I was trying to make a R2R ladder circuit. I am using the resistance "res_iso_pw" from sky130_fd_pr lib. The resistances are really big. Am I doing something wrong? Is there other type of resistances which I should use?

You can increase the resistor width or use multiple instances in parallel.

I could find one poly res. something_0p35. Does it mean a 3.5 volt device?

There are many resistor devices. "0p35" refers to the width of the device, because the device characterization includes the terminals, and the terminals are very dependent on placement of contacts, and there are five good characterizations measured for devices with one to five contacts on the terminals. For any other widths, the models are interpolated and are probably okay but not guaranteed accurate. It's best to avoid well and diffusion resistors unless you really know what you're doing, since biasing them the wrong way results in leakage (in addition to having much greater variation over PVT, as Luis noted). There are three types of poly resistor in sky130:

sky130_fd_pr__res_generic_po

has a resistance of 48.2 ohms per square. This is just poly that has been marked as a resistor for extraction purposes. (corrected)

sky130_fd_pr__res_high_po_*

have a resistance of 319.8 ohms per square and come in five different widths. These have the lowest mismatch and are the preferred ones to use unless you are generating resistances in the megaohm range, in which case the "xhigh" resistors are likely the better choice due to area considerations.

sky130_fd_pr__res_xhigh_po_*

have a resistance of 2000 ohms per square and come in five different widths.

@Tim Edwards Should the first poly resistor type be

sky130_fd_pr__res_generic_po

?

Yes. Missed a line when doing copy and paste and didn't even notice. Corrected above.

Hi @Mitch Bailey @Tim Edwards what type of capacitors shall I use from sky130_fd_pr? I need one for the miller capacitance in opamp. Currently I'm using cap_mim_m3_1

@Lab Lecture Sorry, I’m not qualified to answer that question.

@Lab Lecture: The MiM cap is fine. MiM caps have the benefit of being able to be placed on top of all the other circuitry, so they can can be used without taking up much layout space, if you're careful where the power wiring goes.

Also note that two resistors in series do not equal a resistor of double length, since there are more metal-poly contacts

Yes @Tim Edwards I have observed, It can be placed on any other device and no DRC. How to connect the two terminals? What type of via should be used to connect to m1.

In the top, there is the actual capacitor between M3 and M4. M4 is the top plate. You should route in M4 to the right side use vias down to M1. At the bottom of your screen, there is a via from M3, which is connected to the bottom of the cap, to M4. You should route it from M4 to the right and then route it down to M1 using vias without touching the bottom plate of the cap in M3. You have M1 directly under the M3 to M4 via. If you route from M3 to M1 under it, the M4 pin is useless. Anyway, I don't think there are any rules in SKY130 specifying you can't just route the bottom plate from M3 to M1 directly. It is just a good layout practice.

Hi @Luis Henrique Rodovalho I'm able to connect the mim capacitor. Thanks for the help. One last doubt, does the polarity of a mim capacitor matter? For discrete components (~pf) the polarity is not there.

It matters for matching with other caps. If you're designing a switched capacitor circuit, you should connect everything the same way. There are small errors, as there is a parasitic cap from the bottom plate to the substrate, for example. In an opamp, you should connect the terminal of the bottom plate to the inner node, not the output, but is is a very small difference.

@Lab Lecture it matters for LVS too. The terminals are not permutable by default.

Yes, @Mitch Bailey. I have just seen it now. If I flip the capacitor the LVS passes.

The main reason for the polarity is that that parasitic cap to substrate (and to other layers underneath) differs greatly between the top plate and the bottom plate of the MiM cap. If you are making something like a switched-capacitor DAC, it matters a lot which side is which.