DAC, FPGA and Optocoupler

[john]

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

 

[Randy Day]

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?

 

[Peter Bennett]

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.

 

[David L. Jones]

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.