Can MIMCAPs be used in Series?

[abkarthikeya]

Hi,

I need a small MIMCap (4fF) in UMC130nm process.The minimum allowed size in the PDK is 16fF.Can I put 4 minimum sized MIMCaps in series?

The reason why I am asking this is, I know MOS caps cannot be put in series without a DC bias for the mid-point.
In the case of MIMCaps in series, is it required to bias the mid-point?

Thanks,
Karthik

 

[edabrduser]

MIMCaps are just plain old regular caps. You can connect them anyway you want but be sure to account for the parasitics though. The other option is to draw MoM capacitors in layout and use PEX to extract the capacitance associated with it.

 

[erikl]

... caps cannot be put in series without a DC bias for the mid-point.
In the case of MIMCaps in series, is it required to bias the mid-point?

Just for simulation, possibly yes. Use arbitrarily large values, GΩ, TΩ .

 

[abkarthikeya]

Just for simulation, possibly yes. Use arbitrarily large values, GΩ, TΩ .

Does that mean it should not be used it for real designs (sent for manufacturing) ?
Could you please explain why?

 

[erikl]

Does that mean it should not be used it for real designs (sent for manufacturing) ?

Of course not. Not necessary for real designs.

Could you please explain why?

Simulator software is delicate with nodes which it can't assign a DC value to ("Jacobian singular" error message).
Some SPICE varieties attach low value conductances to those nodes automatically, others don't and utter the a.m. warning/error message.