Re: Capacitor Dummy
What I was trying to say is that the requirements for dummy capacitors are determined by the accuracy needed on the primary capacitors.
I would typically connect both plates of the dummy capacitor to ground, but this will ultimately be determined by what is happening on the other capacitors in the array.
The purpose for the dummy capacitors is to create "equal" surroundings for your primary capacitors. It is simply a question of how equal things need to be to obtain the accuracy that you desire.
I was trying to point out that I would not use dummy devices if I need to match capacitors to 1% of the total capacitance (7 bits or so...). 8 or 9 bits could be done without dummys, but will be easier with dummys. 10 bits or more will most likely need dummys. By 13 or 14 bits, additional dummys, and special care must be taken in layout to keep consistent surroundings for each capacitor. In order to obtain additional accuracy, additional steps must be taken.
I would use the same structure for the dummy capacitors that I am using for the array capacitors. If I only need a little help (8 or 9 bits), I would use smaller capacitors for dummys. If I need to make 13 or 14 bits, I would likely surround each unit device with a shield. This will increase the unit capacitance and will produce equal surrounding for each unit capacitor. I would probably also have a row of dummys outside of the array.
Dummys alone do not provide all the accuracy. You will still need common centroid layout, for example, or all of the effort to add dummys will be lost by gradients.
You, as a designer, must decide whether dummys will help or hurt. Use the matching per total area as a measure of how good various approaches are. There should be matching data for your fabrication. If you are lucky, there will be some data that will demonstrate the additional effects, such as edge of array effects or gradients.
In one process, the gradients and edge effects might be small, but matching not so good. In this case, you want to use your area to make large capacitors, and skip the dummys.
In another process, the gradients and edge effects might be larger, but with good matching on adjacent capacitors. In this case, you would want small capacitors to keep them close together, with dummy capacitors and shielding rings around the array.