Good app notes are great to find; TI is worth mining these days, much like National Semiconductor was in the days of disco. They still have Bob Pease's [1] columns on their site:
http://www.national.com/rap/
[1] You want to know this guy if you're doing opamps.
http://www.national.com/rap/
[1] You want to know this guy if you're doing opamps.
Tuning series resonance with a cap for decoupling purpose is a very bad design practice. Hopefully the guys have measured and build once the things (as I did) they're suggesting. I completely disagree with them. Using very small caps in the pF range for power supply blocking definitely does not work. The impedance of such small caps is some ohms (at about 1 GHz) to some hundred or 1000 ohms (about 1 to 10MHz) in the relating frequency area. As well as the recurrent advisings of using different caps for different frequencies. The impedance of a cap for f >> 1/(2*Pi*sqrt(Le * Co)) is deteminated by its parasitic inductance only: Z = jwLe
For a typical SMD 0603 chip capacitor Le is about 0.4nH, giving an impedance of about 0.3ohms @100MHz and 3ohms @ 1GHz. If someone would increase this impedance by adding an additional capacitive component, go on.
If f << 1/(2*Pi*sqrt(Le * Co)) the impedance is
Z = 1 / jwCo
with C1 = 30pF and C2 = 100nF the larger cap gives 70dB more noise filtering than the smaller one. Only if f = 1/(2*Pi*sqrt(Le * Co)) small band decoupling is sufficiant. Improving power supply decoupling can only be done with lowering the parasitic inductance of the caps, i.e. using smaller cases like 0402 or 0201.
This leads to very simple rules:
1. Use the smallest possible case
2. Use the highest possible value for decoupling
3. Minimize the area for the current loop.
4. Use power supply tiles
I won't get starting a discussion for power supply transmission lines here, but for high frequencies in the GHz range explaining theory with lumped elements is adventurous...
Something I've forgotten: I've the designers have designed their chips wrong (like using pins 4 and 8 for an SOIC-8 case), nothing can be done for improving power supply decoupling, as the intrinsic impedance of the power supply loop is deteminated by its dimensions. For best decoupling results power supply pins MUST be located in direct proximity.
What do you think (or what have you mesured) about BG's
super-E-cap configuration? That is non polar BG in antiparallel arrangement. They claim supression of inductance through mutual cancelation.
My limited experience has been favorable, soundwise. (safe one occasion in wich I got ringing on PS decoupling going from 100nf to 100+100nf in superEcap). Why I go into things that I don't understand anyway
You can expect lower ESL and ESR by paralleling, that is standard electrical engineering. But cancellation? That is hard to swallow!
As an example, ESL in wirewound resistors is cancelled to some 80 or 90% by bifilar winding, i.e. winding with two wires close together and then connecting them in opposite polarity. That appears not possible in caps.
Jan Didden
As an example, ESL in wirewound resistors is cancelled to some 80 or 90% by bifilar winding, i.e. winding with two wires close together and then connecting them in opposite polarity. That appears not possible in caps.
Jan Didden
That's the explanation I understood: each cap has a constant foil "winding" orientation; by antiparalleling them windings are opposite.
http://www.octave-electronics.com/Parts/cap/bg_tech.shtml
Thanks.
M
Anti-parallel windings only work when they are in close proximity to each other as the magnetic fields have to couple in order for the inductance to be cancelled. This is impossible in two separate components.
Note that paralleling two of these, anti-parallel or not is going to result in a reduction of inductance to half the value of the caps by themselves.
The claim seems bogus, the same benefit is achieved afaik by paralleling any two similar caps - the same is true of ESR incidentally.
OTOH I do like black gate capacitors anyway for their apparently good linearity at audio frequencies.
Kevin
Note that paralleling two of these, anti-parallel or not is going to result in a reduction of inductance to half the value of the caps by themselves.
The claim seems bogus, the same benefit is achieved afaik by paralleling any two similar caps - the same is true of ESR incidentally.
OTOH I do like black gate capacitors anyway for their apparently good linearity at audio frequencies.
Kevin
Another thought just crossed my mind. Since these are non polar types that essentially means there are two back to back electrolytics inherent in their construction - this means that the inductance is probably more than twice that of a comparably sized polarized capacitor.
Seems like using polarized types properly might be a better way to go from a purely engineering not subjective viewpoint. Or use a film cap if the value is reasonable and there is room.
Kevin
Edited to fix film cap description
Seems like using polarized types properly might be a better way to go from a purely engineering not subjective viewpoint. Or use a film cap if the value is reasonable and there is room.
Kevin
Edited to fix film cap description
Looking at the curves seems to indicate that at low frequencies they behave just like two conventional capacitors of equal value in parallel.
I am not sure whether or not I believe the curves showing their behavior at higher frequencies. Most el caps exhibit sharply rising impedance above a MHz or so..
The claims made in this advertising copy essentially tell me nothing about the technology employed in the capacitor. Most companies japanese or otherwise have credible technical white papers on their technologies, but this is a bad translation of what is clearly marketing hype aimed at the (japanese) high end audiophile market.
BG capacitors are expensive enough that they have not achieved widespread market penetration in consumer products despite the claims. I have seen them in a limited number of higher end limited production products from Sony, Pioneer and others. (Boutique brands) I used them as well, because of the hype surrounding them and because they did not degrade performance noticeably. Their THD performance is better than average and this is somethign I verified long ago with the NX type. Nichicon Muse and several others are also very good and cost much less. Note they aren't bad caps, in fact as far as electrolytics go they are better than average, but the claims made for them would be hard to believe under the best of circumstances.
I generally don't use electrolytics in the signal path if I can avoid them..
This is just my opinion.. Your milage and opinions may differ.
Kevin
I am not sure whether or not I believe the curves showing their behavior at higher frequencies. Most el caps exhibit sharply rising impedance above a MHz or so..
The claims made in this advertising copy essentially tell me nothing about the technology employed in the capacitor. Most companies japanese or otherwise have credible technical white papers on their technologies, but this is a bad translation of what is clearly marketing hype aimed at the (japanese) high end audiophile market.
BG capacitors are expensive enough that they have not achieved widespread market penetration in consumer products despite the claims. I have seen them in a limited number of higher end limited production products from Sony, Pioneer and others. (Boutique brands) I used them as well, because of the hype surrounding them and because they did not degrade performance noticeably. Their THD performance is better than average and this is somethign I verified long ago with the NX type. Nichicon Muse and several others are also very good and cost much less. Note they aren't bad caps, in fact as far as electrolytics go they are better than average, but the claims made for them would be hard to believe under the best of circumstances.
I generally don't use electrolytics in the signal path if I can avoid them..
This is just my opinion.. Your milage and opinions may differ.
Kevin
Black Gates are not suitable for decoupling purpose in the GHz range. The impedance of a (planar MLCC) cap for very high frequencies is only determinated by its size, no technology can improve this. As the size of that Black Gate Al canned caps is much higher than the small MLCC caps, the inductivity also is. Haven't measured this, but it can't be lower than about 3-4 nH.
But as most ones don't build Cell Phones, Bluetooth or WLAN stuff, that isn't so important here. A decent analog decoupling can be done up 10 MHz or so by using (small) electrolytics, as their inductance is only about 10nH. Analog stages typically don't show cross conduction, as digital stuff does. In this case it's easier to decouple OP-Amps in a decent way than switching, digital ICs. The layout is what matters. Guido has a very good article about this.
Gentlemen:
I value your feedback on superE-cap's controversial topic.
I lack the knowledge and instruments to test the inventor's statement.
Until now, if I understood well, you provided solid arguments based on conventional knowledge about why this superE-cap magical decaying impedance is not possible.
Would it be too difficult to test it?
I appreciate your effort.
Kind regards
Mauricio
I value your feedback on superE-cap's controversial topic.
I lack the knowledge and instruments to test the inventor's statement.
Until now, if I understood well, you provided solid arguments based on conventional knowledge about why this superE-cap magical decaying impedance is not possible.
Would it be too difficult to test it?
I appreciate your effort.
Kind regards
Mauricio
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.