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Optocoupler and probing

Started by john March 19, 2008
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

On Mar 19, 11:44 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. > > 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
The only thing I can think off the top of my head is - when you measured signals on the DAC board was the scope's ground lead on the DAC board's ground or the FPGA board? It should be on the ground of the board you are probing. -Dave Pollum

> The only thing I can think off the top of my head is - when you > measured signals on the DAC board was the scope's ground lead on the > DAC board's ground or the FPGA board? =A0It should be on the ground of > the board you are probing. > -Dave Pollum- Hide quoted text -
Hi, It is on the ground of the board that I am probing. John
"john" <conphiloso@hotmail.com> wrote in message 
news:a897ed32-39ce-4e32-b48c-b0168b7ca9d2@8g2000hse.googlegroups.com...
"It is on the ground of the board that I am probing."

Have you done a "null hypothesis" test, touching the probe itself to the 
ground clip and making sure you don't see anything on the scope?

(If you do see something, you're just picking up induced noise through the 
loop of the probe-body-ground lead-ground clip.)


> (If you do see something, you're just picking up induced noise through the > loop of the probe-body-ground lead-ground clip.)
Hi, If that is the case then what is the soultion? Regards, John
On Mar 19, 3:18 pm, john <conphil...@hotmail.com> wrote:
> > (If you do see something, you're just picking up induced noise through the > > loop of the probe-body-ground lead-ground clip.) > > Hi, > > If that is the case then what is the soultion? > > Regards, > John
Differential probing.
On Mar 19, 4:44=EF=BF=BDpm, 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.
5v battery?
> > 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.
Floating supply, your scope will pick up line. You need to have the probe "ground" clip attatched to the isolated supply return.
> > I then connect the earth ( ground) to the ground of the floating DC > power supply of the DAC board, checked with the =EF=BF=BDoscilloscope 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? >
Did you do this with power still on? Most likely you have fried your fpga. Do not attatch/detatch leads,returns, probes or clips while power is on.
> Regards, > John
<a7yvm109gf5d1@netzero.com> wrote in message 
news:9ac59203-9772-4ca7-bad6-56d685ec1885@d62g2000hsf.googlegroups.com...
> On Mar 19, 3:18 pm, john <conphil...@hotmail.com> wrote: >> > (If you do see something, you're just picking up induced noise through >> > the >> > loop of the probe-body-ground lead-ground clip.) >> If that is the case then what is the soultion? > Differential probing.
...or remove that long ground clip and just use one of those slide-on probe tip covers that contacts the probe's ground and has just a 1/2" or so stiff ground wire coming off of it. (The cover also has a hole in the center for the probe tip to poke of out still, of course.) The idea here is that you're reducing your loop area dramatically, so you'll pick up that much less noise.
> > 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 scope probe GND also earthed the isolated GND, so you're full of shyte- Go away and don't come back until you get your story straight!