doubts in shielding and matching

[vinodhsaminathan]

Hi,

1. What happens when we do matching? Why we are doing inter leave matching for current mirror, resistors and common centroid matching for differential pair, capacitors?what happens when do reverse? how matching prevents device from process gradients?

2. I know 3 types of shielding, is there anything else? what is happening when we do shielding to signals?

Pls explain.

 

[shar2013]

1. Matching is used to overcome random variation, a shift in mask position could cause a difference it threshold voltage of a MOS, which in a current mirror could cause an error in the out current and thus reduce yield. When the devices are interleaved (wisely) a mask shift will affect both branches and will reduce the error.
This holds also for differential stages where interleaving the gm stage will reduce random offset.

2. Shielding is done to decouple noise (usually due to digital signals) from sensitive paths. We provide the digital signal with good return path (ground) . For this reason it is important to keep the resistance of the ground path minimal, or the shielding will be ineffective.

Good Luck !

 

[kenambo]

Hi,

In interdigitised or interleave matching,... The perfect DC matching is obtained, So its good to use interdigitisation for current mirors.. But if you have more devices in current mirror circuits, then you must go for common centroid matching..

Shielding is a process used to decrease noises due to, electrical coupling and magnetic coupling ...

 

[vinodhsaminathan]

hi,
I understood why we are doin matching, but i cant understand how the process gradients getting canceled on x- direction on inter leaving technique and x and y direction on
common centroid technique? we are just placing transistors in different manner, but hw this cancellation is happening?
pls explain

 

[kenambo]

Hi,

take, interleaving technic.... in this technic you arrange moses in x direction with dispersed manner...

Ex... two transistors A - 6 , B-4 then matching is.... ABABA ABABA ok.. then for example process gradients vary in X-direction.... i.e K1,K2,K3,......K10
then A has equal gradients in x direction means the average value... similarly B...

take a look on this pdf..
thanks...

 

[vinodhsaminathan]

hi,

While doing coaxial type shielding, why we are using one higher metal and one lower metal? why cant we use same higher metal or lower metal?