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Optocoupler and probing

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

Reply to
john
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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

Reply to
Dave Pollum

Hi, It is on the ground of the board that I am probing. John

Reply to
john

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.)

Reply to
Joel Koltner

Hi,

If that is the case then what is the soultion?

Regards, John

Reply to
john

Differential probing.

Reply to
a7yvm109gf5d1

5v battery?

Floating supply, your scope will pick up line. You need to have the probe "ground" clip attatched to the isolated supply return.

the

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.

Reply to
cbarn24050

...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.

Reply to
Joel Koltner

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!

Reply to
Fred Bloggs

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