Is it ideal to place a ferrite on LDO output in noise sensitive application - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: November 19, 2020, 01:57:26 pm
content: i have a 3v LDO that is powering a 24 bit ADC, and was wondering if its best practice to place a ferrite bead in series with the LDOs input and output. I have already followed the ADC development board schematics using the same LDO, but it doesnt have ferrites in them. Me thinking that a ferrite is an upgrade** thus this question.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2020, 04:12:10 pm
content: No.It might be justified for a variety of reasons, but I would not call that a default "best practice" step.Tim

author: drakejest
author_rank: Regular Contributor
timestamp: November 19, 2020, 05:08:30 pm
content: Quote from: T3sl4co1l on November 19, 2020, 04:12:10 pmThat is an intriguing answer. So ferrites can be detrimental in some cases? I have always assumed that they were a go to for power supply filtering for high frequencies.The reference design do however has and RC filter on it (0.1 ohms - 1uf - 0.1uf) so maybe thats the reason why its not needed?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2020, 05:43:51 pm
content: Well, they can always be used wrong, exacerbating things like inrush surge or regulator instability. Assuming they're used right, they're still adding BOM items, cost (if slight; they are rather cheap) and layout area.Without any additional information suggesting that your LDO needs the additional filtering, I would not bother.Tim

author: Benta
author_rank: Super Contributor
timestamp: November 19, 2020, 06:58:14 pm
content: Ferrite beads have their place, but this is not the right one.You may think you're filtering the output, but in reality you're preventing the LDO from regulating the voltage. The LDO relies on the voltage on the output pin to operate correctly.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 19, 2020, 07:01:20 pm
content: If anything, use at the regulator input.

author: Northy
author_rank: Regular Contributor
timestamp: November 20, 2020, 03:49:37 pm
content: What LDO are you using?G

author: wraper
author_rank: Unknown
timestamp: November 20, 2020, 03:58:17 pm
content: The most important is to not use it between LDO and decoupling capacitor. Otherwise you may have some real problems such as oscillating LDO. But it may be a valid option if you want to power multiple circuits from the same LDO and want to isolate noise between them. Say isolate ADC from the digital stuff but don't want to use additional LDO. And you may want to use it before LDO input decoupling capacitor if the voltage coming in is somewhat dirty.

author: tszaboo
author_rank: Super Contributor
timestamp: November 20, 2020, 04:37:39 pm
content: Unless you expect the LDO to generate noise in the few hundred MHz region.(Hint: they usually dont.)

author: RoGeorge
author_rank: Super Contributor
timestamp: November 20, 2020, 04:46:49 pm
content: The LDO output should be left with its normal filtering capacitor near the LDO, without any ferrite bead between the LDO out pin and the filtering capacitor.If you want/need to use ferrite beads, put them near the power pins of the ADC. Keep in mind a ferrite bead mostly "reflects the noise back" to the circuit where it came from. You can use ferrite beads before the LDO input capacitor if there is noise upfront (same thing, do not place the ferrite between the input capacitor and the input LDO's pin).The thumb rule is to just follow the manufacturer's recommendation. If it were to be possible to get more performance out of a given chip, the manufacturer would have figure that out already and describe that in an application note. Therefore, any *upgrade* would most probably in fact downgrade the performance, or let the performance unchanged at best.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 20, 2020, 09:01:34 pm
content: Which gets into the whole topic of "used properly". Ferrite beads are simply some impedance. If that impedance is detrimental to the circuit, don't use it. Or put something in parallel (i.e., bulk filter caps?) to stabilize it. Mind also, the impedance depends on bias current, and saturation typically sets in at a small fraction of rated current -- the rating is thermal only, and has nothing to do with its electrical performance. Check for curves; Laird usually provides these including bias effect.If you need to do a lot of filtering at high currents, simply use an inductor, proper -- you can get small values similar to ferrite beads if you like, and rather than relying on the somewhat nebulous impedance of the ferrite, the inductor's Q factor is usually on the high side so that you'll be providing damping explicitly (usually with bulk capacitors, but R||L and R+C pairings are just as handy).Filtering is a matter of source impedance seen by the load, and attenuation through the network (from one source or load to another). The PDN (power distribution network) is a network like any other, and we can solve for certain useful properties reasonably easily*. Once this can be seen, one can set up the equivalent circuit, say in SPICE, and tweak values for best results.*Easy in this case being more to the side of "there exists a solution"... not that a solution is necessarily easy to find, let alone trivial. But it is still simple enough that algorithms exist, i.e., computers can solve it.Tim

author: jkostb
author_rank: Regular Contributor
timestamp: November 21, 2020, 02:48:35 pm
content: As already commented by others ferrites should be used with care. They can saturate or cause oscillation and make things worse. One example where they can be applied is when you need to power sensitive analog electronics from a power rail derived from DC/DC converter. An LDO with high common mode rejection ratio can be powered from this rail. However high frequent switching noise will pass through the LDO and get into your analog electronics. You can reduce this noise by inserting a ferrite bead before your LDO.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 21, 2020, 03:53:50 pm
content: As said above: always put an output capacitor right at the output pin of the regulator. Do NOT place a ferrite bead BEFORE the output cap.With that in mind, the "usual" use of ferrite beads is rather *between* the output of some SMPS and an LDO (so at the LDO's input). You'll notice that it actually forms a Pi filter.Also keep in mind ferrite beads are essentially inductors, and non-ideal inductors of course. In particular, they always have some DC resistance, even if you can find SMD ones with relatively low DCR - still in the order of several tens of mOhms usually. So determining if noise would be lower with rather than without depends on the load and would require proper analysis. You can't just throw a random ferrite bead and some "vanilla" bypass cap to ground like 100nF ceramic, and call it a day.There was an earlier thread about this:https://www.eevblog.com/forum/projects/ferrites-in-power-supply-good-or-bad-how-to-reduce-noise-on-vcc/For a noise reduction technique that can be used with adjustable LDOs, you can read this:https://www.analog.com/en/analog-dialogue/articles/noise-reduction-network.html#

author: drakejest
author_rank: Regular Contributor
timestamp: November 22, 2020, 11:42:40 am
content: Thank you very much for all your inputs. From everything you said i think it would be best to just refrain from placing a ferrite in the first place.For those who are wondering the LDO is an ultra- Low noiseLP5709

author: sorin
author_rank: Frequent Contributor
timestamp: November 24, 2020, 06:57:09 pm
content: AN - Minimizing Switching Regulator Residue in Linear Regulator Outputs

author: KE5FX
author_rank: Super Contributor
timestamp: November 24, 2020, 07:13:21 pm
content: Quote from: T3sl4co1l on November 19, 2020, 04:12:10 pmI do it more or less religiously, but never between the caps and LDO, only on the outside of both. If nothing else, ferrites are cheap, and it's often handy to interrupt power on the input or output side for debugging/optimization purposes.Also nice to have ready-made footprints for actual inductors (or low-value resistors), if you find yourself in a jam at frequencies too low for the beads to be effective.Quote from: NANDBlog on November 20, 2020, 04:37:39 pmYou might be surprised. It doesn't take much noise to ruin your whole day. People underestimate how effective ADCs are at folding noise and EMI from the GHz range down to baseband. And PSRR generally goes to pot around the point where high-Z ferrite beads start to become useful.

author: nctnico
author_rank: Super Contributor
timestamp: November 24, 2020, 07:31:20 pm
content: Quote from: drakejest on November 19, 2020, 01:57:26 pmA ferrite bead does next to nothing in the frequency range a 24bit ADC is operating in. Most regulators (and most analog chips for that matter) suck at filtering high frequency noise; that is true. My default solution is to use a simple RC filter at the input of the regulator. But you have to be careful about line regulation.

author: poorchava
author_rank: Super Contributor
timestamp: November 24, 2020, 07:35:02 pm
content: I tend to us ferrite beads on power supplies to digital chips, or digital parts of mixed-signal chips, near the chip itself. Also, at the output of the LDO block (obviously not before the output caps). For analog chips and analog parts of mixed signal chips, from experience, a low value resistor usually works better, especially that analog parts are usually sensitive to noise in lower bands, where ferrites are ineffective.In general, as my work involves specialized T&M stuff, any trace longer than about 50mm will have some form of lossiness in series, either a bead at the source/driver or RC at the sink/receiver. For longer lines -both. This generally prevents parasitic detection and demodulation of RF during radiated immunity tests and also in normal usage (which in my case involves extremely high mains frequency electric fields gradients, that also often contain all possible kinds of noise from half of a city). This is sometimes omitted, where it would harm functionality, like USB, ETH, SDIO and such.Obviously, if this proves harmful, it can always be 0R'd in production.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 24, 2020, 07:51:49 pm
content: Quote from: poorchava on November 24, 2020, 07:35:02 pmShielding does not prevent interference from radiated immunity test? At what level do you test, more than 10V/m?

author: Northy
author_rank: Regular Contributor
timestamp: November 25, 2020, 10:05:18 am
content: Quote from: drakejest on November 22, 2020, 11:42:40 amIs the LP5907 not low noise enough?To the other people who replied saying they typically use an R/C, I know it depends on the current requirements, but what sort of values do you use?So is a an R/C before the LDO a good idea?G

author: wraper
author_rank: Unknown
timestamp: November 25, 2020, 03:15:44 pm
content: Quote from: Northy on November 25, 2020, 10:05:18 amIn general, no.

author: sam[PS]
author_rank: Contributor
timestamp: December 20, 2020, 09:07:16 pm
content: +1 for the low value resistor,in most case it's all that needed if ever needed. I generally use 10ohm in series on both GNDin and Vin with decoupling cap before and after (balanced PI filter). Ferrite bead is the next step, still in PI topology but most of the time it's overkill. I generally don't do it on the output unless there is a long cable between PSU and the business circuit, then i would put the ferrite on the business circuit board not on the PSU board and beware of the capacitor values to be not too big as it may induce instability as mentioned earlier.As i don't like to redesign things 10times i reuse the same pcb layout where there is place for the two caps and a power resistor footprint with a bit room around. So i can chose to populate juste 1cap and use a jumper for simple cases or populate the two caps and either small value resistor or a bead.Just my 2cts...

author: wraper
author_rank: Unknown
timestamp: December 20, 2020, 09:12:50 pm
content: Quote from: sam on December 20, 2020, 09:07:16 pmResistor value without consideration of input/output voltages and maximum current makes zero sense. Not to say you should use one only if there is actually such need.

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2020, 10:31:37 pm
content: On old Tektronix gear they often used a pi filter with like a 210 microhenry inductor in parallel with a 100 ohm resistor and 0.1 to 0.001 microfarad decoupling capacitors, and that is the sort of thing I have had the best results with by placing it close to both analog and digital loads to decouple them from each other. These have saved my bacon several times.Where low noise really matters, I roll my own regulator from an operational amplifier, reference, and pass element which is usually a cheap integrated regulator for its built in protection. Enclosing the integrated regulator inside of a feedback loop makes its own noise contribution insignificant.

author: bson
author_rank: Unknown
timestamp: December 22, 2020, 09:06:15 pm
content: It's worth noting that if you put the ferrite after the output filter cap you may want another (smallish) cap after it. Otherwise if the decoupling is overwhelmed and ripple in the form of switching/IO driver noise comes back it will reflect off the ferrite, go back out on the rail and very likely couple to signal outputs.

author: KE5FX
author_rank: Super Contributor
timestamp: December 22, 2020, 09:19:36 pm
content: Quote from: David Hess on December 20, 2020, 10:31:37 pmThat's interesting, so you're basically driving the adjust pin of an LM317T or whatever with an opamp instead of a resistor?

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2020, 05:04:18 am
content: Quote from: KE5FX on December 22, 2020, 09:19:36 pmThat is right, although if the operational amplifier is powered from the output for better line rejection, then the 1.25 volt difference might not be quite enough for the operational amplifier to drive, but there are ways to handle that. Below is the example I use but my implementation has some changes; I use a PNP in place of the JFET and at least layout the parts for type 3 frequency compensation which will be required for a low noise operational amplifier because of its higher bandwidth. A JFET might be measurably better but is much more expensive.A few years ago I laid out a board for a 10 volt 1 amp version using a 7805, LT1007, and 6.9 volt reference. Noise was dominated by the reference as expected and load and line regulation was not measurable even at the microvolt level.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2020, 08:21:49 am
content: The only downside to the method, is the feedback necessarily runs out at high frequency; that's the meaning of compensation.And this isn't much of a downside, because the follower (PFET/PNP and LM317/etc.) doesn't have all that much noise up there to begin with (roughly speaking, >10kHz), and that frequency range is easily filtered with LCs if you really need it. At low frequencies (where LC filtering is difficult), the control loop does a very nice job indeed.Tim

author: Miti
author_rank: Super Contributor
timestamp: December 24, 2020, 01:43:27 pm
content: Quote from: wraper on November 20, 2020, 03:58:17 pmWell, as a general rule, this contradicts Jim Williams' AN101. Are you saying never place a bead before the output capacitor, or don't use it in his particular case? What if he uses a 5V switching AC/DC adapter to power this circuit? It would be exactly the case where Jim recommends inserting a bead before the input and output filter capacitors.

author: wraper
author_rank: Unknown
timestamp: December 24, 2020, 02:11:31 pm
content: Quote from: Miti on December 24, 2020, 01:43:27 pmIf you use ferrite bead with fairly low impedance, it might work OK for many LDO. However without extensive testing you cannot be sure that circuit is stable. In other words, unless you will characterize it extensively, do not do this.

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 24, 2020, 08:40:57 pm
content: Quote from: RoGeorge on November 20, 2020, 04:46:49 pmIs that so? So far I understood ferrites use a very lossy (at high frequencies) material which will not impeade low frequency signals but block high frequency by dissepating the energy as heat in the magnetic material?

author: sixtimesseven
author_rank: Frequent Contributor
timestamp: December 24, 2020, 08:45:42 pm
content: I stumbeled over this topic some years ago trying to design a very low noise supply rail for some fog gyros and found this:https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf

author: Northy
author_rank: Regular Contributor
timestamp: December 28, 2020, 01:59:37 pm
content: This thread seems to be somewhat going round in circlesG

author: Simon
author_rank: Unknown
timestamp: December 28, 2020, 07:21:09 pm
content: make sure you have enough capacitance to the regulators output, then if you need to stop other things on the same rail causing a problem have your ferrite followed by some capacitance. The ferrite will slow down the recovery from a voltage drop so could be counter productive without some serious local bypass to the chip, maybe better to make sure the ADC has it's own regulator. Putting a ferrite on the input will not help at all as it will just throttle the response of the regulator.I put ferrites in as you describe but not for the reasons you do it. I do it to stop noise from the chips propagating up the power line like the customers power line.

author: prasimix
author_rank: Unknown
timestamp: December 28, 2020, 11:04:53 pm
content: A snapshot from TIevaluation modulefor 24-bit ADS131E08 ADC with surprisingly big filters with 470 uH inductors on LDO outputs:

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2020, 04:14:34 pm
content: Quote from: 3roomlab on December 29, 2020, 01:54:06 amIt is so place a resistance in parallel with the inductance. Tektronix commonly used 100s of microhenries and about 100 ohms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2020, 08:07:01 pm
content: The resonant impedance is around sqrt(470uH / 10uF) ~ 7 ohms. If the inductors have around 7 ohms DCR, that covers things just fine. Which, with a value that large, may well be the case?Tim