Does anyone here know, if someone has already built a negative rail supply in sky130? (prefferably -1.8V )

@Tim Edwards Tim, Have you seen a negative supply implemented in any submitted designs?

Not that I'm aware of. I left the caravan (analog) chip with no ESD on the pads for the purpose of getting a negative supply on chip. I have fielded other questions about the usage, but I haven't had any follow-up on those questions to know if anything was implmented and/or submitted.

I'm implementing a negative supply for GF180, but it is low power. I'm having issues with isolated NMOS. I still need to implement a LDO, so the negative voltage supply is regulated.

Luis, I do not know if the current user ground it is isolated from the P substrate of the silicon. Tim is checking. (That would be required and there would need to be specific latchup protection to avoid issues between user gnd and caravel gnd if we ran an isolated 1.8V user virtual ground domain.)

All our circuit boards and also the WLCSP chip short the ground domains together so I can't check this with a multimeter---I'd have to work it out from the layout.

The virtual ground wouldn't be connected to the P-substrate. It would be used only for analog circuits, just like this here. https://tangentsoft.com/elec/vgrounds.html

For my problem I acutally found another solution without the need for the negative rail. But still thanks to you guys!