[SI-LIST] Re: input capacitance representation in IBIS


Matt
  C_comp i.e. input capacitance of models is an interesting topic.
Vendors often use very conservative max values in data sheets. I have 
also found numbers in data sheets often do not correlate to the same 
vendors IBIS models (as SUDHEER has discovered).

Excessive conservative input capacitance can be extremely painful for a 
system designer. For example if you are designing a high perfomance 
memory subsystem, driving a number of loads and simulating a variety of 
population schemes. You may find designing to worst case numbers very 
difficult. If the C_comp is really 4pf instead of 6pf and your driving 
10 loads you just needlessly added 20 pf of capacitance, while you where 
trading off between overshoot on a fast corner versus timing on a slow 
corner.

Interestingly after measuring input capacitance on some components 
(using a variety of techniques) I have seen considerably less variation 
and lower values than Max specified, even over a wide range of date codes.

My $.02
Bob

-- 
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15



ruston, matt wrote:
> Hi all:
> 
>  Isn't this why they make provisions top provide Min, Typ, and Max values? I
> know that vendors rarely use these fields though; they usually give one
> number, but you could fix the file to be more realistic.
> 
> Regards,
> 
> Matt
> 
> -----Original Message-----
> From: Ravinder Ajmani [mailto:ajmani@xxxxxxxxxx]
> Sent: Thursday, October 24, 2002 1:29 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: input capacitance representation in IBIS
> 
> 
> 
> 
> The capacitance specified in the datasheet is generally the worst-case
> (highest) input capacitance one may encounter in that family of ICs, and
> will always be higher than the C_pkg and C_comp specified in the IBIS
> models.  Same is true for practically all other parameters in the
> datasheet.   One of the most annoying parameters is the Rise Time.
> Datasheet specifies maximum rise time, whereas the typical rise time may be
> much lower, causing numerous headaches to EMI and SI engineers.
> 
> Regards, Ravinder
> PCB Development and Design Department
> IBM Corporation
> Email: ajmani@xxxxxxxxxx
> ***************************************************************************
> Always do right.  This will gratify some people and astonish the rest.
> .... Mark Twain
> 
> 
>  
> 
>                       "SUDHEER"
> 
>                       <sudheerbs@rediff        To:
> si-list@xxxxxxxxxxxxx                            
>                       mail.com>                cc:
> 
>                       Sent by:                 Subject:  [SI-LIST] Re: input
> capacitance representation   
>                       si-list-bounce@fr         in IBIS
> 
>                       eelists.org
> 
>  
> 
>  
> 
>                       10/24/2002 02:27
> 
>                       AM
> 
>                       Please respond to
> 
>                       sudheerbs
> 
>  
> 
>  
> 
> 
> 
> 
> 
> The 1pf is the sum of C_pkg and C_comp in the IBIS model   and the 3pf
> is the input capacitance  specified in the Datasheet .
> I have seen this kind of difference  for  many IC's eventhough the
> ibis  models have been provided by IC Vendor's Only .
> 
> 
> Thanks and Regards
> SUDHEER
> 
> sudheer@xxxxxxxxxxxx
> 
> --- In si-list@xxxx, "PRIEUR, Olivier" <Olivier.PRIEUR@xxxx> wrote:
> 
>>Is it just a difference between a typical value (~1pF) and a max
> 
> value (3pF)
> 
>>in the datasheet ?
>>
>>
>>OP
>>
>>
>>>-----Original Message-----
>>>From: SUDHEER [mailto:sudheerbs@xxxx]
>>>Sent: Tuesday, October 22, 2002 1:41 PM
>>>To: si-list@xxxx
>>>Subject: [SI-LIST] input capacitance representation in IBIS
>>>
>>>
>>>
>>>Dear SI Gurus
>>>
>>>I have a quection regarding the input capacitance  representation
>>>in ibis models .Is there any parameter other than C_pkg and C_comp
>>>in ibis model to represent the input capacitance ?
>>>If not  then the sum of input c_pkg and C_comp should be
>>>equal to the
>>>input capacitance given in the datasheet ?  I have seen this  kind
>>
> of
> 
>>>difference in
>>>many ibis models/data sheets .Can anyone assist me regarding this
>>
> .
> 
>>>For example take 74abt04(philips) ic,  here ~1pf in ibis to 3pf in
>>>datasheet
>>>
>>>I have to do EMI analysis of an High speed board , I have attached
>>
> the
> 
>>>availible
>>>IBIS models  but i dont have ibis models for analog IC's , i have
>>>pspice models  but my
>>>simulator will not accept pspice models. How to proceed now ?
>>
> These
> 
>>>devices are
>>>availible from many years (lm7805 ,lm7812), does anyone having
>>
> ibis
> 
>>>models for these
>>>devices .I am using Specctraquest SI expert
>>>
>>>
>>>Regards
>>>SUDHEER
>>>
>>>sudheer@xxxx
>>>
>>>
> 

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