Subject: Some questions

From: psd <[email-masked]>
Date: 27 Jul 97 17:32 GMT

Dear Sir.
     
     I have read some documents on the IBIS from the internet and I have 
     some questions to ask:
     
     1- In the simulator you have two curves one for the Pull-up, Pull-down 
     and the other for the Clamp diodes, these two curves are on the same 
     pin I mean you can not get each of them individually so how can you 
     check these curves with the actual Lab measurements. ?
     
     2- When You made these measurements in the LAB ,if you have the IC pin 
     always high when you apply the VCC to that pin, how can you vary the 
     voltage to measure the current variations?
     
     3- Are you sure that the voltage change between -Vcc to 2Vcc will not 
     harm the IC pin??
     
     4- These IC characteristics (IV ,Ramp...) is measured on the pins when 
     all the IC pins are free so are these C/S will not change when the IC 
     is connected inside the circuit or some pins within the IC itself is 
     connected to another pins to the same IC ??

     Thanks for your cooperation.
     Hany Gad.

---

From: Jon Powell <[email-masked]>
Date: Sun, 27 Jul 1997 21:49:04 -0700

>      check these curves with the actual Lab measurements. ?
There are some inherent difficulties trying to make IBIS models from
physical measurement. If you can't probe inside the chip to control or
cut out things (which is VERY expensive) then you have to make some
assumptions about how transistors work and extract your data from that.
>      voltage to measure the current variations?
You are varying the voltage at the input. A programmable voltage source,
CURRENT LIMITED, works good.
>      3- Are you sure that the voltage change between -Vcc to 2Vcc will not
It will melt it to slag. I recommend you stop your sweep very quickly
when the overshoot diodes cut on and LIMIT YOUR CURRENT to around 300ma.
>      connected to another pins to the same IC ??
Sorry, I don't follow this. If your are asking, "can the package
parasitics mask the AC behavior of the part during transition
measurement?" the answer is "yes".
This is why I usually recommend that people make models either from high
quality SPICE like simulations (which presumably come from good
measurements made at the factory) or from raw data generated by the chip
manufacturer.

Special Note:
There are people out there making good models from measurement (I may
even be one of them) but it is very expensive and takes a lot of set up
time, measurement know-how, and spare parts (for when you slag them).

>      Thanks for your cooperation.

Your Welcome,
Jon Powell
Senior Scientist, Viewlogic Systems Consulting services 
(formally of the Quad Design group)