Power Supply for both analog an digital ICs. Ferrite bead? RF Choke? - Page 1
Forum section: electronics / beginners

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 16, 2015, 09:04:44 am
content: Hi there.I have another post open on the same project, but I have another (simple) question.It's about an audio cirquit that includes a ADAT converter, DAC, mixer, filter and headphone preamp. So there is analog and digital circuitry.Everything runs on the same 5V power. It's not ideal, I know. But I kinda set it as challenge to me.Obviously I don't want switching noise in my audio cirquits. I added bypass capacitors nearby the ICs as specified in all the data sheets.However, I have no experience for the actual power supply. I read that I needed to seperate the digital supply from the analog supply. A recommended board layout in tha DACs datasheet (http://www.ti.com/lit/ds/symlink/pcm1680.pdf) on page 31 just says to use an "RF Choke or Ferrite Bead".Now, I don't exactly know what to make of that. An RF Choke is just an inductor, right? Somewhere I found a cirquit using 68µH, but I have no Idea why.Ferrite bead is kinda strange to me. I'm german and the word "bead" is a little unclear to be honest.Is that just a chunk of ferrite? Or is it an inductor with a ferrite core? If I look through my suppliers, they have things with ferrite in the name and they are specified in Ohms. What dimension do I need?The system clock is in the 12MHz region, while the ADAT converter and DAC will run with a wordclock of 48kHz and a bit clock of 3MHz. An ATTINY25 is running at 10MHz. ... If that has any impact on the switching noise supression.Here is my power supply circuit:Power supply circuit imageIn this layout, would I connect my digital ICs to Vcc1 or to Vcc2?For anyone interested, here is the whole circuit that I came up with so far:Whole circuit as pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2015, 10:18:20 am
content: A ferrite bead is just that, a bead of some ceramic, like a necklace bead, if a bit squarer and less attractive.FBs are just about useless for power line filtering, because they saturate easily from DC. Proper inductors are preferred. The main range they're not as useless for is RF (in the 100MHz+ range), where they still retain useful impedance, even when saturated.The average FB is in the 1uH ballpark, so that's a good starting point.Actual design of the power supply filtering network goes as anything else: consider the voltage and current requirements of each load, and try to find what current they draw (AC and DC). The impedance (how many volts ripple are generated for some (AC) amps of load) and filtering (attenuation between load points) is determined by PSRR of each device.If the converters and logic are all conventional CMOS, they'll be at least moderately noisy, so some minor filtering may be helpful. You'd have to check PSRR of the DAC, amp, etc. (versus frequency!) to determine how much filtering is required though.Note that, putting inductors and capacitors together, makes a resonant circuit. You don't want a resonant power supply circuit. If done carelessly (and yes, that includes following many conventional rules of thumb), the impedance can be much higher at unlucky frequencies (resonances). The simplest way to address this is to find the nodes which have the highest impedances (the most ripple voltage for a given excitation, wherever that excitation might be applied at), and bypass those with lossy capacitors: the ESR is key. If you use only low-ESR capacitors, you push resonances around. ESR dampens it, keeping the worst-case impedance lower than would be possible using only low-ESR caps.This magic occurs when ESR ~= sqrt(L/C), the resonant impedance of the circuit. Actual values of L and C may vary (e.g., do you pick the capacitance of one 0.1uF bypass capacitor on its trace inductance, or is it a whole bunch of things in combination?). Sometimes it's better to build a SPICE model of the layout, and test its actual response.Tim

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 16, 2015, 03:19:17 pm
content: Hm, interesting. I could swear I saw an example circuit in some TI-pdf where they used a 5-15Ohm ferrite bead to do just that kind of noise reduction.As for general power supply filtering: I thought that was adressed by using bypass capacitors at my 7805 regulator and those at the power pins of the individual ICs.What I want to do is keep the switching noise of the digital ICs out of the supply voltage for my audio opamps.Will I run into problems with my 68µH choke?I think I am more concerned about audiable noise then about the ICs not working.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: June 16, 2015, 03:56:15 pm
content: lossy ferrite beads come in many current ratings, impedance curvesit really requires some reading, design of the PS component values and locationsand ideally a good look test equipment to see if you have inadvertently created resonators instead of filters

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 16, 2015, 04:54:08 pm
content: Thank you guys, so far!QuoteHm. Well, with the 100nF capacitor I get a resonance at 61kHz which well away from any clock that is working in the cirquit.With the 10µF one I get 6.1kHz, which is in the audio range. That would be bad, right?

author: SeanB
author_rank: Super Contributor
timestamp: June 16, 2015, 04:58:32 pm
content: Then damp the inductor with a parallel resistor, you probably will be looking at a resistor in the range of 10-50R to do so. Otherwise a resistor in series with the inductor if the current is low enough that the voltage drop is not going to be an issue.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: June 16, 2015, 06:47:06 pm
content: Forget about following datasheet recommendations of "place one XXX cap at each power pin". Rather,engineera proper Power Distribution Network.Check out these links as starting point:Lee Ritchey: FAQ Ferrite Beads:http://www.innofour.com/8440/news/literature/lee-ritchey-faq-ferrite-beadsSignal & Power Integrity - Power Distribution Network - Altera:https://www.altera.com/support/support-resources/support-centers/signal-power-integrity/power-distribution-network.htmlLike others point out, the goal is to achieve a steady enough voltage level as to not exceed the specified voltage ripple on the power lines. Local "decoupling" capacitors (really coulomb buckets aimed at providing quick, local electrons) are used to compensate for IR drop (DC drop due to power rail resistance) as well as dI/dt drop (DC drop due to inductance in power rails).Never put additional inductance in digital supply power rails as this will result in a lower capability of the power rail to provide power to fast switching circuitry. Better to properly calculate the inductance in each power rail and properly design each rail.More links explain how to do this properly:https://www.google.com/?gws_rd=ssl#q=power+distribution+network+design

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 16, 2015, 08:15:50 pm
content: Those are some interesting links!So ferrite beads prevent the power supply to react to sudden current requirements by the digital ICs. That would flatten the transients of all the digital signals, clocks, ect. It would decrease high frequency noise, but overall performance aswell. Not what one would want. To provide charge at "short notice", capacitors are used at each pin. That would make faster switching and steeper transients possible again.In the Lee Ritchey link, this solution is described as not beeing great. However, I do not understand why? Wouldn't this prevent the high frequency noise from propagating through the ferrite bead /choke while still providing the needed peak current for the fast switching?My fastest digital clock is 12.3MHz. So there is not even the need for extremely fast switching, right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2015, 08:37:37 pm
content: Quote from: Chaos_Klaus on June 16, 2015, 08:15:50 pmPerhaps. But read the list of four requirements.There's an extremely high likelihood that another component (like a proper inductor) meets those better. Thus, no ferrite bead design-in.Those same four rules should be applicable to every point in your circuit. Some kind of don't matter: pull-up resistors might collectively be chosen as 10k, because it's a common value. A value of >= 1k and <= 100k might be desirable for actual electrical reasons, but within that space, who cares. This should be the weakest justification in the entire design.Every component should have at least this level of justification: all resistor values and ratios in the amplifiers, all capacitors or inductors in the filters (both power supply and analog signal), all filtering / protection for external connections (inputs and outputs), all choices of ICs -- and a laundry list ofthings which are intentionally absent:surge protection, over/under/reverse voltage protection, ESD, EMI, shielding, etc.Obviously this doesn't matter a lick for simple or amateur circuits, but for developing a professional edge, you should be able to build your circuit like, well, like you'd write a theorem in geometry!So, this is why I asked earlier: what do the ICs require? Because our collective response fundamentally hinges on this quantity. We could instruct you that it's okay with nothing at all, but we'd be lying. Or we could "CYA" and tell you to build a massive filter between every chip, knowing it would be outright overkill -- but arguably less of a lie.This is the essence of engineering. Designing all parts of a system so they do their job optimally.QuoteThe edges of that clock will be much less than 10 nanoseconds, carrying significant energy into the 200MHz range! It need not go terribly far (many chips these days have integrated clock handling circuitry, so very little digital signaling actually meets the PCB), but it's absolutely a possibility of getting that coupled through the power supply, or self-radiating through traces, or transmitting RFI to other devices. This is, again, the domain of good design: keeping everything sealed up tight.Tim

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 16, 2015, 10:06:13 pm
content: QuoteOk, that makes sense.QuoteOK, So here are the ICs involved.digital supply:CoolAudio V1402(ADAT Decoder)ATTINY(Microcontroller for bitbanging the settings into the DAC)analog supply:PCM1680(DAC)OPA337(Opamp)TL974(Opamp)TPA152(Headphone Amp)I don't know what value to look for in the datasheet. Can someone explain?I'm sorry, you don't have to work it all out for me.I just add all the datasheets here for completenes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 17, 2015, 06:55:25 am
content: Quote from: Chaos_Klaus on June 16, 2015, 10:06:13 pmAh let's see.. it is as I feared. You have a shitty datasheet.Everything up to section 3 seems okay, but then this:Absolute Maximum RatingsSupply Voltage Vcc max 4.0VThere's no Vcc pin on the stupid thing! What idiot filled out this table!DC ParameterSupply voltage Vdd4.5-5.5VAssuming Vcc == Vdd, they're overvolting the thing by their own admission!I also don't get why Temperature should warrant being a DC Parameter. They already specified Tamb = 25C as a measurement condition. And 70C max is in the ratings. (BTW, 70C is kind of low, and you may be surprised to reach it under otherwise fairly ordinary conditions, depending on just what all goes into your project. If it starts misbehaving on a hot day, or something, there's not much you can do about it, it's simply not rated for that kind of use.)Other basic checks:V_IH, V_IL are kind of loose. 0.3 and 0.7 of VDD are more typical of CMOS devices. This implies they used a shitty CMOS process that's cheaper and less reliable (but hey, it only goes up to 70C, and it's presumably ancient 5V capable, so what do you expect?). These still shouldn't be hard to meet though.C_IN says 5pF typ, 1pF max. Which is it?V_OH, V_OL are fairly reasonable, but it's not stated what current they are measured at. Nor what voltage I_OH, I_OL are at. In general, the two pairs of specs are not measured at each others' level.Useful info: Supply current Idd = 5-8mA, I guess depending on mode. This is probably going to be accompanied by sharp spikes of some amount greater than this, but after the local bypass cap(s), it's probably going to look pretty flat.The AC Parameters are pretty brief. I would think to expect some basic stats about the protocol it decodes (how long does it take to process a packet, how good is the PLL, how stable is the resulting clock...). They could obviously go to considerable length documenting just what the heck it does, but sometimes datasheets are intentionally terse like that ("oh sorry, you have to buy the $10k license and NDA to get the full datasheet, this is just a teaser" -- or the Chinese ones just don't give you anything and you have to figure it out yourself!).Conclusion: crappy chip, but if it does what you want, it's probably OK. Special purpose brainy things often do that, sadly.QuoteGood, or good enough. Won't add much to digital noise. Could probably throw some analog functions in as well if you wanted (ADC temp reading..?! Idunno).QuoteOooh, 100mA, she's a biggun!Beware the logic pins are TTL compatible. TTL inputs can be connected to TTL or CMOS; TTL outputs cannot be connected to normal CMOS. It seems this is only a concern for ZERO1 and ZERO2, so if you don't need them, it doesn't matter.As a DAC, the performance specs are pretty appalling. They don't tell you this, of course: 100dB of dynamic range or SNR or whatever, and 0.002% THD+N, sounds great (and is more than sufficient for audio purposes). But 105dB is only 17.4 bits. 0.002% THD is 15.6 bits.So be fully aware that "24 bits" is only marketing wank. You'll never be able to measure, let alone hear, the difference of those additional 8 bits. They do not give any traditional DAC measures, like INL, DNL, SINAD, etc.A proper, professional duty, precision 24 bit DAC can get down to a few LSBs, but I don't think any are out there that claim a few LSBs at anything more than a few Hz bandwidth (i.e., pretty much DC). The SNR of space itself becomes dubious at that kind of precision (quantum fluctuations and all)!Because ears have about 100dB of dynamic range (below the pain threshold), and far less under average ambient conditions, it should be no surprise that 16 bit audio has been as pervasive as it is, since its introduction. Yes, you can play some gimmicks with extra bits (mostly numerical, for intermediate processing), but ultimately there's very little gained.QuoteThis looks good, but mind the downsides of CMOS amps: they're noisy (26nV/rtHz). I do like the PSRR. Note that the input common mode range goes not include the positive supply.QuoteHmm, this thing is really weird. 1.4mA source but 80mA sink? What the hell?The equivalent circuit shows PNP inputs, but these will not function up to +V as claimed. Their circuit is patently wrong. Very likely, a complementary design is used, but they should've shown that; they should also have a Vos vs. Vicm graph, which usually gives some evidence how the NPN and PNP combo works together. (Often, non-complementary parts will have this graph too, which usually suffices to show how the device fails when you exceed Vicm -- useful info when designing a circuit.)The GBW varies massively with supply voltage. It's advertised for its wide supply range, but if you actually used it over that range, you'd probably get an oscillator at some point! It's not hard to keep GBW stable. This thing is poorly designed.The only thing that is reasonable is the noise level, which is low as op-amps go. But it's not painfully so; Fig. 6 shows it rising below 1kHz (1/f flicker noise), and more steeply than usual. For comparison, the TL071 claims:http://www.ti.com/lit/ds/symlink/tl074.pdfabout 45 nV/rtHz at 10Hz, while the TL971 claims ~55. And that's an otherwise noisy JFET amp (its advantage is high impedance, so it's less noisy around high impedance sources -- noise is, in part, an input matching thing).The input bias current is also fairly high (~1uA). Most bipolar amps are in the 200nA range. (FET and MOS amps are in the nA to pA range, but they're also better for high impedances, like I said.)Here's a better "audio" op-amp for comparison:http://www.ti.com/lit/ds/symlink/lm833-n.pdfSame 4nV/rtHz noise spec, but it extends down to much lower frequencies (~7 at 10Hz, meaning very much less rumble/flicker noise than the '971). This is achieved with the same input bias current. PSRR is also much better (Fig 14 and 16, CMRR and +PSRR), which means less demands for filtering those supplies.Of course, the problem is, you can't use LM833 because it requires +/-5V supplies to really begin to be useful.You should seriously consider this: if you are going to the trouble of making a 24 bit system, it will be well worth the added expense and difficulty of using boosted voltage supplies. 24 bits is HARD. And I mean for the experts, it's hard. You have essentially no chance of getting true 24 bit performance here (even with a DAC that was fundamentally capable); you have even less chance of being able to measure and verify how many bits it's actually doing!QuoteHmm, kind of heavy on current draw for battery operation, if needed. Pretty noisy, but, you probably won't notice because it's a high level output.That said, out of a +/-2.5V range, 6uV noise is 40 LSBs at 24 bits, not counting the noise of anything else. 40 is a bit more than 2^5, so it's more like 19 bits already.I don't like how it's documented: they're only giving parameters and curves directly corresponding to its intended purpose, not its actual function -- which is a specially configured power op-amp pair, so it should really have op-ampy data. But like I said about special things, if it works it works.QuoteAt a basic enough level, you're going to have to look up the definitions in the first place. Sometimes these are provided in the datasheet (i.e., what negative current means, where to measure currents and voltages, what test circuits/jigs are required for DC and AC tests, etc.). Sometimes, it'll be typical for the family: a 74HC00 datasheet might be pretty short, but somewhere, the manufacturer will have to say that, yes, their product line meets the de-facto standard of everyone else's 74HC00s, and that everything else in that logic family meets the same input and output pin characteristics. So sometimes you can absorb it, piece by piece, after looking at an awful lot of datasheets and app notes. (No, this isn't very directed instruction...) Having a working understanding of the characteristics (using them in your own breadboarded circuits, and measuring DC parameters and AC waveforms), their range of behavior (even outside what the datasheet says), is also helpful, and reinforces that knowledge.Tim

author: Chaos_Klaus
author_rank: Contributor
timestamp: June 17, 2015, 08:51:36 am
content: Ok, WOW! Thank you for taking the time to look through the datasheets!!QuoteOh well. Even to my eyes that sheet looked a little funky.The main design criteria for my device was that it could decode an ADAT datastream. I found two, solutions. The first one used an FPGA and as soon as I looked up what that was, I got so scared that I immediately went for option nr 2 which was this CoolAudio chip that claims to do it all with an ordinary 5V supply and no programming involved.The problems started with the DAC. The ADAT stream (in my application) has 48kHz and 24bit. In terms of audio quality, I'd happily go with 16bit. 24bit is really just for beeing able to do heavy digital processing on the signal - which I don't want to do at all.Now, unfortunately this DAC forgets it's bloody setting everytime I power down the device. So I had to get that ATTINY in there just for that purpose.QuoteI will not use those. Should I connect them to ground, or leave them foating?QuoteI use this OPA337 to establish my virtual ground at +2.5V. I actually do not want that to be noisy. This opamp was used in the application circuit of the DAC ... I should really stop trusting those.The TL974. Well, I went for that one because I needed a rail-to-rail capable opamp for having enough headroom in my +5V environment. That one was reasonable cheap. They are not driving any high currents as they are all connected to other opamps.QuoteWell, I could just go for a higher voltage wall wart for that matter. I kinda wanted to keep it simple. I don't need extremely high performance from the device. 24bit is overkill basically. I just went for that because the ADAT stream I am tapping into happens to be 24bit.Now for the power supply question: How would I go about seperating the power supplies when I did not want to go with two voltage regulators? Do I even have to use an in line inductor? The switching noise is going to be outside the audiable range anyway, right?