Why use 2 decoupling capacitors?

[raptor31]

I have a question on the caps...i have a .1uF and a 100uF (i ordered the 100uF's instead). Would that be ok even with the one cap being 10 times bigger???

[polymorph]

I recently found some 10µF MLCC types in an 0805 package that are very competitively priced as compared to the 100nF caps that I've been using. Looking at the charts in the datasheets, the 10µF caps actually have lower impedance than the 100nF caps. The datasheet for the 100nF caps doesn't have anything to say about ESR, but the ESR for the 10µF caps is 0.01? or less from 20kHz to 20MHz. Conclusion: one 10µF near the MAX7219.

Jack, do you have a link or part number you can share with us for that 10uF 0805 capacitor?

[polymorph]

http://www.intersil.com/content/dam/Intersil/documents/an13/an1325.pdf

http://www.altera.com/support/devices/power/integrity/pow-integrity.html

[JChristensen]

I recently found some 10µF MLCC types in an 0805 package that are very competitively priced as compared to the 100nF caps that I've been using. Looking at the charts in the datasheets, the 10µF caps actually have lower impedance than the 100nF caps. The datasheet for the 100nF caps doesn't have anything to say about ESR, but the ESR for the 10µF caps is 0.01? or less from 20kHz to 20MHz. Conclusion: one 10µF near the MAX7219.

Jack, do you have a link or part number you can share with us for that 10uF 0805 capacitor?

Glad to share, but you may not want them now. I don't understand this, but it's happened on quite a few parts now from Mouser. I bought those caps a year ago, paid $3.50 for 100 of them. Now there's a minimum quantity of 3000.
http://www.mouser.com/ProductDetail/Yageo/CC0805KKX5R6BB106/?qs=%2fha2pyFaduhNpscxq1stixAvSpVvOL5tPk3BrNc%2f60pzrCxT5ov20OuMxn74az3N

These look pretty much similar though:
http://www.mouser.com/ProductDetail/TDK/C2012X5R1A106K125AB/?qs=sGAEpiMZZMs0AnBnWHyRQFzBYxg9rzNc7DwMbOY8Gy8%3d

Lately for 10µF caps I've switched to a higher voltage rating in a larger package, with the aim of minimizing the voltage coefficient:
http://www.mouser.com/ProductDetail/Taiyo-Yuden/TMK316BJ106KLHT/?qs=%2fha2pyFadugJ4Z%2f39Rolq8%2fR7W%2fklDrTsUBP%252b1Z%252bxHytiuk%252bdOpkbLjTwO2u36lB

HTH.

[CrossRoads]

@Guy Cothal - yes.

[polymorph]

Those ceramic high value capacitors can reduce to as little as 1/10th their rated capacitance when you come close to their rated max voltage.

Looking at the datasheet, a 10uF MLCC capacitor's impedance reaches resonance at about 1.5MHz and goes up from there. So I'd never use that without something like a 1nF for higher frequencies.