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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
Message 1 of 7 , Dec 6 3:15 AM
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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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• ... 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 1 of 7 , Dec 6 4:11 AM
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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.

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
>

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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
Message 1 of 7 , Dec 6 4:41 AM
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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.

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@...

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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
Message 1 of 7 , Dec 6 7:40 AM
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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.
>
> 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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• 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 1 of 7 , Dec 6 9:20 AM
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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.

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!

• 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 1 of 7 , Dec 9 4:25 AM
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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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• 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 1 of 7 , Dec 9 9:35 AM
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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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