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[OT] Capacitor value

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  • Paulo Castro
    Hello ListMembers, I have a very off-topic subject, maybe the correct destination is [EE]. I hope You could bring some light on this. The question is: How can
    Message 1 of 7 , Dec 6, 2004
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       Hello ListMembers,

      I have a very off-topic subject, maybe the correct destination is [EE]. I hope You could bring some light on this. 

      The question is: How can I determine the value of a capacitor between amplifier stages, or between a transdutor and an operational amplifier? Of course, the capacitor would permit only the AC component of the output of the transducer to be amplified, but what should be the best value?

      In the case of the transdutor, its resistance is about 500 ohms, and the center frequency is 40 KHz.

       

      Best regards,

       

                                       Paul


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    • H.C. Croon
      ... Hi Paul, Your question is, in my opinion, not OT, but in fact here in Europe the answer to such questions belong to the education in electronics. Two spec
      Message 2 of 7 , Dec 6, 2004
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        > Hello ListMembers,
        >
        > I have a very off-topic subject, maybe the correct destination is [EE]. I hope You could bring some light on this.
        Hi Paul,

        Your question is, in my opinion, not OT, but in fact here in Europe
        the answer to such questions belong to the education in electronics.

        Two spec items are important:

        - Which frequency range has to be transmitted.
        - What is the total resistance of the source plus the destination.

        Inserting a capacitor will limit the transmitting of low frequency
        signals. So what is the lowest frequency you will transmit and what
        attenuation is alowed for this frequency?

        When you know these facts, you can calculate the capacitor value.

        For instance, if -3 dB is allowed and R is the total resistance of
        source and destination together C=1/(2.pi.f.R). For other
        attenuations use the formula

        attenuation= R/(SQRT(SQR(R)+SQR(1/2.pi.f.C))

        Be aware that the attenuation caused by the internal resistance is
        freqency independent and is not contained in the frequency dependence
        of the specs. Frequency independent attenuation can easely be
        compensated by augmenting the amplification of an opamp circuit.

        Things are a little bit more complicated if the transducer has a
        frequency dependent inner impedance. Than you must draw a model
        circuit and calculate from there.

        I hope this will help you.

        Regards, Harry

        >
        > The question is: How can I determine the value of a capacitor between amplifier stages, or between a transdutor and an operational amplifier? Of course, the capacitor would permit only the AC component of the output of the transducer to be amplified, but what should be the best value?
        >
        > In the case of the transdutor, its resistance is about 500 ohms, and the center frequency is 40 KHz.
        >
        >
        >
        > Best regards,
        >
        >
        >
        > Paul
        >

        --
        Author: H.C. Croon
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      • Friedel Bruening
        First engineers aproach is calculate (go and study electronics), if that fails ask yourself what could go wrong using the try and error method, if there isn t
        Message 3 of 7 , Dec 6, 2004
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          First engineers aproach is calculate (go and study electronics), if that fails
          ask yourself what could go wrong using the try and error method, if there
          isn't much what could go wrong, get yourself some parts and try.

          Paul, on need more information to be able to answer this,
          whats the frecuency, is it continous or pulsed operation, what are the impedances,
          etc. we can't really discuss this by e-mail.

          Or I am wrong ?

          Friedel


          On 6 Dec 2004 at 3:15, Paulo Castro wrote:

          >
          > Hello ListMembers,
          > I have a very off-topic subject, maybe the correct destination is [EE]. I hope You
          could bring some
          > light on this.
          > The question is: How can I determine the value of a capacitor between amplifier
          stages, or
          > between a transdutor and an operational amplifier? Of course, the capacitor would
          permit only the
          > AC component of the output of the transducer to be amplified, but what should be
          the best value?
          > In the case of the transdutor, its resistance is about 500 ohms, and the center
          frequency is 40
          > KHz.
          >
          > Best regards,
          >
          > Paul
          >
          >
          > Yahoo! Mail - Agora com 250MB de espaço gratuito. Abra uma conta agora!


          Friedel Bruening
          Bolivian Instruments
          Casilla 4856
          Santa Cruz
          Bolivia
          Mobile : ++591-773-92119
          Tel/Fax : ++591-3-3580572
          e-mail : bolinst@...
          --
          Author: Friedel Bruening
          INET: bolinst@...

          Fat City Hosting, San Diego, California -- http://www.fatcity.com
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        • Alois Bauer
          hello I m from the HF (High Freq.) and microwave community. My rough estimate for coupling capacitors is: impedance of the coupling cap should be about 1/10 of
          Message 4 of 7 , Dec 6, 2004
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            hello
            I'm from the HF (High Freq.) and microwave community.
            My rough estimate for coupling capacitors is: impedance of the coupling cap
            should be about 1/10 of the characteristic impedance (more acurately spoken:
            reactance) at the lowest frequency to have neglectible losses.

            Your case: what capacitor has a reactance of 50 Ohms (1/10 of 500) at 40 kHz?

            X_c = 1/(2*pi*f*C) ==> C = 1/(2*pi*f*X_c) (units Hz, Ohms, Farads)

            C = 1/(6.283*40000*50) F = 79.6e-9 F = 79.6 nF = 0.0796 uF

            rounded to next practical value: ==> 0.1 uF (or 0.22 uF)

            attention:
            such large cap. values as SMDs are ceramic caps made of material with high
            dieelectric constant. X7R ceramics is in most cases ok (0.1uF 50V available in
            size 0805); Z5U or Y5V change their capacitance value over temperature and
            with applied (DC) voltage, in worst case more than 50% of the value at 25 deg
            C.
            Best (highest stability, highest Q, lowest resistive losses) is NP0 or C0G
            ceramics (maybe max 10nF available in size 1206 with 50V), and at low frequs
            (Audio Freq) some foil caps.

            hope this helps
            regards
            Alois

            > First engineers aproach is calculate (go and study electronics), if that fails
            > ask yourself what could go wrong using the try and error method, if there
            > isn't much what could go wrong, get yourself some parts and try.
            >
            > Paul, on need more information to be able to answer this,
            > whats the frecuency, is it continous or pulsed operation, what are the impedances,
            > etc. we can't really discuss this by e-mail.
            >
            > Or I am wrong ?
            >
            > Friedel
            >
            >
            > On 6 Dec 2004 at 3:15, Paulo Castro wrote:
            >
            > >
            > > Hello ListMembers,
            > > I have a very off-topic subject, maybe the correct destination is [EE]. I hope You
            > could bring some
            > > light on this.
            > > The question is: How can I determine the value of a capacitor between amplifier
            > stages, or
            > > between a transdutor and an operational amplifier? Of course, the capacitor would
            > permit only the
            > > AC component of the output of the transducer to be amplified, but what should be
            > the best value?
            > > In the case of the transdutor, its resistance is about 500 ohms, and the center
            > frequency is 40
            > > KHz.
            > >
            > > Best regards,
            > >
            > > Paul
            > >
            > >
            > > Yahoo! Mail - Agora com 250MB de espaço gratuito. Abra uma conta agora!
            >
            >
            > Friedel Bruening
            > Bolivian Instruments
            > Casilla 4856
            > Santa Cruz
            > Bolivia
            > Mobile : ++591-773-92119
            > Tel/Fax : ++591-3-3580572
            > e-mail : bolinst@...
            > --
            > Author: Friedel Bruening
            > INET: bolinst@...
            >
            > Fat City Hosting, San Diego, California -- http://www.fatcity.com
            > ---------------------------------------------------------------------
            > To REMOVE yourself from this mailing list, send an E-Mail message
            > to: ListGuru@... (note EXACT spelling of 'ListGuru') and in
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            > (or the name of mailing list you want to be removed from). You may
            > also send the HELP command for other information (like subscribing).

            - - - - - - - - - - - - - - - - - - - - - - - - - -
            Alois Bauer
            WORK Microwave GmbH
            Raiffeisenstr. 12, D-83607 Holzkirchen, Germany
            Tel. {+49} (0)8024-6408-0 / FAX (0)8024-640840

            --
            Author: Alois Bauer
            INET: aba@...

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          • Fred Townsend
            Paulo this is the type question I love to teach in an advanced circuits class because the freshman student is likely to give a very different answer than the
            Message 5 of 7 , Dec 6, 2004
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              Paulo this is the type question I love to teach in an advanced circuits class because the freshman student is likely to give a very different answer than the experienced engineer. Your specifications are a minefield of problems.

              First: You stated your CENTER frequency is 40 KHz.  Capacitors for this type application are selected by the MINIMUM frequency therefore the lower frequency or bandwidth of the transducer is required.

              Second: There are many types of capacitors. Capacitors are NOT ideal components. What is the value of the DC voltage you are trying to block? This will determine the working voltage rating of your capacitor. Also what is your tolerance to leakage. Some types of capacitors have relatively high values of leakage.

              Third: What is the nature of the transducer? Is it a wound component? If so it will have a relatively high value of inductance.  The inductance will resonate with the coupling capacitor. Granted with 500 ohms resistance the Q, figure of merit, will be low.  This means the resonate effects will not dominate unless you have some critical flatness or ripple requirements.

              All things said I think you have over simplified your question. It is critical to know the nature of the application even if you can't supply all the missing parameters.

              Please resubmit you question.

              Fred Townsend


              Paulo Castro wrote:

               Hello ListMembers,

              I have a very off-topic subject, maybe the correct destination is [EE]. I hope You could bring some light on this. 

              The question is: How can I determine the value of a capacitor between amplifier stages, or between a transdutor and an operational amplifier? Of course, the capacitor would permit only the AC component of the output of the transducer to be amplified, but what should be the best value?

              In the case of the transdutor, its resistance is about 500 ohms, and the center frequency is 40 KHz.

               

              Best regards,

               

                                               Paul


              Yahoo! Mail - Agora com 250MB de espaço gratuito. Abra uma conta agora!

            • Paulo Castro
              Hello ListMembers, In first place, thanks for your responses. It was very Instrutive. Now I am looking for X7R and COG caps for outdoor applications (Mr.
              Message 6 of 7 , Dec 9, 2004
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                 Hello ListMembers,
                 
                  In first place, thanks for your responses. It was very Instrutive. Now I am looking for X7R and COG caps for outdoor applications (Mr. Alois) !
                 
                  Reposting my question:
                 
                  Looking all over books and in the Internet, I found many different values for same applications. This, also for capacitors between amp-op. stages and for transducer input.
                 
                  In the case of the transducer, it is a piezo one. I have now a different supplier, with more specs.
                 
                  Its center frequency is 40 KHz (+ - 1 KHz), bandwith (-6 dB) 1.0 KHz. The capacitance@(1 KHz + - 20%) is 2400 pF. Impedance at 40 KHz is 15 K Ohm (!)
                 
                  The DC voltage is about 8V, this will deppend on the circuitry chosen, and leakage is not a issue for now.

                  Best regards,
                 
                                          Paulo 


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              • H.C. Croon
                Hi Paulo, Now there is more clearness. I suspect the impedance of 15KOhm which you mentioned. I think it should be 1.6K. It is 1/(2.pi.f.C) with C=2400 pF. By
                Message 7 of 7 , Dec 9, 2004
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                  Hi Paulo,

                  Now there is more clearness.

                  I suspect the impedance of 15KOhm which you mentioned. I think it
                  should be 1.6K. It is 1/(2.pi.f.C) with C=2400 pF. By the very small
                  bandwidth of the transducer it is not likely that their will be
                  another Ohmic impedance. You will have a resistor between the
                  transducer and the DC supply voltage, but for the signal this
                  resistor is seen in parallel with the capacitance of the transducer.
                  Also this resistor should be large compared with the 1.6 KOhm of the
                  transducer itself. (BTW is this resistor the 15KOHM?) So the
                  capacitance of the piëzo makes the impedance.

                  As for your question: You have to take a coupling capacitor which is
                  large compared with the 2400 pF of the transducer. So 0.1 uF would do
                  the job.

                  But .. a very important point is that the impedance of the opamp
                  circuit is large compared with the 1.6 KOhm of the transducer
                  capacitance in order not to have much attenuation and not to much
                  fase shift. Because the impedance of the transducer is at right angle
                  to a resistive input of an opamp circuit, a factor of 10 will do. But
                  a larger ratio is beter. I would suggest 47KOhm at least.

                  Regards, Harry

                  > Hello ListMembers, In first place, thanks for your responses. It
                  > was very Instrutive. Now I am looking for X7R and COG caps for
                  > outdoor applications (Mr. Alois) ! Reposting my question: Looking
                  > all over books and in the Internet, I found many different values
                  > for same applications. This, also for capacitors between amp-op.
                  > stages and for transducer input. In the case of the transducer, it
                  > is a piezo one. I have now a different supplier, with more specs.
                  > Its center frequency is 40 KHz (+ - 1 KHz), bandwith (-6 dB) 1.0
                  > KHz. The capacitance@(1 KHz + - 20%) is 2400 pF. Impedance at 40
                  > KHz is 15 K Ohm (!) The DC voltage is about 8V, this will deppend
                  > on the circuitry chosen, and leakage is not a issue for now.
                  >
                  > Best regards,
                  >
                  > Paulo

                  --
                  Author: H.C. Croon
                  INET: h.c.croon@...

                  Fat City Hosting, San Diego, California -- http://www.fatcity.com
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