LC filter op amp supply (ultrasonic range PSRR) - Page 1
Forum section: electronics / beginners

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 03:58:41 am
content: Are there any practical limitations for L values if one wants to add a LC filter to an op-amp supply rail in order to improve rejection?A 100uH+4.7uF ceramic at the input would enhance the PSRR of my op-amp nicely at the ultrasonic frequencies, which I am interested in. Also maybe a 10R resistor to kill the resonance.1 mH + 4.7uF ceramic at the input would be even better.. only 40 cents for the 1mH inductor...Is there any problems with going even higher in inductance? I have only ever seen it talked about in the context of higher frequencies, so orders of magnitude less inductance. I noticed that in general in electronics the "practical" implementation of the component in question usually has hidden demons once compared to the 2 orders of magnitude different solution.chip inductors are pretty cheap, 30 cents for 100uH that saturates at 15mA (i expect the op-amp to draw 2.5mA per rail).the PSRR of my amplifier gets pretty bad at these frequencies. How about 1mH inductor?I assume the inductor needs to be sheilded, is this correct? (I am limiting my search options to shielded on digikey).I realize the 1mH would saturate lower, and that DC saturation current must not be exceeded. but this part I found is good for at least 10x of what the op amp needs..I intend to have it the RL before the decoupling capacitor connection.or should I keep the L small and go for a bigger capacitor (10uF ceramic 50V should be good even with voltage derating? even bigger (when to start decoupling with 100nF? I'd like to avoid buying 100nf capacitors too..)).. or is it so easy that I could make both big? 1mH + 10uF... 0.6 dollars per op amp only, and 1210 packages... sounds like a good deal if it works.my op amps are running at <100KHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2014, 09:24:37 am
content: It'll also resonate like a <Samuel L. Jackson's favorite word>.May not be a problem with a soft start supply, or line frequency rectifier, but it will be a problem if the rails ever short, or the amplifier sees a particularly unlucky frequency. These will excite the resonance and give you exactly the opposite of what you were hoping, or create destructive voltage overshoot!If you need a *lot* of attenuation, you're probably better off with something like a capacitor multiplier (an RC filter into an emitter follower), LDO (or normal regulator), or some combination thereof.The C mult. will be fine above whatever the cutoff frequency is, as will the LC filter. Residual (i.e., it's not actually going to be -160dB at 100MHz or whatever) should be in the -40dB or better range; the transistor is limited by capacitor ESR and Early effect, and the inductor by parasitic capacitance and losses.LDOs tend to be poor at high frequencies, but some do reasonably well (maybe -20dB to -40dB). Skim lots of datasheets.As for the LC, damping is best provided by a big beefy cap, which has relatively high ESR.For signal purposes, this is how supplies work: you want enough series inductance to achieve the filtering you need, with low enough DCR to keep the voltage in tolerance. You want enough low-ESR filtering (ceramics, film, aluminum polymer) to achieve the high-frequency attenuation you need. And finally, you want 2-10 times that capacitance, in a not-really-low-ESR type (such as tantalum or electrolytic; otherwise, a low-ESR type with an explicit series resistor). You certainly don't want too high ESR either; the idea is to get the right value.The bulk capacitor does very little to supply current to the circuit, and rather serves to dampen resonances.The overall circuit looks like this:Code:In o--LLL--+-------+---o Out| |=== C1 === C2| |--------+-------+---- GNDcall C1 the ceramic and C2 the bulk.A reasonable equivalent would be:Code:R2+----RRR-----+| L1 R1 |In o--+--LLL--RRR--+--+-------+---o Out| || === C2=== C1 || R R3| R| |----+-------+---- GNDShowing:- R1 = DCR of L1. Helps dampen resonance. Use as large a value as you can, given your supply requirements. Example: a 5.00V +/- 5% supply delivering 1A. 5% is 0.25V sag, at 1A is 0.25 ohm -- but notice supply tolerance and droop should be subtracted from that tolerance first.- R2 = EPR of L1. Limits HF attenuation. Can be increased by adding more (smaller value, higher Q) inductors in series, to make a composite inductor that's really good, or by using multiple LC stages (follow the same approach, with each cutoff frequency maybe a decade higher, and minding the effect of the adjacent filters on impedances and stuff).- C1 is the low-ESR cap(s), shown without ESR because it's going to be small relative to the other impedances here (big ceramics, and aluminum polymer, are usually in the 50mohm range or below).- R3 is the ESR of C2, the lossy bulk cap.Any time you have an inductor and capacitor together, there's a resonant impedance Z = sqrt(L/C), and if your circuit has resistance about as much, the Q will be low, and so will the overshoot/ringing. R1 and R3 act to dampen the resonance, so, choose R1 + R3 such that sqrt(L1/C2) < (R1 + R3) < sqrt(L1/C1).If the resistances are poorly matched, you will get resonances, something that's easy to miss following a cookbook method ("place bypass caps at the supply pins!!!!11"). This analysis can be extended to every single capacitor, trace, ground plane and inductor in the power supply, but following a more nuanced approach will probably suffice.The cutoff frequency is somewhere between 1 / (2*pi*sqrt(L1*C1)) and 1 / (2*pi*sqrt(L1*(C1 + C2))). Since the circuit is well damped, attenuation will probably drop gradually over that range, and only really get going at a few times the cutoff. You'll get great attenuation (>20dB) at >10 times higher. So, if you need a lot of attenuation above some frequency range, plan on setting the cutoff frequency well below that band.At low frequencies (DC up to maybe half the cutoff frequency), R1 is dominant and you only have to worry about series losses.At medium-low frequencies (near cutoff -- which is also near resonance), C2 becomes a small impedance, and effectively, R3 gets connected in parallel with C1. The impedances of L1 and C1 are similar. This isn't ideal damping (better would be in series, so you can get a critically damped series resonant circuit, L-R-C from In to GND -- but that may be impractical), but it's good enough.At medium-high frequencies, L1 and C1 dominate, and you get a -40dB/decade filter slope.At high frequencies, L1 becomes a very high impedance, and R2 dominates; C1 is very low impedance, and its ESR, though small, also becomes important. The attenuation levels off at the resistive voltage divider of R2 into C1 ESR.At still higher frequencies, L1's parasitic capacitance, C1's ESL, and the geometry of connecting traces and vias, all become important and attenuation may have complicated peaks and dips. For most components, this will be in the >100MHz range.If you need good attenuation well into that range as well, consider using a linear layout (trace enters capacitor's pad from one side, leaves through the opposite side; vias to ground placed either side of the cap, not lengthwise off the end, and certainly not on thin traces), multiple parallel caps in a row (the ESL between caps helps mitigate the ESL of each cap to ground), and using multiple size inductors in series (but be careful with ferrite beads, they saturate at insanely low currents, which will screw with the response as a function of load current).As for the bulk cap, if your circuit doesn't require hot plugging or other massive power-line transients, tantalum are probably okay (in the sense of long term reliability). If not, you should probably settle with electrolytic (use a much larger capacitance multiplier, C2 > 10*C1) or use a low-ESR type (aluminum polymer or really big ceramics) with an explicit series resistor to set ESR.You can also choose "any" bulk cap and ignore the "fast" cap, if the inductor is lossy enough. Example: suppose you're looking at a 100uH 0.2 ohm choke. That has a resonant impedance of 0.2 ohms with a capacitance of 2500uF. Kind of a lot, but not unavailable. A 25V 2200uF electrolytic might be 40 mohm ESR, so doesn't affect damping very much (if DCR is already enough, any additional ESR just dampens it further; being so small already, it's also fairly non-critical of ESR variation, which is a big downside to electrolytics, especially when cold). The cutoff frequency would be a mere 340Hz.Something else you can tell from the resonant impedance: peak inrush current. Any applied voltage at the resonant frequency turns into a current, whether RMS, peak, transient or continuous. If the inductor and capacitor were ideal, a step voltage from 0 to 12V would draw 60A peak, overshoot to 24V peak within 1.4ms, and continue ringing forever. (This is also the Thevenin impedance, so you can guess that, if you try clamping this with a TVS, it must have much less than 0.2 ohms internal resistance. You can also calculate the excess energy from E = 0.5 * L * Ipk^2 or 0.5 * C * Vpk^2, which is energy the TVS would have to absorb.) Now... that's the ideal case, but there's 0.24 ohms ESR in this tank, so the peak current will certainly be at least half, and the peak overshoot may not be much at all. (But do mind that the DCR has to absorb all that charging energy, some 0.3J or so -- be careful!) Mind, also, this assumes an inductor that doesn't saturate within 30A or more; a rather beefy choice for 100uH. When the inductor saturates, its inductance drops, changing the dynamics suddenly; with L down, the resonant impedance is down, the peak current is much higher, and the capacitor charges much faster. So you might expect a hot-plugging current as high as VCC/DCR or 50A in this case.As a final step to this approach, you can add some low-ESR, low-ESL caps in parallel. 2200uF is way huger than a couple uF of support, so this won't change the transient response any. Bonus points if you pick values, and run traces (or add more inductors), such that the subsequent resonant impedance lands in the vicinity of ESR (i.e., 40mohm), perhaps, 2.2uF and 14nH, so it gets damped by the ESR of the massive cap it's working against. Which is probably as much as you'd get placing a couple ceramic caps at the base of an electrolytic, so that's handy to know.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 11:09:24 am
content: i wonder if this thread will turn into a massive decoupling fighti like what you have wrote, it is very clear and easy to understand.However, how does the bulk capacitor resonance with choke work when there are multiple units in parallel?If I wanted the 1mH chokes, could I use a capacitance multiplier on the bulk capacitor to get all my 1mH chokes on that rail to work right? How tricky could it be to get a bunch of op amps working at the same time (lets say 4-5) if they used the same LC circuit? (it would be nice to cut down on parts near the op-amps so its less crowded where the magic happens).If so I would probably make the seed capacitor for the multiplier something that wont drift, like a tantalum?I tried a example4.7uF decoupling bulk (using cheap 30 cent electrolytic, esr 28 ohm)470nF fast decoupling (ceramic)1mH choke, 20 ohm resistanceresistance should be betweeen 14-46 ohm28+20=48 ohmcuttoff between1/(2*pi*sqrt(0.001*(0.00000047+0.0000047)) = 2000 and 7000 Hzcost = ~2$ per op amp.does that look like it would work ?it would definatly turn that PSRR datasheet frown upside down if it did.. it makes me sad, like a stroke patient... one sidejust droops..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2014, 06:41:34 pm
content: Quote from: SArepairman on April 19, 2014, 11:09:24 amMultiple from the same common "dirty" supply? Depends on the supply. If it's a big bulk cap or a known low impedance switcher (meaning, impedance around the filter's cutoff frequency), there will be little opportunity for crosstalk.QuoteCap multiplier doesn't act like a cap. Well, it kinda does at the output, but you can't use it that way because it's already biased. You can't just connect an inductor across it, that obviously doesn't work. You cascade it, out to in, with other things as needed.QuoteThe only downside to sharing a power rail would be crosstalk. Use enough bulk to minimize that. If the op-amp loads are on the order of 1k+, you've got at least 40dB suppression (1/100th voltage divider) if ESR < 10 ohms, plus whatever the fast cap does.If you don't have to tolerate current spikes (a 50A inrush isn't going to feel too good to a tantalum), I would recommend it. In your example, 12V into a 20 ohm choke wouldn't even be 1A peak and would probably be fine. Use at least 25V rated, 35V preferred.QuoteYes. I'd use less L and more C, partly for cost and partly for impedance reasons. You'll also get lower ESR from a tantalum of similar or greater value, suggesting somewhat lower L... which will have lower R too, but that'll all come out fine.QuoteHave you verified that your supply really is that noisy?There are better amps out there.Or you can reference the signal to the dominant rail so that PSRR doesn't matter.Or you can figure out an elegant way to do..... this whole series of postsTim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 07:22:26 pm
content: i think that other people will read these posts and learn, I did read other decoupling threads before, lets just say if I read this post last year some of my designs would be a bit better..there are a few threads that seemed to have useful information in them but there are many gaps in information.. i think you wrote the LC filter bible right here... if anyone googles "lc filter eevblog" they will find it.I have not actually measured power rail, i expect the power rails to be pretty good anyway, my plan was to put down pads for the inductors/fatty capacitors and purchase zero ohm jumpers (pennies) so I can see the difference, typically I used 100nF capacitors CUREALL solution to decoupling, which never broke anything, so I figure I could leave in the 470nF on all my amplifiers (close enough to the tried and true 100nF) and experiment with the effects of LC filtering sensitive (gain) stages first, by adding the bulk cap + inductor... if I notice nice improvements I might also try it on other stages.or maybe I will do 1uF/10uF/0.5mHhopefully everything will be good enough so I don't need to worry about Rp.. unless they sell cheap multistage inductors so I can do it in one partthat inductive kickback does worry me though... the supplies are soft start but if something does short out that could blow out many things i imagine, adding a TVS diode across every inductor is not something that I anticipated., so it would be a good reason to step down on the inductance value.will a short circuit condition in a circuit happen fast enough so that inductor can damage something? If a 15V rail tantalum shorts out (I assume this is the most likely thing to short out) how does the "switching" look?it also makes probing the circuit hazardous, it is easy to short things out when you are messing with them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 19, 2014, 10:44:46 pm
content: I think a tantalum is going to burn out gradually, once it begins to fail (of course, accompanied by that lovely exotherm they're notorious for), but the "switching" of a short circuit is nearly instantaneous (back in the day, mercury wetted reed relays were used in pulse generators to achieve rise times under 1ns). So, if any given part of a circuit shorts to ground, expect peak currents corresponding to that node's cap's ESR plus the delayed reaction of whatever connects to it (inductors, resistors, more caps..). Either the peak current or voltage overshoot is what tantalums are particularly sensitive to.TVS goes to ground, not across the inductor. Or, if you must, you could put a regular diode backwards across the inductor to prevent it from producing too high a voltage, for the same idea, but not being ground referenced. But if shorting or hot-plugging is a thing anyway, better to put in a TVS somewhere just because.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 19, 2014, 10:59:04 pm
content: ok, if anyone is interested, I have a 1uF ceramic, 10uF tantalum (i chose tantalum because of the nice form factor and SMD, ill take the price hit), 470uH inductor.the tantalum is 2 ohms, the inductor Rs is 15 ohms, the ceramic is 1uF 35V. I am going to use the 1uF ceramic for general decoupling all over the board.I calculated the R should be 6-20 (we have 17), cutoff frequencies look good and it should not need extra series R to operate.I have some through hole equivalents of these parts (with roughly similar R) that I will put together and test the over shoot from various short circuits (using different values of shorting resistors) on a sacrificial op amp on some solder board before I buy anything.but, as you said, the saturation current of the part matters, so a part that saturates at 10mA would spike alot less then a part that saturates at 10Aas for high current low resistance fast rise time pulses... whats the best way I could do it now? I think I have a old tilt switch with mercury that I could use. ill test some switches.or a puddle of mercury and a wire for some nice out doors circuit testing (im not opening that inside the house)*tests will be done with a 40V psu so the inductive spike cannot break it too easily.**I will also test a AC sweep into the decoupling network to see if I can get the "evil" resonance effect going.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2014, 12:01:53 am
content: Just use a jumper wire and poke it around... observe with the scope on NORM trigger.Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 20, 2014, 01:02:09 am
content: well it turns out I don't have a tantalum capacitor on hand, so I just tried it out with a electrolytic that has 1ohm ESR 10uF.Inductor, 10K load, L=470uH, C= 10uF, C = 1uF (ceramic). (load is where op amp should be). VDD = 15VI shorted out the power supply, simulating a shorted tantalum capacitor (short across capacitor, not from Vdd to gnd) , I can measure a undershoot of -1.25V on the op amp input once the capacitor is shorted.when I use a 1k ohm load the undershoot goes to 2V.when I measured it with a 1Kohm load it undershoots by 2V. I am triggering on a negative voltage using long memory mode on my DSO and zooming in, shorting it by tapping an alligator clip to the ground point in my circuit by ~1 foot long alligator/alligator wireI plugged in a function generator to the VIN of this circuit and added a sweep wave (set the function generator to ~10Vpp (50 ohm output impedance). I made a bode plot with this *using the FG as a tracking generator* and I found that there is no nasty resonances from 0-100KHz. I also tested 0-10KHz and 0-1MHz and found no problems @ the load resistor input. It seems to kill the noise pretty well across the band, even on a bread board (should I bother soldering it up to test it further?). at 0-10 MHz there are some resonances formed at what looks like ~7MHz.Anyway, I don't think a momentary 2V undershoot is much threat to Vdd and Vss (so I assume that Vss would get +2V if Vdd gets -2V) (right???)Done with ahttp://www.mouser.com/ds/2/54/78F_series-42602.pdfthis is a 90mA inductor. So if the SMD one that I use is 15mA the over shoot should be even less right (or should I say undershoot).and I know tantalums are kind of feisty but I don't think its enough to worry about a cascade failure from a single -2v spikei dead bugged soldered the filter together, the performance is the same a on the breadboard, including at the 10MHz zone, I lack a FG that goes past 10MHz.*the tracking generator is sweeping slowly**the "bumps" at 7 and 10MHz are pretty damn tiny in magnitude too.**I tried adding more R to the electrolytic capacitor, no change noticed. (7.5 ohms).***no change noticed adding 47 ohms to Rs.you wrote that:At high frequencies, L1 becomes a very high impedance, and R2 dominates; C1 is very low impedance, and its ESR, though small, also becomes important. The attenuation levels off at the resistive voltage divider of R2 into C1 ESR.Is the 7MHz behavior I am seeing probably related to RP becoming dominant? dollars to doughnuts its probably ugly math...but it will be ultimately ignored because the effect is minor, unless someone says something I am going to go with this solution, it seems pretty good (using only cap/inductor no additional resistors)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2014, 09:01:19 am
content: Under/overshoot should be fine. I don't like your testing method, but the conclusion shouldn't be much different I think.Yes, the squigglies in the MHz are probably evidence of the inductor playing funny, as well as layout of course.SRF > 2.5MHz suggests Cp < 8.6pF. Q ~ 60 at 0.79MHz suggests ESR ~ 39 ohms (14 of which is DCR, which will be higher at AC due to skin effect, but probably not much more than double -- which is pretty close, really), or EPR ~ 140k (the EPR due to losses in the ferrite core is probably very small).Cascading with one of those in the 10uH value would probably clean up some of the HF-VHF squigglies, and some ferrite beads would extend that further into the VHF-UHF range, if it mattered that much. At which point you'd also want to explore shielding and feedthrus too.Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 07:06:46 pm
content: Using an excessively large value inductor for power supply decoupling will have some other side effects:1. The series resistance will be higher which will compromise load regulation.2. The resonate frequency will be lower and parasitic capacitance higher so high frequency filtering will be compromised. Multiple stages can get around this.At relatively low frequencies but extending into the ultrasonic range, active filtering using a point of load regulator may be a better choice.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 21, 2014, 12:14:31 am
content: Quote from: David Hess on April 20, 2014, 07:06:46 pmcan you explain the load regulation penalty?my op amps are driving 1000-10000 ohm loads, I thought that the 20-40ohm RS would be nothing, so long its not a power op amp driving 50 ohms.well, by explain I mean give me some basic mathematical concept relating to opamps resistor on supply rails, i am vaguely familiar with the idea that resistance on supply rails is bad.. is there a rule of thumb?like V+/V- Rs should be X times smaller then load impedance for negligible performance penalty

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2014, 03:13:08 am
content: Quote from: SArepairman on April 21, 2014, 12:14:31 amCMRR and PSRR involve the same type of analysis. The input offset voltage of the operational amplifier will shift by so many microvolts per volt of change in common mode voltage or in power supply voltage. In a precision application, this error becomes significant. One thing that makes precision operational amplifiers "precision" is high CMRR and PSRR. If you know the load regulation of the power supply and the change in load current then you can calculate the change in supply voltage. That combined with the PSRR will reveal the change offset voltage.A 10 volt change across 1000 ohms would be 10 milliamps which would yield a change of 400 millivolts in the supply voltage with 40 ohms of resistance. 400 millivolts of supply voltage change could change the input offset voltage of an LM324, minimum PSRR of 65 dB, by up to 225 microvolts. An LT1014 which is an improved LM324 would change by only 4 microvolts maximum because of its much higher minimum PSRR of 100 dB.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 21, 2014, 12:59:01 pm
content: Quote from: David Hess on April 21, 2014, 03:13:08 ami wanted to find a picture from die hard where the fbi guy says i can live with those numbersnow to re-read this thread 15 times and find 10 questions that i missed out on asking

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2014, 05:28:15 pm
content: Quote from: SArepairman on April 21, 2014, 12:59:01 pmKeep in mind that the error is referred to the input. If you were operating at a gain of 100, then that 225 microvolt change becomes 22.5 millivolts at the output.CMRR and PSRR degrade with frequency and of course operational amplifiers with higher gain-bandwidth products maintain better CMRR and PSRR at higher frequencies. At 40 kHz, an LM324 will be down another 10 dB so the error would increase to about 720 microvolts. Incidentally, it is not uncommon for operational amplifier datasheets, or at least old ones, to show the wrong CMRR and PSRR curves versus frequency because the GBP was inadvertently measured instead. If the curves look identical, then the wrong measurement was almost certainly made.CMRR and PSRR apply to noise as well. If you know what the common mode and power supply noise is, then you can calculate how much of it shows up as input referred noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 22, 2014, 06:05:31 am
content: For that matter, it's not uncommon for op-amps to have "power supply gain". That is, PSRR / Av > 1 at some frequency (not necessarily including DC).Which power rail also matters. If one rail is good and the other sucks, hopefully the difference is CMRR. If it's not, God help you...Tim

author: brabus
author_rank: Frequent Contributor
timestamp: April 22, 2014, 11:00:04 am
content: The complete analysis and dimensioning criterion can be found in the magic book "Optimizing OpAmp Performance", by Jerald Graeme.http://www.amazon.com/Optimizing-Amp-Performance-Jerald-Graeme/dp/0071590285

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 23, 2014, 04:54:00 am
content: great now I will never finish this design thanks to those EMC and optimization booksbut this stuff is so much more of a pleasure to learn when you are actually working on it...not that I am calling you guys 900 year old gremlins