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Old 11th July 2012, 09:28 AM   #31
qusp is offline qusp  Australia
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how do the latest high performance micas and thin film caps compare vs c0g marce? was there any talk of them? they would seem to be more linear again vs F/S, but as yet the thin film types are not available in large enough values, topping out around 20pf, where the same caps start being mica. have you started playing with embedded thin film types fabricated right into the PCB.

@barrows: I would actually be more likely to use a larger cap or none at all with the Crystek for this reason, or at the least not adding more than is on the PCB already, unless you have access to a scope it would be pretty easy to cause resonance with the internal decoupling. follow the app note I guess, but something to be aware of.

i'm very interested to play with the new DSPLL types, like the Si570 from silicon labs, hopefully Ian becomes motivated in that direction again soon. finding suitably fast 22.1x and 24x parts for sync mode with 8xOSF enabled on es901X is not easy, with NDK being oretty much the only suitable set frequency model you can buy in singles.
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Old 11th July 2012, 12:16 PM   #32
marce is online now marce  United Kingdom
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For decouplingit is the size of the cap that is critical, as soon as the peak in the frequency Vs ipedance curve is passed, the cap becomes inductive and stores energy, not what we want. It is not so much actual capacitor value, but reducing inductance to the smallest possible value, and that means small packages and short leads.
I have done embedded components, actual 0201s in the PCB and planar capacitance using very very thin dielectrics (the problem of doing this is the signal propagation goes down because of Er so everything has to be nearerto cater for this, no free ride with high speed). I have also done 0201s between the pins of BGA components on the same side of the board, so you place the caps first then the BGA's. This combined with your top two layers as power and ground gives the lowest loop area and thus the lowest inductance , with micro vias you have only 0.1-0.05mm of travel for the loop, whereas a standard 1.6mm board with central power and ground has 0.8mm X2 loop area approx to travel through the board.
At the moment this is only a minority of boards where we do these sort of things, and usually where pockets are deep and reliability is paramount, probably over the top for audio, but when I get some spare time!!! I would like to do the ultimate DAC layout using all the tools I have at my disposal, just as a fun exercise. At the moment though I seem to always on site, spent 7 of the last 10 weeks away from home (but I do get to design boards for some pretty impressive kit.
Due to their small size almost all decoupling on comercial designs is performed by X8R, X7R - X5S multi layer caps, the better kit uses X7R's and above, throw away commercial products will use the lower dialectrics for cost. MLCC's X7Rs are the best choice for local decoupling, and with uF values becoming available are starting to replace elecs for the locla bulk caps (1-10uF), plus SMD electrolitics dont survive ballistic shock tests or extreme acceleration tests (150G +).

Last edited by marce; 11th July 2012 at 12:22 PM.
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Old 11th July 2012, 03:57 PM   #33
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"@barrows: I would actually be more likely to use a larger cap or none at all with the Crystek for this reason, or at the least not adding more than is on the PCB already, unless you have access to a scope it would be pretty easy to cause resonance with the internal decoupling. follow the app note I guess, but something to be aware of."

I am going to try and have a look at Crystek app notes if they have any. I am dealing with a commercial product, so there is only so much I can do, but adding/changing a single decoupling ceramic at the pin of the oscillator is pretty easy.

marce, hope that you have time to produce a DAC board design sometime, it sounds like you have the skills to really get it right!
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Old 11th July 2012, 04:08 PM   #34
marce is online now marce  United Kingdom
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I wish I didn't as I never seem to stop working, apart from the odd weekend I have worked every day since the new year!!! Been doing PCB design for 27 years now so I hope I have picked up something along the way
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Old 23rd October 2012, 12:59 PM   #35
zinsula is offline zinsula  Switzerland
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The most important parameter for clock supply is an as low as possible 1/f noise.
You cannot achieve this with a noisy supply and throw (small) decoupling caps at it. This helps for noise higher up in frequency, but at low f not, obviously.

The regulator or the battery has to have very low 1/f noise itself.
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Old 23rd October 2012, 01:28 PM   #36
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Quote:
Originally Posted by zinsula View Post
The most important parameter for clock supply is an as low as possible 1/f noise.
You cannot achieve this with a noisy supply and throw (small) decoupling caps at it. This helps for noise higher up in frequency, but at low f not, obviously.

The regulator or the battery has to have very low 1/f noise itself.
If you have a low noise, high output impedance supply, the clock is going to stutter, you're going to get logic indecision. That's just the way the physics works.

Since RS has some LiFePO4 in stock, I will test the Zout with the same apparatus used for the regulator bake-off featured in the Linear Audio V4. From what I've read in manufacturers' data sheets I'm going to guess it will be in the tens of milli-Ohms.
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Old 23rd October 2012, 01:35 PM   #37
qusp is offline qusp  Australia
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no, you cant just pick any lifepo4, some are pretty ordinary. the one most people are usually talking about is a specific variant made by A123 and the model # is ANR26650M1A or M1B. They have significantly lower impedance, better current delivery and lifetime than most. but watch out, its often faked and sold on the bay, you cant buy direct from the manufacturer and there are only a coupe of distributors that will sell legit new stock direct. if it wasnt so expensive to send it via Auspost as they havent upgraded their policy on polymer cells, I would send you one. perhaps someone over there will?

Last edited by qusp; 23rd October 2012 at 01:38 PM.
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Old 23rd October 2012, 01:47 PM   #38
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by jackinnj View Post
If you have a low noise, high output impedance supply, the clock is going to stutter, you're going to get logic indecision. That's just the way the physics works.[...]
Well, it depends how you define high output impedance. 10, 100mOhms? And at what frequency?

Btw, as far as I have seen, clocks usually do have quite a constant demand at LF, there will not be much sag even with highish output impedance.
As soon as you go into the MHz, output impedance is mainly depending on the decoupling network. Feedback based regulators behave inductively, because the loop gain will reduce with frequency.
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Old 23rd October 2012, 01:47 PM   #39
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Quote:
Originally Posted by zinsula View Post
The most important parameter for clock supply is an as low as possible 1/f noise.
You cannot achieve this with a noisy supply and throw (small) decoupling caps at it. This helps for noise higher up in frequency, but at low f not, obviously.

The regulator or the battery has to have very low 1/f noise itself.
Sounds like a case for maybe a Belleson with Bybees on its output! I know, some of you are laughing... but I have a couple of small Bybees here already...
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Old 23rd October 2012, 01:59 PM   #40
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Sounds like a case for maybe a Belleson with Bybees on its output! I know, some of you are laughing... but I have a couple of small Bybees here already...
Bad choice.

Bybees ranked last in our survey of 13 regulators. Belleson is prone to oscillation.

Edit: A123 went bankrupt 3 weeks ago -- with guess who as their largest creditor.
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