I'm still struggling trying to make a simple bandg...
# analog-designp
Patrick Van Oosterwijck
10/12/2020, 9:14 PMI'm still struggling trying to make a simple bandgap. I just ran into an interesting find. My simulation results for sky130_fd_pr__pfet_01v8_lvt with l=1 w=5 m=2 are different from the same model with l=1 w=10. Shouldn't they be the same???
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Weston Braun
10/12/2020, 10:02 PMWhats that a plot of? drain voltage vs something? Or is this your bandgap output?
Weston Braun
10/12/2020, 10:04 PMI believe that the models are based on collected data from test structures, and might represent some variation in nominal and width as fabricated, where the true sizing of the w = 10 and w=5 m=2 fets do not match
Weston Braun
10/12/2020, 10:05 PMbased on the texts I have been reading, if you need different widths you chose one width and just change the number of fingers, that gives better matching between FETs
Weston Braun
10/12/2020, 10:08 PMalso, I think the 1V8 fets models are only valid for voltages up to 1.8V. I see the X axis goes up to 5V so you might be running into issues with those models.
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Patrick Van Oosterwijck
10/13/2020, 4:24 PM@Weston Braun Yeah this is the bandgap output. Not very good I know, but seeing things like this happen makes me think I can never get it anywhere near good.
To get good matching you need more than just fingers, the devices need to be split into a centroid layout and maybe with opposing current flow AFAIK. But that may be old info, I don't know about newer processes.
Anyway, I'm nowhere near the point where I worry about layout, I just want a fricking simulation to work well and I can't even get to that point so...
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Weston Braun
10/13/2020, 9:50 PMit should be possible. My best guess is its just due to the difference between nominal and actual transistor sizes between the two bins impacting matching
Weston Braun
10/13/2020, 9:51 PMdo you have your work online anywhere? I am interested in what others are doing (and I need a bandgap reference....)