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how to select capacitor based on freq impedance

Is it possible to calculate a capacitance value from a required impedance at a frequency, say for example

"impedance for C7 was chosen to be roughly 100=D9 at 500 kHz. That calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

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I know caps have that impedance vs frequency curve that slopes down to a minimum impedance then slopes back up at higher frequencies.... but can you be given a frequency and impedance and calculate a capacitance out of that? How the hell did they come up with 3300 pF?

much thanks!

Reply to
panfilero
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Xc = 1/(2*pi*f*C)

-or-

C = 1/(2*pi*f*Xc) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

thank you.

Reply to
panfilero

Keep in mind that any R in series with a CAP is going to produce an overall different Z (impedance), that is, the X of c (Xc) with the R in series added to it, will not produce exactly that.

To get the final Z of that network.

Z = Sqrt( Xc^+R^);

It's also use to find the C line of a right/left triangle. :)

Jamie

Reply to
Jamie

Jim's correct for an ideal capacitor. And you're correct that non-ideal capacitors have an effective internal serial inductance that becomes dominant at high enough frequencies.

The key is the "high enough" bit. At 500 KHz, pretty much all modern ceramic caps are well below self-resonance and can be considered as ideal capacitors for most applications, so the ideal formula applies.

The size, both package size and how much capacitance, also plays a roll in where the self resonance shows up. A 0.0033 uF would be expected to have a higher self resonance than, say, an otherwise similar 0.1 uF.

So, your concern is quite valid and in applications where the frequency dependent effects become visible, head for the datasheet.

--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

Correct. I didn't want to jazz up my getting to a 3300pF value with the complexities of "real life" ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

You need to learn some basic theory. That's straight from the equation that Jim Thompson quoted (which is for an ideal cap -- the plots you see where it dips down close to zero and then rises is for a real cap that has some series inductance).

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Reply to
Tim Wescott

Sorry: and the way to learn that basic theory, if you don't want to take a four-year course in EE, is to get yourself a copy of the ARRL Handbook. It's main thrust is about radio -- but most of radio is just basic electronics, so it is a very useful self-teaching document.

Not everything in there is presented the way a guy with an EE degree is used to -- they leave out some detail so that you can absorb it out of a book and get useful results quick -- but it'll certainly get you there.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Reply to
Tim Wescott 

--
Z = sqrt(R^2 + (Xl-Xc)^2)
Reply to
John Fields

This calculator will work out the capacitance, frequency, reactance, and resonant freqs for various capacitors at different conditions. It's for ideal capacitors and inductors.

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-Bill

Reply to
Bill Bowden

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