Could deviations from nRF52832 reference design cause chip to die during DTM testing?

[wes_nCase]

We recently ported to the nRF52832 and our electrical engineer had to make a couple changes from the reference design due to space constraints. The port has worked well during internal testing but unfortunately a unit running the standard DTM firmware died on the company that is testing it for CE Marking. The testing company is using nRFgo Studio with our unit being powered by UART and a lipo battery. They said it worked correctly for one day of testing and then the next day it wouldn't power up.

We have since run a lot of tests internally on multiple units with the DTM firmware controlled by nRFgo Studio and unfortunately have not been able to recreate the issue. The testing company thinks the issue could be caused by our deviation from the reference design, specifically the changes on the capacitors on VDD. 

I've included our schematic and layout below. Could our minor deviations from the reference design cause the nRF to die during DTM testing? What (if any) issues would you expect these changes from the reference design to cause? Please let me know if any additional information will help. Thanks for any input!

[Jay Tyzzer]

Hello Wes,

Can you clarify this please?  " Unit being powered by UART and a lipo battery."  Do you have a external LDO or DC/DC supplying the 1.8vdc?

[wes_nCase]

Hi Jay,

Our board is usually powered by a 3V CR2450 coin cell battery through a 1.8v LDO to the VDD pins. We also have a port for testing that directly exposes pins that the UART plugs into and puts 3.3V directly on the VDD pins. When they were running the DTM testing they had the battery connected and the UART plugged in so VDD was receiving 1.8V from the battery/LDO and 3.3V from the UART. Does that make sense? Thank you for any help.

[wes_nCase]

Jay, thanks so much for reviewing our circuit. That makes sense about the UART at 3.3V and it sounds like that is probably what broke the part. We originally had C12 populated and were having signal strength issues. When we removed C12 the signal strength improved quite a bit and our engineer considered it a basic form of network matching. Would the items you pointed out only cause signal strength/efficiency issues or could it cause issues with chip functionality and/or other issues?

[wes_nCase]

Thanks so much for the help Ketiljo. If our design was able to pass CE testing and we are getting acceptable signal strength do you think we still need to make changes? Would the items you pointed out only cause signal strength/efficiency issues or could it cause issues with chip functionality and/or other issues?

[ketiljo]

C12 is part of the matching network and does some harmonic filtering. It might be that the layout you have there does some of the filtering by design and luck. So if you're happy with the range and are able to get it certified, by all means use it as it is. 

[wes_nCase]

That is what I was hoping. Gene Rittenhouse recommended that I ask you about sending our hardware to you for RF analysis. Is that something we can do and if so how do we proceed? Thanks again for the help!

[ketiljo]

As the design is now, we can't do any conducted measurements. Simply because there are no place (ground plane) to solder the coaxial cable to.