How to construct dummy caps around a set of caps?

This is going to be a novice layout question, but I'm not sure how to construct dummy caps around a set of caps that I've drawn for a circuit. I've attached a JPEG screenshot of the caps in my layout in this post. These are poly-poly caps. Specifically, I'm interested in those caps that share the same poly1 backplate.

Color key:
red = poly1
dark red = poly2
blue = metal1
grey = metal2

Please look at it and give me tips/advice/instructions.

Thank you!

Re: Dummy capacitors

I dont know what cap performance you want. Basiclly, dummy is only a match methods for cap ratio, where you need the related cap have a ratio, dummy caps are around the ratio cap.

Please also be noted that if you need dummy cap, you must use the same cap.

Re: Dummy capacitors

smilodon said:

I dont know what cap performance you want. Basiclly, dummy is only a match methods for cap ratio, where you need the related cap have a ratio, dummy caps are around the ratio cap.

Please also be noted that if you need dummy cap, you must use the same cap.

Click to expand...

I know that already. My question is HOW should I arrange dummy caps given the configuration of caps I have drawn in my attachment? I don't understand what you mean by cap performance when my primary concern is for cap matching.

Maybe I should be more specific: Several of my caps all share the same bottom plate as they are drawn in the layout now. If I want draw dummy caps, around these caps all at once, can I just use a cap of some arbitrary size (probably the same size as my smallest cap) and arrange them around all the caps at once?

Re: Dummy capacitors

If you concern is matching, the layout you showed here doesn't even satisfy very basic matching rule. What I will do here is to use a smallestu cap(All other caps are some integer factors of this cap); form a square array(or something close to square); put smaller caps in center and larger caps outside; finally surround the array with exactly same unit cap. For example, if you are using 4x4 array, the actual array you should draw is 6x6.

The space between every two caps should be same. You can either use one big, single bottom plate for all caps(including dummy), or small, saparate plates for each of them.

I am not sure if I answered your questions...or you already know this.

Re: Dummy capacitors

you can use dummy poly & ground them later....from screenshot the layout not match at all ..try use unit cap array them...

Re: Dummy capacitors

bageduke said:

If you concern is matching, the layout you showed here doesn't even satisfy very basic matching rule. What I will do here is to use a smallestu cap(All other caps are some integer factors of this cap); form a square array(or something close to square); put smaller caps in center and larger caps outside; finally surround the array with exactly same unit cap. For example, if you are using 4x4 array, the actual array you should draw is 6x6.

The space between every two caps should be same. You can either use one big, single bottom plate for all caps(including dummy), or small, saparate plates for each of them.

I am not sure if I answered your questions...or you already know this.

Click to expand...

From the comments I've been getting, I gather that people are confused about why my caps as shown aren't matched. So I'll explain the background to the layout.

The capacitors belong to a RC oscillator circuit with bit trimming, and the idea is to use trimming to compensate for any variations in oscillation frequency. The capacitors I showed are not the same size because they are not supposed to. They are binary sized capacitors that are ratioed to give the amount of bit trimming I need. Each capacitor is a power of two multiplied by the size of the smallest capacitor.

The fact that the capacitors as shown are not matched is not the issue. What I really want is some way of minimizing size variations of these capacitors after processing. In other words, I want to make SURE that these capacitors as maintain the same ratio with each other after processing.

What I gather from what you said is this: There is no way for me to draw the dummy caps for matching given the configuration I showed. It seems like the only way I can guarentee matching is if I draw each of the different sized capacitors seperately, and surround each with dummy caps (hence each cap is really an array of 3x3 caps).

Am I right?

Dummy capacitors

I think the above comments suggest u should use unit cap to construct a bigger cap u really want... i.e.
for a cap 4X of unit cap,u shouldnt directly draw one whole 4X cap,but 4 seperated unit caps in 2X2 array pattern then suround them with grounded unit caps as dummies.I think this method is good for cap ratio matching u want in ur application.
but maybe the small and odd volume cap like 2,3,5,7,the array pattern are more difficult to draw...

ps: u want 1:2:4:8 cap ratio in you application,but u didnt consider the edge fringe cap...so even the caps u have layout are perfectly matched on real silicon,their capicitance ratio still wont be 1:2:4:8,the unit cap method above would be better in this case for the cap ratio consistancy