paralleling film caps with electrolytic caps
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 5th December 2011, 02:45 AM #222 gootee   diyAudio Member   Join Date: Nov 2006 Location: Indiana Yeah, I think I used peak-to-peak amplitude to calculate the "equivalent frequency" from the max slew rate, when I should have used 0-peak amplitude. I still can't remember which way is correct but I'm guessing 0-peak is the "amplitude" that I used when deriving the max slew rate equation. So a bunch of stuff in the example might be off by a factor of either 2 or 1/2. Last edited by gootee; 5th December 2011 at 03:03 AM.
 5th December 2011, 05:56 PM #223 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders The matter of whether the example has an error of 2 or 0.5 can be sorted later. What really matters, to me, is that the analysis shows that a 1us 10Apk signal can be supplied by a ~220uF cap, if esr and inductance are low enough. By scaling we can see that a 10Apk 10ns pulse can be supplied by a 2uF cap. and similarly that a 10Apk 1ns pulse can be supplied by a 200nF cap wrapped very closely around the device terminals. __________________ regards Andrew T.
 6th December 2011, 12:43 PM #225 marce   diyAudio Member   Join Date: Jun 2007 Location: Blackburn, Lancs Another good reference: http://www.interferencetechnology.co...ia/ITEM_01.pdf PCB inductance: http://www.polarinstruments.com/supp...ts/IPC1999.pdf Most if not all published figures for PCB inductance are based on a ground plane being present, as the return path is critical in calculating inductance. I'm currently on pages 436 onward of Mr Otts book as we are looking into planar capacitance and whether we use to many power islands as shown in figure 11.22...or whether a uninterupted plane pair with planar capacitance would be better. Last edited by marce; 6th December 2011 at 12:54 PM.
 6th December 2011, 12:51 PM #226 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders The flow and return paths are equally effective in resisting flow of current. __________________ regards Andrew T.
 6th December 2011, 12:56 PM #227 marce   diyAudio Member   Join Date: Jun 2007 Location: Blackburn, Lancs Yes but inductance is probably the most important, especialy as speed goes up, and inductance has to ttake account of the return path, trace resistance is a minor consideration for decoupling providing there is adequate copper for currents required.
 6th December 2011, 01:05 PM #228 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders the effective inductance of the circuit, as seen by the load, must take account of both the flow path and the return path. Neither is more or less important than the other. __________________ regards Andrew T.
 6th December 2011, 01:45 PM #229 marce   diyAudio Member   Join Date: Jun 2007 Location: Blackburn, Lancs I am aware (very) aware of the impotance of the return path, but to calculate the inductance of a trace without a ground plane is more complex, inductance is of the loop, and with decoupling of digital signals or for RF decoupling the inductance loop is of paramount importance, you cannot calculate a PCb trace inductance without taking into account the return path, and if you dont have a ground plane the calculations are harder and the trace inductance will be higher. So for star grounded layouts as you often do on the amplifiers you realy need to use a 3D field solver such as those from polar intruments or simple calcs can be done using one or two of the options in the calculator before. The figure Gootee quoted is for a microstrip trace. Inductance Calculator http://www.speedingedge.com/PDF-File...ance%20(2).pdf Just out of interest I am also collating inductance figures for small (0603-0402) chip capacitors and there routing, as with moving to HDI (high density interconnect) we can halve the inductance for a placed and routed 0402 capacitor.
 6th December 2011, 03:10 PM #230 gootee   diyAudio Member   Join Date: Nov 2006 Location: Indiana I'm sure that I made a lot of wrong assumptions and some errors. Still learning. Yes, all of those calculations (above) are probably quite optimistic unless a ground plane is used. With a one-sided PCB, probably the best we could do would be to put two traces as close together as possible. And even then, the characteristic impedance (sqrt(L/C)) won't get below a hundred Ohms. e.g. Two flat conductors, .0027 inch thick, each 0.020 inch wide, separated by 0.04 inch, on a glass-epoxy PCB, would have a characteristic impedance of 120 Ohms. For our power distribution loops we would rather have 1 Ohm or less! And THAT'S one reason that we need decoupling capacitors. They make a power/ground pin pair see a low impedance supply of current. Also note that without a low-impedance DC power rail, the voltage noise induced across the power rail impedance by time-varying current demands could feed into the amplifier input, which could cause oscillation. (Imagine a simple transistor amplifier and the resistors from rail to transistor base and base to ground. Power and ground rails are coupled to amplifier input, there.) So we basically need to provide a short circuit for AC from rail to ground, across the entire band at which the amplifier has gain, to avoid having voltage disturbances feed into the amplifier's input. Our decoupling capacitors should also provide that. That's also another good reason to use a low-pass input filter, if the amplifier has gain at frequencies past what we need. I still need to do some more learning, and simulations and experiments, to better understand how we might be able tio apply these types of concepts and calculations to simple DIY configurations. I am hoping that someone who already knows will jump in. I'm already imagining having to arrange and solder a dozen small electrolytic decoupling capacitors like the pistons in a twelve-cylinder radial aircraft engine and then solder two leads from the assembly to my board. Last edited by gootee; 6th December 2011 at 03:16 PM.

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