Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast

raymondl@qsa.qualitysemi.com
Thu, 15 Jul 1999 13:39:17 +1000

I have been a little bit curies about using a series resistor to control the
edge
rate. In my past projects, I usually used the resistor to control the
equivalent
output resistance, and left the edge rate controlled by some other internal
circuits in the IC. Especially in point-to-point connection, well controlled
output
resistance at the driver already means termination. I have tried to reach to
the web site of Analog Devices but was unsuccessful to get any information
about 21020.

About the series resistor, it is of course more cost effective by putting it in
the IC. However, the process and temperature variations can easily change
the diffusion resistor by more than 25% (some foundry spec. even looser).
Together with the PCB trace tolerance, it is very difficult to use diffusion
resistor to provide a good matching of the driver resistance to the PCB trace.

Regards,
Raymond

Steve Weir <weirsp@a.crl.com> on 15/07/99 10:04:03

Please respond to si-list@silab.eng.sun.com

To: si-list@silab.eng.sun.com
cc: (bcc: Raymond Leung/QSA/AU)

Subject: Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast

I have been looking for a fabricator who supports this, but have not found
one. Does anyone know of one?

Regards,

Steve.
At 02:20 PM 7/14/99 -0500, you wrote:
>
>
>All,
>
>I'm also hearing that the microwave guys are using thick film printed
>resistors,
>as part of the etch, for termination elements - even on internal signal
layers.
>Anybody out there willing to share experiences?
>
>
>Regards,
>
>Roy
>
>
>
>
>
>
>Philip Gantt <pgantt@cisco.com> on 07/14/99 12:42:09 PM
>
>Please respond to si-list@silab.eng.sun.com
>
>Sent by: Philip Gantt <pgantt@cisco.com>
>
>
>To: si-list@silab.eng.sun.com
>cc: (Roy Leventhal/MW/US/3Com)
>Subject: Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast
>
>
>
>
>Has anyone considered that if a more resistive material (other than copper)
>were
>used for PCB laminates, the inherent resistance of the conductors would slow
>the
>edge rates and also reduce EMI? This might eliminate the need for
>external resistors as well. Just a thought...
>
>Jon Keeble wrote:
>
>> I've seen one or two interesting approaches to controlling edge speeds.
>>
>> The first was a method of controlling the edge speed:
>> The Analog Devices 21020 DSP chip has an external resistor for setting
>> edge speeds (all outputs). I've no idea how this works internally.
>>
>> The other method reduced the production spread of edge speeds:
>> It seems that the parts that run faster also have faster edges.
>> The method involved
>> - an onboard oscillator that ran at a speed determined by the speed of
>> the device
>> - internal control on the output drivers to vary the edge speed
>> - a circuit that translated the oscillator speed to more or less drive
>> on the outputs
>>
>> As a designer, it is obvious tpo me that there is a real need for
>> control over edge speeds.
>>
>> I find it really exhausting to have to add hundreds of external
>> resistors, not to mention the additional costs, numbers of parts,
>> numbers of nodes to test etc etc.
>>
>> I'd encourage the IC designers to be a bit more inventive.
>>
>> Jon Keeble.
>>
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