[SI-LIST] Re: pkg model usage
- From: "Brad Brim" <bradb@xxxxxxxxxxx>
- To: <tonywai34@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 12 Apr 2012 21:11:15 -0700
hi Tony,
Hook-up your real power delivery network to the pwr/gnd ports at both the
pins and the pads regardless of whether you are doing AC or transient
simulations.
Your goal is NOT go get the AC response to match the S-parameters from HFSS.
Your goal is to model your real system - and your real system does not have
a frequency-independent 50 ohm impedance for the source driving your power
delivery network from the board-side nor a 50 ohm load at the die-side.
Please recall that S-parameters (along with their corresponding reference
impedances) can be converted to Y- or Z-parameters, which are independent of
the reference impedance. The information content of any of these three forms
is exactly the same and is applicable only to the geometry simulated but
includes nothing outside of it.
You real PDN circuit will be much different than a 50 ohm load. For example,
an ideal voltage source will have a zero ohm source impedance. More
realistically, you will have a VRM on your board and it (along with its bulk
caps) will drive the impedance to a low value on the order of 1 to 10 mOhms.
The board will have decaps mounted on it and over the mid frequency range
(maybe <1MHz to >100MHz) these will keep the impedance low but not
necessarily as flat w.r.t. frequency as the VRM and bulk caps did for low
frequencies. Then at higher frequencies your package will see inductive
behavior looking into the board. On the die-side of your package you will
have to first order an RC load for the chip. This shunt capacitance will
drop the PDN impedance at high frequency and may even resonate with the
package inductance.
You would be "better" served to short the PDN rather than terminate it with
50 ohms. You would be "best" served to hook-up something a little more
realistic on both sides of your package, as I described above.
cheers,
-Brad Brim
Sigrity
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Zhang Tony
> Sent: Thursday, April 12, 2012 8:23 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] pkg model usage
>
> Hi
> I got a package model from HFSS, which is s8p file, including
>
>
> two differential signal run up to 10Gbps one power pin and
> one ground pin close to the differential signal.
>
>
> For AC simulation I need to add 50 ohm termination in both
> sides of power/ground pin/pad to get same AC response match
> with S-parameter.
>
>
> My question is if I want to run trancient simulation with
> this S-parameter, whether I can use the power/ground ports in
> the package model to provide power supply from pin to pad?
> My feeling is this will make transcient simulation
> un-accuraccy due to no termination.
>
> Thanks
> Tony
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