Where do you usually place dummy transistors?

[ccw27]

Dummy Transistor

Where do you usually place dummy transistors? Should dummy transistors be placed next to switches used for control (i.e. they go either low or high)?

Thanks

 

[ambreesh]

Re: Dummy Transistor

Dummy transistor fine application in switch cap circuit as you have mentioned and also in layout. In layout they would help in problems faced in process .

 

[itheman]

Re: Dummy Transistor

Dummy transistors are usually placed in the layout for better matching.,etc.

If you are talking about the dummy transistors in the switched cap circuits(or for switches) they have a different purpose. They are used to reduce charge injection. They are usually used in series with the switch, and the gates of he dummy devices are controlled by the inverted control that controls the main switch. The drain, bulk and the source are all shorted together.,and is in series with the main switch.

 

[ccw27]

Re: Dummy Transistor

For Dummy transistor I short the gate, source and bulk together. Should I connect it to ground or substrate for NMOS? Also I guess for regular switches not to be used in switch cap circuits there is no need fo dummy, right?

Thanks

 

[ambreesh]

Re: Dummy Transistor

Dear ccw27,
We are talking digital CMOS process and not twin tub. As for NMOS in digital cmos process (N-well) substrate is shorted to ground.
FOr NMOS u could do 3 things,
1. Tie all together and tie to VDD
2. Tie all together and tie to GND or Substrate
3. Tie D and S to Gnd and G to VDD get a CAP

I would prefer using 3 as it would aid my decoupling caps.
Hope it helps

 

[nomad]

Dummy Transistor

dummy transistors are used for switch cap circuits

 

[ckt.designer]

Re: Dummy Transistor

Dummy transistors are placed in the layout for better matching. They are required if you are using interdigited or common centroid stuctures in your design.
The layout software i work with is L-Edit from Tanner EDA and it has a default p-substrate (since i use single n-well CMOS process). If your dummy transistor is NMOS, then the body of it is automatically connected to ground.

 

[ccw27]

Re: Dummy Transistor

In CMOS bandgap is it advisible to surround diodes with dummy transistors? If yes, does it have to be the same size? Wouldn't that increase the size a lot?

Thanks