Hi all!

I am currently designing a buck DCDC converter (I have already received the first silicon). During design, power and command ground have been separated in order to minimize noise on command side circuit. The DCDC oscillate in some cases at about a 1/8 of switching frequency. I have tried to reproduce this behaviour in simulation and I now think oscillation are due to the bandgap ground coupling with power ground (a few mV of noise on bandgap ref generate oscillations).

I have tried to measure substrate coupling of the two grounds but I am not sure if the method is valid. As there silicon already exists, I have simply used a ohm-meter to mesure the resistance between the two ground. The ohm-meter injects 1mA and mesure the voltage. I have found about 16 Ohm of coupling. This mean there is 16mV between the two ground while measuring, it seems like the same conditions as when circuit is running (junction bias).

Do you think the method is valid?

Matthieu