Guard ring for PMOS with its well shorted to source:

[zopeon]

Hi,
I am new to analog layout. I have a PMOS diffamp with its Body shorted to source. So how should I draw the guard ring around it. Is the way I have drawn correct? Can I remove the 2nd Well ring? Is there any other method? Also please explain difference ( if any ) between n/p tap and n/p guard ring (well guard ring) ?
What should be the spacing between the contacts (metal) on the rings?

Thanks.

zopeon

 

[Saravanan.G]

a guardring is a complete ring structure of n/p tap place around a circuit whis is to be protected agains noise.
where as a simple n/p tap is a piece of ohmic contact to a psub or nwell

I your case 2nd nwell guard ring is needed if ur analog block is critical/sensitive one and u have digital circuit near to u r analog block,
for non critical blocks u may ignore the 2nd nwell guard ring.having 2nd nwell gaurd rind is better in terms of noise isolation but occupies area.

 

[zopeon]

hi Saravanan,
thanks , Well its a cell which is a part of an GmC filter. The chip has only analog circuits and if any mixed types will be far away from this. I am making the layout for OTA which I will integrate in the filter. So can I ignore the second nwell? as I can have a simillar guard around the filter. its a low frequency circuit 250 Hz max. N area is an issue also my circuit has maximum current of 300nA with power of around 5uW so is there any issue in using 90 degree metal runs for routing of the circuit and 45 deg for I/o and Power riuting of the entire chip.( diff analog blocks included).

Thanks