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What is the significance of SRF and ESR ,while selecting SMT capacitor.

Question asked by btv_murthy@rediffmail.com on Aug 23, 2013
Latest reply on Aug 25, 2013 by volker_muehlhaus

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Dear Sir,
              I want to select the 82 PF SMT capacitor at 2GHz of creating the layout of LNA.When I downloaded the AVX company market available 82 PF SMT capacitor

(1) I have noticed the series resonant frequency (SRF) vs capacitor and ESR vs frequency. I am confused, how I have to select the series ( Example 0402,0805,0603,1210) for my 82 PF at 2GHz and ESR. What is this SRF and ESR graphs, is the significance, while selecting the particular value capacitor. For 82 PF , which series capacitor I need to select. On what basis we have to select the series for particular value capacitor.

(2) How we have to read the capacitor value at particular frequency. Similarly how I have to read the ESR value.
                      Anybody knows, please can you explain me in detail.

Edited by: btv_murthy@rediffmail.com on Aug 23, 2013 11:54 PM

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volker_muehlhaus Aug 24, 2013 8:03 AM

http://tiny.cc/rdtb2w
http://tiny.cc/oftb2w

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btv_murthy@rediffmail.com @ volker_muehlhaus on Aug 24, 2013 8:55 AM

Volker Sir,
Thank you very much for your help.

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btv_murthy@rediffmail.com @ volker_muehlhaus on Aug 25, 2013 12:31 AM

Dear Sir,
Thank you for your help to show the usefull link. I have gone through those two documents, those are not much useful. I need specifically, how we have to find the SRF( Series Resonant Frequency) for the particular capacitor value and how we have to read the SRF value for particular capacitor value using company datasheet graph and what we can learn from the ESR. I didn't find the significance of SRF & ESR much information.I am sorry.

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volker_muehlhaus @ btv_murthy@rediffmail.com on Aug 25, 2013 7:57 AM

But these documents have very clear, detailed answers to your question on SRF and ESR. Try to read it again.

For interpretation of the SRF graph, to find your SRF value for given value and package size, maybe the picture helps where I have added some notes.

General comments on choosing the capacitor package size:

- All capacitors have parasitics, as seen in their equivalent circuit models. Two of these parasitics are effective series resistance (ESR) and series inductance.
- If you use circuit models or S-parameters for the capacitor, and have the exact model for your capacitor type, all these parasitics are included.
- Series inductance leads to a frequency dependent effective capacitance. At the self resonance frequency (SRF), the parasitic inductance cancels the capacitance. Above, the capacitor behave inductive.
- If you need the capactor to block DC, and don't care much about the exact value, you can use it in wide frequency range.
- If you need an exact capacitance value, the parasitic inductance and resulting SRF is important, because they change the effective capacitance.
- Series resistance for your AVX capactors is about the same for different package sizes.

So which is best for you?
- The package code indicates size. Smaller number = smaller package. Picture: http://tiny.cc/bnnd2w
- Some values might be available only in small/large package dimension.
- Smaller packages have smaller parasitic inductance, and higher SRF, so that they behave more like ideal capacitors.
- Bigger packages have higher parasitic inductance, but can usually handle higher voltages (not a problem with your LNA).
- Bigger packages might be easier to handle/solder.

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readSRF.PNG193.5 KB

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volker_muehlhaus Aug 24, 2013 8:03 AM
Mark Correct
Correct Answer
http://tiny.cc/rdtb2w
http://tiny.cc/oftb2w
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- btv_murthy@rediffmail.com @ volker_muehlhaus on Aug 24, 2013 8:55 AM
  Mark Correct
  Correct Answer
  Volker Sir,
  Thank you very much for your help.
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- btv_murthy@rediffmail.com @ volker_muehlhaus on Aug 25, 2013 12:31 AM
  Mark Correct
  Correct Answer
  Dear Sir,
  Thank you for your help to show the usefull link. I have gone through those two documents, those are not much useful. I need specifically, how we have to find the SRF( Series Resonant Frequency) for the particular capacitor value and how we have to read the SRF value for particular capacitor value using company datasheet graph and what we can learn from the ESR. I didn't find the significance of SRF & ESR much information.I am sorry.
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  - volker_muehlhaus @ btv_murthy@rediffmail.com on Aug 25, 2013 7:57 AM
    Mark Correct
    Correct Answer
    But these documents have very clear, detailed answers to your question on SRF and ESR. Try to read it again.
    
    For interpretation of the SRF graph, to find your SRF value for given value and package size, maybe the picture helps where I have added some notes.
    
    General comments on choosing the capacitor package size:
    
    - All capacitors have parasitics, as seen in their equivalent circuit models. Two of these parasitics are effective series resistance (ESR) and series inductance.
    - If you use circuit models or S-parameters for the capacitor, and have the exact model for your capacitor type, all these parasitics are included.
    - Series inductance leads to a frequency dependent effective capacitance. At the self resonance frequency (SRF), the parasitic inductance cancels the capacitance. Above, the capacitor behave inductive.
    - If you need the capactor to block DC, and don't care much about the exact value, you can use it in wide frequency range.
    - If you need an exact capacitance value, the parasitic inductance and resulting SRF is important, because they change the effective capacitance.
    - Series resistance for your AVX capactors is about the same for different package sizes.
    
    So which is best for you?
    - The package code indicates size. Smaller number = smaller package. Picture: http://tiny.cc/bnnd2w
    - Some values might be available only in small/large package dimension.
    - Smaller packages have smaller parasitic inductance, and higher SRF, so that they behave more like ideal capacitors.
    - Bigger packages have higher parasitic inductance, but can usually handle higher voltages (not a problem with your LNA).
    - Bigger packages might be easier to handle/solder.
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    readSRF.PNG193.5 KB
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What is the significance of SRF and ESR ,while selecting SMT capacitor.

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