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"Ignore Instances" not completely ignoring capacitor instance effects

MahdiMohammadi

I'm simulating a CS-DAC, and was experimenting with the effect of an extra capacitor on the tail node. 

To do that, I placed an ideal cap from analogLib on the node, and then measured in three cases:

  • No capacitor installed,

  • 100fF capacitor installed,

  • 100fF capacitor installed but ignored

Here are the results:

As you can see, there's almost no difference between ignoring or not ignoring the cap! I assumed the ignore instance option would cause it to act like the case without any capacitor.

I would appreciate your suggestions.

Best regards,
Mahdi

Parents

  • Mahdi,

    That seems rather odd. Ignoring the capacitor this way should cause it to be omitted from the netlist - can you check if there is a capacitor in the netlist sent to the simulator?  Which simulator are you using?

    Regards,

    Andrew

  • in reply to Andrew Beckett

    Dear Andrew, 

    I just played with the cap and realized that even placing a 10fF mimcap has the same effect. So, I think the placement of this cap somehow changes the simulator's behaviour.

    And I confirm the cap is omitted from the netlist when ignored.

    I tried both Spectre and APS (with ++aps enabled).

    Best regards,
    Mahdi

  • in reply to MahdiMohammadi

    I realized what was going on.

    I was using bus instantiation (I don't know if that's the correct term) for the tail and input transistors. So without the capacitor, there are 15 different tail nodes. However, when I place the capacitor, regardless of whether it is ignored, since the cap is instantiated only once, all the tail nodes are shorted together, causing the circuit to behave completely differently.
    I'm sorry about the mistake.

Reply
  • in reply to MahdiMohammadi

    I realized what was going on.

    I was using bus instantiation (I don't know if that's the correct term) for the tail and input transistors. So without the capacitor, there are 15 different tail nodes. However, when I place the capacitor, regardless of whether it is ignored, since the cap is instantiated only once, all the tail nodes are shorted together, causing the circuit to behave completely differently.
    I'm sorry about the mistake.

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