On 09:49 PM 9/09/2002 -0400, Brian Guralnick said: >DAMN ISP HEADACHES!!! > >Re-send... > > This is one of the decoupling caps between VCCINT 1.8v & GND. > > > > As you can see, the 1.8v dips down to 1.625. This creates havoc with > the processing inside the Altera EP20K200E which has clocks > > running at 133MHz, 125MHz, 95MHz, & 24MHz. > > > > 16KB > > ftp://ftp.point-lab.com/quartus/Public/ProtelUsers/NoiseVcc18-2ns.png > > 14KB > > ftp://ftp.point-lab.com/quartus/Public/ProtelUsers/NoiseVcc18-4ns.png > > > > Here is my current decoupling caps for the 1.8v. > > > > 4 x 0.1uf smd 0805 ceramic. > > 3 x 470uf electrolytic. > > > > See the problem?
0805 is supposedly somewhere around 0.5 nH - add a bit for via inductance - lets say 1 nH. With 100nF this resonates at about 15 MHz. So you are on the rising X curve with your noise glitches. What about paralleling 100pf caps across the 100nF decouplers. I suggest changing the order so the 100p's are on the board and the 100n on top - or do what the very high freq guys do and mount the caps on their side to reduce inductance (have a look at the way multi-layered caps are made to figure this out). You could also try pick a capacitance such that the total net cap and inductance is series resonant at your major noise freq. This will put an AC short, limited by capacitance Q (assume inductance Q is high), across the supply at this freq. I have seen this done but usually on supplies with narrow band noise. Using high-Q caps may help as well as they have much lower ESR and usually lower ESL. More $ of course. Murata and Philips make ceramic caps designed for high freq work. And of course companies like ATC make porcelain caps for microwave work but I am not sure how high they go in capcitance. Does paralleling another 100nF cap (or 10 nF or 1 nf or 100 p ...) on each of the existing caps make any difference? The sorts of noise freqs you are seeing look like small HF caps may help rather than bulk capacitance. Guessing, Ian Wilson ************************************************************************ * Tracking #: 688322440672634DB4E78E976578EAE7F7C43FB3 * ************************************************************************ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[email protected] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
