Hi Harry and Paulo, the value of 100nF sounds very reasonable for this application.
However, the 15 kohm which Paulo cited might be correct as well. These ultrasonic piezo trsnaducers are operated at or near their resonance frequency which means that they do not behave as simple capacitors but more like an RLC circuit with a comparatively large Q value. Uwe. HCC> Hi Paulo, HCC> Now there is more clearness. HCC> I suspect the impedance of 15KOhm which you mentioned. I think it HCC> should be 1.6K. It is 1/(2.pi.f.C) with C=2400 pF. By the very small HCC> bandwidth of the transducer it is not likely that their will be HCC> another Ohmic impedance. You will have a resistor between the HCC> transducer and the DC supply voltage, but for the signal this HCC> resistor is seen in parallel with the capacitance of the transducer. HCC> Also this resistor should be large compared with the 1.6 KOhm of the HCC> transducer itself. (BTW is this resistor the 15KOHM?) So the HCC> capacitance of the pi�zo makes the impedance. HCC> As for your question: You have to take a coupling capacitor which is HCC> large compared with the 2400 pF of the transducer. So 0.1 uF would do HCC> the job. HCC> But .. a very important point is that the impedance of the opamp HCC> circuit is large compared with the 1.6 KOhm of the transducer HCC> capacitance in order not to have much attenuation and not to much HCC> fase shift. Because the impedance of the transducer is at right angle HCC> to a resistive input of an opamp circuit, a factor of 10 will do. But HCC> a larger ratio is beter. I would suggest 47KOhm at least. HCC> 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: Uwe Zimmermann INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
