. Likewise, to get sensible readings from the FPGA board, the scope ground _must_ be connected to the FPGA board ground. Therefore, if you want to scope signals on both boards at the same time, the grounds must be connected together. When you have finished testing, you can separate the grounds of the two boards, and things should continue to work. > > >The FPGA gets programmed by DB25 connector connected to the computer. >I removed the cable and get my optoisolation back but the problem is >that I can not program the FPGA any more. Can anybody advice me what >to do? > > >Regards, >John > -- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
On Mar 20, 4:38 am, john <conphil...@hotmail.com> wrote: > Hello, > > I am trying to interface my FPGA board to the DAC board. The DAC > needs > four lines from the FPGA to work properly . I optocially isolated the > four lines using four HCPL 2400. I powered up the FPGA board and DAC > board separately using two different DC Lead acid battery (+5volts) > inorder to preserve isolation. Why? What is the purpose of the isolation? Isolation is generally only required in special cases. > Now, I probed the power pins of the DAC and the optocoupler with > oscilloscope to check the power supply and found sine waves of 60 Hz. > The output of the optocoupler also producing sine wave of 60 Hz. > > I then connect the earth ( ground) to the ground of the floating DC > power supply of the DAC board, checked with the oscilloscope and the > sine wave went away. The signals look a little crapy, the power pins > look good. But this solution shorted the return paths of both the > lead > acid battery so eliminating the optoisolation. > > The FPGA gets programmed by DB25 connector connected to the computer. > I removed the cable and get my optoisolation back but the problem is > that I can not program the FPGA any more. Can anybody advice me what > to do? Simply remove the optocouplers and your problems will go away. If you really need the optocouplers for some reason then you will need a non-mains earthed oscilloscope to view signals on the DAC board while the FPGA board is connected to the PC. e.g. a differential probe or a battery powered scope. Alternatively you could use a battery powered notebook PC with a parallel port. Dave.