Tlaloc wrote on Nov 12th, 2008, 9:07am:In just about every SAR, Delta-Sigma converter, and other switched cap circuits, the summing junction (SJ) plate of the capacitors are always shielded from the top of the chip. With poly-poly caps, this was easy since you could use whatever metal layer you wanted. With MIM caps, it now looks like the bottom plate of the cap, i.e. the plate that sees the parasitic to substrate, is used for the sensitive SJ nodes since the top plate is shorted to top metal above which is effectively a shield. I'm assuming that the MIM cap must be made on the second highest level of metal with the capacitor top metal (CTM) sitting above that with vias up to top metal.
First off, why shielding a necessity? Is it to protect from piezo-electric effects? Is it to absorb EMF before it disrupts the SJ nodes? Is there some other reason?
With many switched cap designs using poly caps, I have seen them split the cap into two and arrange them back-to-back. This has a number of positive effects, not the least of which is eliminating the linear term contribution of the voltage coefficient of those caps. What can be done when using MIM caps that cannot be placed on arbitrary metal layers, i.e. fixed to the second highest metal line? Can we no longer place these caps back-to-back? If not, are there any other ways to compensate for the linear voltco?
Hi,
I don't know if this is also true to delta-sigma converters, but I would shield the SJ only to ensure that only the intended caps are coupling to it, and there are no strays from other interconnects which run around that add to the SJ. If these interconnects are carrying signal, then they change the gain from the signal or reference in such a manner as to introduce nonlinear distortion for a Nyquist ADC. For a delta-sigma, I am not sure. If you are coupling to a noisy line, then that is surely a disadvantage and you want to shield out anything but the caps you really want.
However, the price you pay (you always pay one) is that the additional parasitics from the shielding will slow down the overall system, and worsen the overall performance noticeably.
By the way, keep in mind that with MIM, you have a very very small parasitic cap on the bottom plate. Depending on your layout, you may be able to put the shield in the well below. Again, that may not be helpful depending on your layout.
Regards
Vivek