Re: [SI-LIST] : Ground gap problem

Mike Hughes (mike.hughes@analog.com)
Tue, 04 May 1999 08:44:04 -0400

I am also interested in Rajkumar's question. Mainly the "why" part of it.

If an IC's power and ground pins are connected to planes there are 3 ways to
connect a decoupling cap:

1. connect a cap directly to the planes (using via's) in the vicinity of the IC
(without connecting to the IC).

2. connect a cap to the IC power and ground pins (using traces) without
connecting the cap to the planes.

3. connect a cap to the IC power and ground pins (using traces) AND connect the
cap to the planes (using via's).

The cap should be as close as possible to the power and ground pins to lower
inductance. #3 doesn't seem to be a good idea because of ground loops(?) The
only difference between #1 and #2 seems to be the inductance. Could it be that
using #1 and #2 doesn't matter as long as you get the lowest possible
inductance?

Sorry if this is an old question.

Thanks,
Mike

> -----Original Message-----
> From: Rajkumar C [mailto:craj@india.ti.com]
> Sent: Monday, April 26, 1999 12:11 PM
> To: si-list@silab.eng.sun.com
> Subject: RE: [SI-LIST] : Ground gap problem
>
> Hi guys,
>
> I have a very basic question.
> Generally, there is a guideline that every
> IC's used in the PCB should have a decoupling
> capacitor near the power and ground pins.
> Here is my question:
> In the PCB, I have a separate power plane and
> ground plane. For all the IC's I am using the Decoupling
> capacitors.
>
> * connect the power and ground pins to the Decoupling
> capacitors first and then connect the capacitors to Planes
> (or)
> * Connect the Power and ground pin to the plane directly and
> have decoupling capacitors connected to power and ground plane
> and also placed near the IC's power supply pin.
>
> In the above two methods which is best and why?
>
> Expecting your answers.
>
> Thanks and regards,
> Rajkumar.
>
> **** To unsubscribe from si-list: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list, for more help, put HELP. si-list archives are accessible at
> http://www.qsl.net/wb6tpu/si-list ****
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
================================
Mike Hughes               
Product/Test Engineer      
High Speed Conversion       
Analog Devices, Inc. 

Phone 781-937-2370 Fax 781-937-1011 mailto:mike.hughes@analog.com http://www.analog.com ================================

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****