I thought those diodes also provide ESD protection

— hifiduino on 09.11.2012 at 00:55 (UTC)

They are supposed to. But because they are in parallel with the FPGA's own diodes, I believe that they don't actually improve the protection at all. The FPGA's diodes have a lower voltage threshold.

The external diodes might have higher current rating than the FPGA's own diodes. However, if they ever kick in, the FPGA has been burned already. The FPGA pin max voltage is 3.6V, and the external diode kicks in at 5 V.

— Petteri Aimonen on 09.11.2012 at 06:33 (UTC)

The ESD diodes can take 5A of pulse current and 8000V. It is to protect against static discharge rather than a constant voltage.

— hifiduino on 14.11.2012 at 00:18 (UTC)

The ESD diodes can take 5A of pulse current and 8000V. It is to protect against static discharge rather than a constant voltage. 

BTW, thanks for your contributions...

— hifiduino on 14.11.2012 at 00:18 (UTC)

It is true that D5 probably has a higher ESD rating than the FPGA pins themselves. However, would the FPGA pin burn anyway, because the D5 will cut too high?

Another factor is that the awfully high capacitance of D5 would surely limit the ESD spike.

— Petteri Aimonen on 14.11.2012 at 20:01 (UTC)

I replaced my diode with a TPD2E009DBZR - very low capacitance and fits perfectly.

The digital bandwidth seems exactly the same as without it.

— fungus on 03.06.2013 at 21:55 (UTC)

OK, i'm new to electronics and my DSO will arrive tomorrow.
I'm thinking about to do your "hack" but I'm not sure with two things:
After removing D5 you bridge the contacts on the board or leave them open?
And is it useful for someone who need no "scientific results" but better "Noop-protection"?

— SmenTrick on 31.03.2015 at 07:18 (UTC)

@SmenTrick You leave the pads open. It is useful if you want to use the digital channels for any signals > 1MHz.

— Petteri Aimonen on 31.03.2015 at 07:30 (UTC)

Hi. Thanks for your writeup.
Would it make sense to change all of the diodes with BAT54A's? Not only D5, but also D6 and D8?

— Kim on 02.06.2015 at 13:17 (UTC)

@Kim No, the other signals have much lower impedances (50 ohm and 22 ohm) so the diode's capacitance doesn't matter much there.

— Petteri Aimonen on 02.06.2015 at 13:21 (UTC)