I recently had a PCB designed which has several analog, digital, and power elements. The board is powered by 24Vdc which is stepped down using an LDO to 3.3V and 5V. These voltages are used to power the many IC and MCU on the board. The issue I am having is when I touch the ground layer on the board to measure voltages with my multimeter a very intense voltage oscillation occurs. This first picture shows the voltage spike on a 1 volt signal on the board.
Some other pieces of information:
I first noticed this issue while debugging the MCU code. I was verifying the DAC output and noticed when a single probe on my meter touched the board the debugger would lose connection. Thus I used the oscilloscope to see this disturbance.
This phenomenon is very random when it occurs. It is not instantly repeatable and when it occurs the amplitude of the disturbance varies.
We've used 2 different 24Vdc supplies and both create the same issue.
I've also recreated the issue using a stray wire as opposed to a multimeter probe.
My current thoughts:
I haven't provided the PCB layout because I don't need someone to solve this problem for me (although I will provide pieces of it if need be). What I would like to know is what could cause an issue like this? To me it seems that because it only takes a single open wire to create this issue, the stray wire is acting as an antenna or some sort of conductor for a massive discharge on the board. The spike always starts with a voltage dip and it seems like it takes time to "recharge". Let me know what you guys think about this. I have seen grounding issues on a PCB before that cause signal integrity issues but never anything that cause the high voltage spikes.