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projectwhite 9th February 2010 01:32 AM

PCM1704 decoupling
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Hi guys. Please see the attached circuit and impedance graph showing what I have so far for the decoupling caps. BTW, my clock runs at 8.4276Mhz, and the oversampling at 264Khz.

I'm having trouble choosing one cap: a 1uF cap.

The problem with this cap is its ESR between 1-10mhz needs to be between 0.03 and 0.2 Ohms. Inductance must be less than 5nH.

Does anybody know of such a capacitor? Quality is not a factor since it will only be used for digital decoupling.

Many thanks.

projectwhite 9th February 2010 04:06 AM

By the way, there's this 1uF controlled-ESR 200mOhm cap, but it's only rated at 4V! Argh! What terrible luck :(

Do you think it's ok to use it at 5V for digital decoupling?

Nazar_lv 9th February 2010 08:26 AM

projectwhite, use SMD1206 X7R or X5R (better 10-47u 6.3v), it has ESR ~0.04 and ESL 1nH or use 0612 SMD it has ESL ~0.2nH

projectwhite 9th February 2010 08:32 AM

All the other caps will be SMD X7R/X5R as you suggested. It's only that particular 1uF cap that is a problem - ESR must be between 0.03 and 0.2Ohms.

I found a couple of electrolytic with 0.2Ohm ESR but the ESL is larger than 5nH since they're not SMD parts. That TDK cap above is the only one that would work, if only it could take 5V. I thought about putting a 0.05Ohm resistor in series with an X7R/X5R but I suspect the ESL would rise above 5nH, especially since this cap will be behind a smaller 0.33uF cap, which must be closest to the pin.

If someone tells me I can use that TDK cap at 5V, I'll be over the moon. Doesn't even matter if the accuracy is reduced.

Nazar_lv 9th February 2010 11:19 AM

1u is crytycal? if not use 47u X5R 6.3, he has very low ESR and ESL (1nH)
for example

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