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Doubt about decoupling capacitors in high-freq PCBs

Hi,

Im a total newbie to PCB design, i have to build a high-speed PCB for a board containing an Atmel At91 microcontroller operating at 200MHz, with RAM, Flash, etc, so im trying to collect info on PCB design.

The question may be a bit misleading, heres the actual dope: Ive seen high-speed PCBs where not all of the decoupling capacitors are included on the final board, even though the schematic may mention all of them.How does one decide which decoupling caps can be safely omitted in the final design?

Also, in an earlier design using the same controller as above, we had the RAM and the controller on seperate boards, connedted with a 20cm long ribbon cable. But the RAM wasnt functioning correctly, we were getting garbled data when we tried to store stuff on the RAM..could it be due to the fact that the RAM and the uC were on seperate boards??

Thanks in anticipation, Mayank

Reply to
Mayank Kaushik
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You look at the amount of noise on the supply rails with them removed/not fitted. A reasonable test is to see if the circuit continues to function normally with the supply rails turn down. eg Does your 5V design still work at 4.5V? 3.75V?

Yes for a whole bunch of reasons. It's one reason why modern hard drives now use a serial interface and low voltage signals (SATA).

Reply to
CWatters

Calculation for minimum capacitor value is here:

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How to terminate a Cable / PWB trace is here;

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All other design pitfalls are here;

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Reply to
www.interfacebus.com

Hi guys..

One more thing..In such a design, is it absolutely necessary to use surface mounted caps? Whats the advantage these surface mounted caps have over normal ones? We have used only normal ceramics and electrolytic caps till now (both on the main board and the RAM board). They seemed to work fine on the board, though i cant say anything abt the RAM

Regards

Mayank

Reply to
Mayank Kaushik

Why not use them? They can be fitted by hand on prototypes.

They can usually be located closer to the pin that they are decoupling. They make layout easier because you can run tracks underneath them.

Electrolytic caps are only usually used at low frequencies. "Normal" ceramic capacitors may not always have a good enough high frequency response.

Define "fine".

Tried submitting your product for EMC compliance testing yet?

Reply to
CWatters

Er..no, its only my final year undergrad project :-)

Reply to
Mayank Kaushik

Lucky you. It can be one of the more frustrating aspects of product design. I've done my time down the salt mine!

Reply to
CWatters

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