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Old 19th October 2007, 06:53 AM   #121
GK is offline GK  Australia
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Quote:
Originally posted by Eva
Of course you can get 20dB lower impedance at 50Mhz (or at 500Mhz for that mather) but it is at the expense of 20dB higher impedance around 5Mhz... This is not an advantageous trade for audio because most amplifiers have its unity gain around this frequency and rail resonance will degrade phase margin. Every added capacitor produces a resonant peak. Measuring impedance at a single frequency tells very little information.

Note that I'm making my best effort to believe you because I have *never* seen dissimilar capacitors paralleled locally and directly in RF circuits like TV and radio tuners, though. Two or more dissimilar capacitors placed together and connected in parallel are only routinely seen in exotic audio equipment.

I just gave a single frequency as an example and I did actually mention the relevance of parallel resonances over a broad frequency range.
If you have never seen dissimilar capacitors paralleled locally in RF circuits then you honestly can not have looked at many - it is a standard RF design practice.

Would you like me to start posting up circuits?
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Old 19th October 2007, 07:56 AM   #122
AndrewT is offline AndrewT  Scotland
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Hi,
if we do ignore Eva's advice (at our circuit's peril) and parallel caps, then presumably we fit the smallest at the opamp/chip pins and fit the electroltytic somewhere down stream.

What is the significance of the trace length between these local and distant bypass caps?
Is there a recommended gap between them?
How much inductance and resistance is necessary to avoid resonant interaction?

What if we add a medium value film cap between the pin mounted ceramic and the distant electrolytic?
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Old 19th October 2007, 09:46 AM   #123
GK is offline GK  Australia
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Quote:
Originally posted by AndrewT
Hi,
if we do ignore Eva's advice (at our circuit's peril) and parallel caps, then presumably we fit the smallest at the opamp/chip pins and fit the electroltytic somewhere down stream.

What is the significance of the trace length between these local and distant bypass caps?
Is there a recommended gap between them?
How much inductance and resistance is necessary to avoid resonant interaction?

What if we add a medium value film cap between the pin mounted ceramic and the distant electrolytic?

I'm logging off the net for the weekend now, but here are a few quick comments:

If you tried to bypass some fancy real high bandwidth audio opamps with electrolytic capacitors, you'd most likely have great deal of success at building an oscillator.
Don't just take my word for it, download the datasheet for just about any high performance opamp (audio or otherwise) and read the bypassing recommendations. 100nF at the supply pins are recommended virtually universally. This is not because 100nF caps are cheaper than 10uF electro’s, but because a low impedance supply bypass is required at rather high frequencies. If the supply leads are long, then parallel electrolytic capacitors are always recommended in addition. For really wide bandwidth opamps (especially video drivers, which require bypassing from nearly DC to 100+MHz), 10uF(or greater)+100nF+1nF might be the recommendation - but this is getting beyond audio.
Look up the bypassing recommendations for the 50MHz LM6161, for example – an IC which isn’t even that fast. The recommendation is 100nF in parallel with a 2.2-10uF tant.
Look up the bypassing recommendations of some audio power IC’s (say, 1W and up). You’ll find similar recommendations - but the electro’s in parallel with the recommended 100nF will be much larger.

Generally, when paralleling caps of different values it is generally necessary to keep the tracks between them as short as practicable to keep parallel resonances at bay.

Cheers,
Glen

Oh, and here is a pic for eva (see MRF1517T1 datasheet):
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File Type: jpg eva.jpg (68.2 KB, 758 views)
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Old 19th October 2007, 10:16 AM   #124
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Quote:
Originally posted by G.Kleinschmidt




Nearly anything over 100uF falls in a heap above a few tens of MHz, and degrade significantly quite a lot earlier.
I do a lot of mixed signal RF / audio stuff where stages are decoupled from each other with RC, LC (or combination) filtering of the supply rails, where it is necessary to decouple the rails for both audio frequencies and RF frequencies. A small 100nF MKT capacitor (for example) in parallel with a typical 100uF electrolytic can easily give a 20dB or more reduction in the impedance bypassing the rail to ground at 50MHz.
I also came to the conclusion that the large e-caps act inductive at higher frequencies, see my early posted measurements.
But these measurements also showed poor HF properties of MKT caps. How do you get good HF behavior from them?
The only thing during my measurements that showed good capacitor properties up to 110MHz was a SMD chip cap. Furtheron I found another very nice property in the X7R ceramic chips. Quite some HF losses resulting in nice damping of oscilations.
http://www.diyhifi.org/forums/viewto...t=1203&p=30452
I am becoming more&more a fan of X7R chip capacitiors!
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Old 19th October 2007, 03:51 PM   #125
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Hi,
Thanks to all for the discussion so far. I'm trying to learn here.
I'm interested in SMD film caps.
I found this (commercial) info which compares dielectrics:
they state that MKT (PET-HT ) and MKI (PPS) perform better than NPO and X7R (and Tant for that matter) in several, IMHO, critical features, like ERS, dielectric absortion, aging, mode of failure, non-linear distortion (3rd harmonic)...

http://docs-europe.electrocomponents...6b80066c93.pdf

Who has info to contrast this statements?

Regards,
M
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Old 19th October 2007, 05:25 PM   #126
Eva is offline Eva  Spain
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Would you, at least, agree that the common practice shown in this randomly chosen picture, which describes the particular way in which most audio related people understand the concept of "stiffening supply rails at high frequencies by paralleling capacitors" is completely useless?

Click the image to open in full size.

Of course, if you stack two 0603 SMD chip capacitors under an electrolytic and near the load, you are leaving little room for resonance, but this is not what audio people does.


AndrewT:

If you connect capacitors in parallel, any wiring inductance between them will make resonance much worse. They must be paralleled together, like soldering a SMD chip ceramic to the legs of a thru-hole electrolytic. If you parallel them away from the IC, then wiring inductance cancels any improvement in HF supply impedance. If you parallel them near the IC, then you need a set of capacitors for each IC and you have to isolate each set from the others with inductors and/or resistors to prevent mutual resonance.

Figure out what happens when you have several small supply bypass capacitors randomly spread along a large PCB and all them are connected together in parallel through long tracks. This usually results in a system with several resonant modes and unexpectedly high HF supply impedance in the 100Khz to 10Mhz range. A system with less capacitors is likely to exhibit lower impedance in this frequency range.
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Old 19th October 2007, 09:59 PM   #127
jnb is offline jnb  Australia
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Quote:
Originally posted by Eva
Would you, at least, agree that the common practice shown in this randomly chosen picture, which describes the particular way in which most audio related people understand the concept of "stiffening supply rails at high frequencies by paralleling capacitors" is completely useless?
What if the electrolytic is large enough that it becomes resistive at the top of the audio band, and Cbypass is chosen to be small enough not to have significant reactance below that frequency, and what if at that frequency, the wiring to the circuit does not have significant inductance?

Further bypasses at the circuit being optional and a separate question.
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Old 20th October 2007, 12:47 AM   #128
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@Maxlorenz:
The key to your question is: '...perform better...'
Low ESR and especially low dissipation factor at high frequencies is not always desired. Low dissipation factor in HF is resulting in low damping of parasitic resonances.
What you rate good is always depending on the application. Normally I would rate a formula 1 race car to be the better car than the Chinese Landwind. But I you have to go on unpaved roads ... you know... the Landwind will make the race, no question. (BTW: Opposite from common thinking China has a lot of very good roads and they are pushing their infrastructure in an absolutely unbelievable way!)

Also your data sheet gives no information about the HF properties. No graphs about Z vs. f ....
Coming from the stacked construction I would guess that these SMD caps have fortunate high resonance frequencies, but having this specified (or measured on your own) would be a good idea if you want to use it HF filtering.
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Old 20th October 2007, 12:54 AM   #129
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Quote:
Originally posted by jnb

What if the electrolytic is large enough that it becomes resistive at the top of the audio band, and Cbypass is chosen to be small enough not to have significant reactance below that frequency, and what if at that frequency, the wiring to the circuit does not have significant inductance?

Further bypasses at the circuit being optional and a separate question.
In my measurements there was not a single e-cap, which I would call 'resistive above xxx'. They all went inductive at HF. Resistive Z I only found around the resonance frequency.
Nevertheless I was positively impressed about the behavior especially of the lower capacitance e-cap types for higher voltage...
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Old 20th October 2007, 01:39 AM   #130
jnb is offline jnb  Australia
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Quote:
Originally posted by ChocoHolic
They all went inductive at HF.
What if we were to define a threshhold. e.g. that it is resistive +/- 45 degrees. This is how I understood Eva when, IIRC, she suggested that it never goes inductive.
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