forwarding for George ____________________Reply Separator____________________ Subject: RE: Charge moving from decoupling capacitors Author: [email protected] List-Post: [email protected] Date: 5/17/00 11:16 PM
Barry, This is true on most boards. But that is because the caps are tied to the planes through vias and lead traces, which are inductive. Also, the caps are further away from the IC power pins than the planes. This does not mean that the cap by itself cannot work at 1 GHz. If you have a 50 mil trace above a gnd plane terminated to a cap without using vias or other lead traces and measure VSWR and reflections from the cap, you will see that the cap works well at 1 GHz and beyond. You must solder the cap to the gnd plane and to the 50 mil trace directly. Regards, George Tang -----Original Message----- From: Barry Ma [mailto:[email protected]] Sent: Wednesday, May 17, 2000 9:16 AM To: [email protected]; EMC-PSTC Cc: [email protected] Subject: Re: Charge moving from decoupling capacitors Steve, Thanks a lot for the very nice hierarchy description below. If there's a 10 mil or less spacing between pwr and gnd planes, the plane cap is available. The plane cap and the decaps are complementary in whole frequency range. Plane cap takes care of high end, and decaps cover low portion. Then the location of decaps are not critical. And then decaps can be shared by other chips, according to the excellent research conducted by EMC lab at UMR. Please allow me to modify a bit your description as follows. The capacitance inside the device supports the chip first, but usually not enough. Charge from the planes also supports the chip and replenishes the device capacitance, Decaps replenish the plane on low frequency portion, while plane cap responds itself on HF end, Bulk capacitors replenish decaps and plane cap, The voltage regulator replenishes the bulk capacitors. Please correct me. Thanks. Regards, Barry [email protected] ----------------- From: "sweir" <[email protected]>, on 5/11/00 9:28 PM: .... The capacitance inside the device supports the chip, Charge from the planes replenishes the device capacitance, HF capacitors on the board replenish the planes, Bulk capacitors replenish the HF capacitors, The voltage regulator replenishes the bulk capacitors. ....[edited by bm] ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
