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Old 4th February 2013, 08:57 AM   #1741
fas42 is online now fas42  Australia
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Originally Posted by gootee View Post
Thanks, Frank. I'm not really worried as much about the higher MHz range; only up to the reciprocal of Pi times the fastest rise-time.
I would worry about the Mhz region from the POV of shorting out any RF interference that happens to intrude. So, don't get fancy but work to avoid any resonance peaks, that's the key thing.

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I was "assuming", without really thinking about it, that Z_target was a constant from say 20 Hz to 318 kHz, and then didn't matter, except that we would want it as low as possible, and with no impedance peaks, so that no HF resonances would get too excited.

So now I'm wondering, above what frequency DOES the z_target curve cease to matter?

Does the stuff above really mean that if the PSU impedance at 15.9 MHz is too high, a 0.1 Amp or smaller transient cannot be produced at the maximum slew rate without causing a rail disturbance that is greater than the Δv_max that dictated that impedance at that frequency?
From the POV of getting the amp to work properly, and very specifically the FB side of things, a rule of thumb I would use is to make sure up to 200kHz is good, the 10th harmonic of 20kHz. The key thing is to make sure that where the PSRR of the amp starts to look sad, the higher frequencies, that the power supply itself is still exhibiting excellent, low, impedance.

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Frank, you mention 1 mOhm; 0.001 Ohm. For the example with 5A/μs slew rate and 0-5A range, that would mean a worst-case Δv_max = 0.005 Volt.

Ignoring ESR, which, at very low frequencies, would be swamped by the capacitance when paralleling lots of electrolytic caps, anyway, to get 0.001 Ohm at 20 Hz we would need

C = 1 / (2∙π∙20∙0.001) = 7.96 million μF; almost 8 Farads!
Which is why I use regulation here to do the job ...

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PSRR is usually very good at lower frequencies, though. So as long as we have enough capacitance to produce the demanded current, then how much ripple we create shouldn't matter much, within reason
Yes, I'm not fussed about low frequency "wobbling" of the supply, it shouldn't be a problem, I just don't want it to sag beyond a certain point ...

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I guess we would need a plot of PSRR vs frequency, to be able to create a plot of acceptable ripple amplitude vs frequency, for a chosen maximum distortion level (or something like that).

...

We would just need to know what effect it might have, and how low the ripple needs to be at each frequency, and then we could decide how much capacitance was actually needed. Again, the PSRR might tell us those answers. I'll have to go investigate PSRR and ripple-induced distortion, now, I suppose. DF96, Frank, anyone? Any ideas about that?
One simple way of looking at it is considering the worst case, of a maximum amplitude 20kHz sine wave. If you know the PSRR for that frequency, and work out the supply ripple in your worst load for that signal, you then want to keep the ripple below the level where it will inject an error signal of say -100dB into the output. If 20kHz is OK then every other frequency should also be OK, unless your amp has bizarre characteristics ...

Frank

Last edited by fas42; 4th February 2013 at 09:00 AM.
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Old 4th February 2013, 03:51 PM   #1742
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gootee liked the ref in post 1739 . The use of polyprop in small values to lower impedance of the power supply above 220k hz looks good. Grant it that will be adding an other pole and zero to the equation given the addition. If the multi resonance are spread out over the band a wider and net flatter curve can be produced. Turning the power supply in to a multi unit device much like a speaker crossover does with a multi-driver speaker. The curve from a 3577a would I believe show this . Sorry I do not have one at this time to show it off. 3 or 4 stage of progressively smaller caps. to flatten out the curve will work. To get really low a regulator the choose however when current is high it may not be that easy to do and have hold up long term .

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Old 4th February 2013, 10:39 PM   #1743
fas42 is online now fas42  Australia
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The use of polyprop in small values to lower impedance of the power supply above 220k hz looks good.
If you use such it must be placed precisely at the point in the circuit where the low impedance at high frequencies is required. Say, across the legs of the power supply pins of the opamp -- otherwise, it's useless and may in fact create problems ...

Frank
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Old 4th February 2013, 11:14 PM   #1744
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Well, with a flex PCB (thin polyimide) and wide 3oz copper planes on both sides one could have the cap's (and the chip's) intrinsic inductance dominate the scenery even when it's an inch or two away... alas, not an option for DIY, and quite probably overkill (for chip amps).
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Old 5th February 2013, 01:58 AM   #1745
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Originally Posted by fas42 View Post
If you use such it must be placed precisely at the point in the circuit where the low impedance at high frequencies is required. Say, across the legs of the power supply pins of the opamp -- otherwise, it's useless and may in fact create problems ...

Frank
As I understand it yes. The inductance of the board traces would set up a resonance circuit if the cap is to far away from the ic.
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Old 1st March 2013, 12:55 AM   #1746
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Long time since any chatter here ...

This is a nice little, tight summary of what can be done about the inductance of power supply traces, here is about as good a place to link to it as any: Power Tip 56: Estimate PWB interconnect inductance.

Frank
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Old 1st March 2013, 09:53 AM   #1747
AndrewT is offline AndrewT  Scotland
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The Ti Author in the link from post1746 has got his cm mixed up with his mm (see Fig2).
How do we get these US based Engineers to think straight?
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Old 1st March 2013, 01:35 PM   #1748
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Originally Posted by fas42 View Post
Long time since any chatter here ...

Frank
I think we ran out of things to talk about :-)
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Old 1st March 2013, 02:17 PM   #1749
gootee is offline gootee  United States
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Originally Posted by AndrewT View Post
The Ti Author in the link from post1746 has got his cm mixed up with his mm (see Fig2).
How do we get these US based Engineers to think straight?
Yes, the units for the left half of that table (and in the accompanying equation) should be mm, not cm, and the inductances/mm in the third column should be divided by 10. OR, the numbers in the first two columns could be divided by 10.

The inductances are lower than I expected. It's looking like decoupling capacitor distances from power pins would not be as much of a concern as I had thought, if a ground plane were used, especially with a board thickness of 0.06 inch (1.52 mm) or less.
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Old 1st March 2013, 02:17 PM   #1750
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Andrew,
If the rest of the world would just capitulate and go to Imperial units all would be right with the world again! Just kidding, it is funny that anyone still is having troubles with metric units, we have been using them long enough for that to not be the case even if we are still using Imperial units also. There isn't really any excuse anymore, I can move back and forth without a problem and we have been doing it for long enough not to make silly unit mistakes.

On another note, I would still love to request that all the information that Gootee developed here integrated into a simple computer program would go a long way to solving many problems of implementing the solution that he developed. A fill in the blanks program that would be as simple as using a Thiel Small program for figuring out an enclosure problem or a simple network solution. A few fields such as power output, impedance and input voltage and a result of transformer size and capacitor requirements and move on to the rest of the circuit. I guess I can only wish that someone who is a computer programmer steps forward and writes the routine to do this and uses all of Gootee's hard work.

Steven
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