For some reason I was trying to use the register "res_iso_pw" from sky130_fd_pr. This is a 3 terminal resistance. Where should the third terminal be connected to?

Usually GND because it's the substrate so that's what it should be biased to. If you have a pwell inside a deep nwell and you want your resistor there, then it could be different.

yes, if the resistor is in a p doping, gnd it. see the resistor gds once. if its covered in p

This is the resistor. sky130_fd_pr_res_iso_pw. Where should the 3rd terminal (substrate) be connected to?

gnd

@Lab Lecture Actually for the p-well resistor (which as mentioned in another thread, is probably not what you should be using), the bulk terminal is the deep-nwell connection which should probably not be ground. Here’s the extraction rule from magic showing

dnwell

as the third terminal.

device rsubcircuit sky130_fd_pr__res_iso_pw   rpw \
        pwell dnwell error l=l w=w

The reason you do not want the bulk connection to be grounded is because the pwell-dnwell connection is inherently a diode. If the dnwell level is less than the pwell level, you could have a forward biased connection causing unintentional leak and therefore an unexpected resistance. The dnwell level should be higher than the highest voltage on either p-iso resistor terminal.

Thanks for the correction @Mitch Bailey ,I misunderstood Pwell as bulk. Logic is correct , we just need to make sure body is reverse bias so current does not go through body