Thank you alvadep. I will try to get the recommended diode.
Frank
It would be not bad to try TVS diodes array in SOT-143 (SEMTECH SR3.3 or similar). The capacitance is a little more, but more reliable protection. In my opinion even it is not bad possible to establish with the minimum alterations in board (PCB).
I was thinking of doing this replacement but I can’t find a source for the RCLAMP0502 in UK.
What about TPD2EUSB30, which is designed for USB3 protection?
ti.com/lit/ds/symlink/tpd2eusb30.pdf
Package is a bit smaller which would be my main concern.
I’ve just spent a bit of time looking for something suitable and my conclusion is:
a) 3.3V
b) SOT23 (or anything similar shape)
c) Low capacitance
Pick 2.
The only part that’s anywhere near is the RClamp0502 but that’s 5V, it’s the wrong size and I have no idea where to get one.
So…I was thinking: How about a hybrid solution?
Put a PESD3V3U1UT where the existing diode is, it gives 3.3V clamping on both channels plus ESD protection on one of them. Then add a 2-pin ESD protection diode across the input connector on the other channel (where the connector is soldered to the board). It doesn’t matter really what the voltage is for this diode so it’s no problem finding something with single picofarad capacitance (or less).
Any thoughts?
Using two devices sounds a bit messy but if you try it let us know how it goes.
My thoughts on the USB3 device is it is very low capacitance and actually has a superior protection strategy in using ordinary diodes combined with a clamp diode so that the higher capacitance of clamp diodes is effectively shielded by the low capacitance ordinary diodes.
It is in a smaller pack which may make it a bit fiddly to fit. I’m not overly concerned about any differences in protection voltage as the series limiting resistor will lower fault current levels that could possibly get to the actual device before the main protection kicks in.
I’ve ordered one to give it a go.
The problem is that the protection diode is on the same side as the series resistor.
Consider for example if there is 10V input voltage. Then the FPGA diodes will start clamping at 3V (koti.kapsi.fi/jpa/stuff/pix/dso_ … urrent.png). Because the voltage is clamped at 3V, absolutely no current will flow to the 5V protection diode. Such a diode is completely useless, it won’t do anything until the FPGA input is already broken.
I’m not sure if 3.3V protection diodes would be effective either. The FPGA input diode forward voltage will probably rise a bit when the current rises, so it might still help something.
Optimal solution might be replacing the R41 and R42 with a series connection of two resistors, with the clamp diode in between them. But this is probably quite difficult to do.
Yes. You’re right that the input resistor configuration doesn’t allow for best protection here.
I thought though that the existing external diodes were also higher stand-off than internal and so it would not be making anything worse.
I’m no expert but I think that main risk is during ESD transients where the situation is quite complex with the transient conditions being significantly different from what a simple stable low voltage analysis indicate. So for sustained low voltage overload the internal will be taking the current but the series resistance will be fairly effective in limiting energy. Obviously putting on medium voltages for sustained periods is not going to do the series resistor much good depending on its rating but that is a low risk when using these inputs for what they are intended. For high voltage pulse the external diodes may help redirect the energy depending on the transient behaviour of the internals and provide better protection
If the external diodes have high capacitance, they ruin the digital input bandwidth. Because the digital inputs otherwise go directly to FPGA, they should be good for up to 72MSps without the external diode.
The TPD2EUSB30 pack I have ordered is designed for protecting 6GB/s USB 3 connections and has 0.7pf capacitance to ground. It should have negligible impact on achieved bandwidth.
I completely removed my diode a week or so ago and the effect on the digital inputs was like night vs. day.
There’s no way I’d go back to the way it was before, ESD protection or not.
The only problem is finding a replacement…
(and being able to solder it in - I only have a very basic setup).
Problem is … it’s only 1mm long.
Yes. It will be fiddly / difficult and need to be done under a microscope.
I am also interested in other methods before I have a go. You had mentioned using the PESD3V3U1UT in conjunction with a secondary ESD diode but I am not sure how that works. Doesn’t the low capacitance of that device rely on the internal diode so that if it is connected as in the original circuit then the channel connected to the ESD diode will still have a high capacitance.
OK. What about this one? TPD2E009DBZR
uk.farnell.com/texas-instruments … /1755715RL
This has pretty much the same characteristics of the USB3 one I mentioned before but the DBZ is in a SOT-23 package.
Looking at the photo in previous posts carefully it looks to me like the board was actually designed out for a SOT-23 package and the SOT-523 device has been stretched by the soldering to fit.
Anyway I’ve now ordered the SOT-23 device as it will be much easier to handle than the smaller usb30 one.
We’re waiting with baited breath. Let us know how you get on…!
No problem. I ordered it from eBay for UKP 2.50 and should get it in the next couple of days. Hope to fit it next weekend.
OK. So I have fitted this part (TPD2E009DBZR) now replacing the previous one. It is a perfect fit and was fairly easy to do, but obviously one needs a very fine tip soldering iron, good magnification and a steady hand. I actually found the most time consuming bit was getting the unit back into the case juggling all the buttons.
Initial tests show high frequency logic now goes through but I haven’t explored the limits yet.
I’ve just done mine as well with the same part. I used one of those stands with magnifying glass and crocodile clips and a fairly average soldering iron, it wasn’t too difficult.
I’ve been using mine for a week or so with no diode. Adding this diode didn’t seem to be any difference, which is good! An 8mHz square wave generated by my Arduino displays perfectly. I haven’t got a higher frequency signal to look at, that’s basically the bandwidth I needed though…
Before the mod I had trouble seeing a 1mHz signal and a 2mHz signal showed a flat line (I thought it was a bug in my software at first, or a problem with the Arduino outputs!)
I removed the D5. Negative side protection may be a bit worse, but otherwise looks good: