[SI-LIST] Re: Bypass Capacitor Selection
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "SI List" <si-list@xxxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Tue, 30 Dec 2003 10:06:45 -0800
Martin,
You have two questions in your message. First, package size and second
capacitor value.
Package size first: The motivation for small package size is lowest
possible parasitic inductance. One must include mounting inductance as
part of this. When this is added, the difference between the total
inductance of an 0402 and 0603 is so small that 0603 is a better choice, as
it makes assembly easier.
Capacitor value: Usually, the objective is to achieve a low impedance
across a very broad range of frequencies. Using a single value capacitor
will not achieve this. There are several papers that address this and give
insight into how to determine the values and quantities of capacitors. If
you send me a separate e mail I can supply some of them. One of them is on
my web site www.speedingdge.com.
Lee
> [Original Message]
> From: SI List <si-list@xxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 12/30/2003 5:23:45 AM
> Subject: [SI-LIST] Bypass Capacitor Selection
>
> Dear All,
>
> After studying many books, the bottom line for high frequency bypass
> capacitor selection seems to be the following:
>
> Use the smallest possible package and then take the largest valued
> capacitor available in that package.
>
> Although 0201 capacitors are available, they are kind of hard to
> assemble. So in our design, we are thinking of using 0402 X5R parts.
>
> In a 0402 package, the largest value available seems to be 1 uF.
>
> My frequency range of interest is DC to 500 MHz. In addition to the
> bypass capacitors, I know that there must be a pair of closely spaced
> VCC/GND reference planes.
>
> My questions:
> 1. Does it make sense to only use 1 uF 0402 capacitors for bypassing?
> Other people often use a combination of 1 nF/10nF/100nF. 2. I have been
> looking at the frequency characteristic of 100 nF 0402 X5R capacitors
> given in the data sheets of different manufacturers. The impedance vs.
> frequency plot often looks quite different from manufacturer to
> manufacturer. Should I choose the manufacturer with the best frequency
> characteristic or are all such parts almost equivalent? 3. Does it make
> sense to use surface mount capacitor arrays for bypassing?
>
> Thanks for all your expert feedback.
>
> Best regards,
> Martin Heimlicher, heimlicher__at__enclustra//dot//com
>
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