Re: Optocoupler and probing

On Wed, 19 Mar 2008 09:44:14 -0700 (PDT), in sci.electronics.design
john <conphiloso@xxxxxxxxxxx> 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
Linear.com AN70 app note p30 on how to scope things is very good


martin
.

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