How to connect the substrate?

[ripoo82]

How do you connect the substrate?

Every building block I use has a separate net for Vdd, Vss and Vsub. The Vsub net connects to the Vss pad using a star connection. Since I assume that the substrate doesn't run any current, this is a small connection.

Is this correct, and if not, what kind of resistance is allowed from a building block to the vss pad?

 

[csolis_GT]

The DC current that can flow in the substrate should be very small. However, a noticeable amount of current can flow through the substrate during transitions specially if you have any fast switching signals and having low resistance path can create large voltage drops. It is good to directly connect the vsub in a star connection to avoid debiasing, but allowing high resistance at some portions of the vsub routing may affect your performance.

 

[hobbyiclearner]

The DC current that can flow in the substrate should be very small. However, a noticeable amount of current can flow through the substrate during transitions specially if you have any fast switching signals and having low resistance path can create large voltage drops. It is good to directly connect the vsub in a star connection to avoid debiasing, but allowing high resistance at some portions of the vsub routing may affect your performance.

Was is meant by star routing here pls....As I understand, in a layout when the mos transistor substrate has to be connected, it is either done by connecting the substrate terminal to source or to gnd for nmos and vdd for pmos. In physical layout terms, the substrate is connected with substrate contact and metal line to the appropriate point (vdd rail, gnd rail etc).

 

[csolis_GT]

You are correct. In this case vsub which is the substrate connection is connected to the same pad that vss is connected (source and substrate connect to the same net). Problems may arise depending on where do you make that connection. Suppose that at the pad you have 0 volts. If your substrate is connected to ground but in the middle of the ic, there is going to be IR drops because the net vss carry dc & transient currents and there is finite resistance in the routing. This can lead to debiasing and excessive noise coupling. To avoid these ground loops you should connect the substrate in a point where you avoid seeing IR drops. In the case that you have only one pin for ground, the substrate should be connected at the pad directly instead of doing it in the middle of the ic or where it seams easier.