how to select capacitor based on freq impedance

[panfilero]

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 100Ů at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

 

[Jim Thompson]

On Thu, 15 Mar 2012 14:49:38 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

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? at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

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 | |
| Voice480)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.

 

[panfilero]

On Mar 15, 4:55 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:

On Thu, 15 Mar 2012 14:49:38 -0700 (PDT), panfilero









panfil...@gmail.com> wrote:
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? at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

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  |             |
| Voice480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon athttp://www.analog-innovations.com|    1962     |

I love to cook with wine.     Sometimes I even put it in the food.

thank you.

 

[Jamie]

panfilero wrote:

On Mar 15, 4:55 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:

On Thu, 15 Mar 2012 14:49:38 -0700 (PDT), panfilero









panfil...@gmail.com> wrote:

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? at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

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 | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon athttp://www.analog-innovations.com| 1962 |

I love to cook with wine. Sometimes I even put it in the food.


thank you.
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

 

[Rich Webb]

On Thu, 15 Mar 2012 15:09:40 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

On Mar 15, 4:55 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:
On Thu, 15 Mar 2012 14:49:38 -0700 (PDT), panfilero









panfil...@gmail.com> wrote:
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? at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

Xc = 1/(2*pi*f*C)

-or-

C = 1/(2*pi*f*Xc)

                                        ...Jim Thompson
thank you.

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

 

[Jim Thompson]

On Fri, 16 Mar 2012 08:31:28 -0400, Rich Webb
<bbew.ar@mapson.nozirev.ten> wrote:

On Thu, 15 Mar 2012 15:09:40 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

On Mar 15, 4:55 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:
On Thu, 15 Mar 2012 14:49:38 -0700 (PDT), panfilero









panfil...@gmail.com> wrote:
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? at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

Xc = 1/(2*pi*f*C)

-or-

C = 1/(2*pi*f*Xc)

                                        ...Jim Thompson
thank you.

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.

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 | |
| Voice480)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.

 

[Tim Wescott]

On Thu, 15 Mar 2012 14:49:38 -0700, panfilero wrote:

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 100Ί at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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?

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

 

[Tim Wescott]

On Fri, 16 Mar 2012 12:20:06 -0500, Tim Wescott wrote:

On Thu, 15 Mar 2012 14:49:38 -0700, panfilero wrote:

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 100Ί at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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?

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).

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

 

[John Fields]

On Thu, 15 Mar 2012 18:54:40 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

panfilero wrote:

On Mar 15, 4:55 pm, Jim Thompson <To-Email-Use-The-Envelope-I...@On-My-
Web-Site.com> wrote:

On Thu, 15 Mar 2012 14:49:38 -0700 (PDT), panfilero









panfil...@gmail.com> wrote:

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? at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

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 | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon athttp://www.analog-innovations.com| 1962 |

I love to cook with wine. Sometimes I even put it in the food.


thank you.
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

---
Z = sqrt(R^2 + (Xl-Xc)^2)

--
JF

 

[Bill Bowden]

On Mar 15, 1:49 pm, panfilero <panfil...@gmail.com> wrote:

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 100Ů at 500 kHz. That
calculates to around 3300 pF"

that's a quote from page 6 of this datasheet

http://www.ti.com/lit/an/snva166/snva166.pdf

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!

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.

http://www.bowdenshobbycircuits.info/XLC.htm

-Bill