DC-accurate Low-pass-filter - Page 1
Forum section: electronics / general

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 11, 2024, 05:49:04 pm
content: Hi voltnuts,for my voltage-reference(s) i'd like to add a DC-accurate Low-pass-filter. (See PDF)Okay, we have to deal with the OP-Amp-offset-voltage, i know.Here we have the standard Sallen-Key Butterworth 2nd order LP(fc10Hz) modified to cancel the leakage-current through C2.Step-response is 0.2 seconds.Meassured leakage of foil-caps is 100...200 pA at 45 to 50 Voltsthat result in 225...500 Gigaohms.And there are my precious ppms escaping to ground. 8-OIn the modified LP there is no DC over C1 and C2 anyway, nice. :-)What do you think?Is this a good idea, or did i overlook something?Should i make a 1Hz- or even 100mHz-Version?Best regards

author: macaba
author_rank: Regular Contributor
timestamp: July 11, 2024, 07:37:26 pm
content: Have you investigated the attached topology? I use this in practical DC accurate circuits a lot.Attached image shows a 3rd order response, it can be extended to further orders but my experimentation shows it tends to be better to chain multiple 3rd order stages instead.

author: trtr6842
author_rank: Regular Contributor
timestamp: July 12, 2024, 12:39:04 am
content: I have been working on a DC accurate low-pass filter too! I actually just got my prototype boards made, but I have yet to test them.I got the idea when I was working on myLNAs. I figured if you can measure the noise on a voltage reference, then you can cancel it out!Obviously a low-pass filter will not help long term stability, but I do think it could have great value in reducing measurement times, or helping measurements that have to be fast (i.e. scanner cards) be a little less noisy.The main challenge is making it such that the filter doesn't add more noise than it filters out...@Roehrenonkel I did a quick simulation of your design, and it appears that it would have a noise floor of approximately 19nV/√Hz. This would be about 600nVpp in the 0.1-10Hz range, even with a shorted input, which is great for bandgap or LM399 references, but falls short of LTZ/ADR1000 territory. To reduce that you have to increase all capacitances and reduce all resistances, that's the only way around it. This simulation even ignores opamp input voltage and current noise too.I also simulated @macaba's topology, and it has similar noise results, about 500nVpp form 0.1-10Hz.I attached the schematic for the active filter I'm working on, I decided to call it "PARF" for Precision Active Reference Filter, just because I thought it was kindof funny.Based on simulations I think it should get to about 175nVpp of input referred noise, including contributions from the opamps, and it is effectively a single pole 15mHz low-pass filter.I essentially took my LNA design, modified the input for a 15mHz high-pass cutoff, then feed it's inverted output back into the buffered input to cancel out noise. Since the noise is amplified, I can AC couple it back into the DC path using ceramic caps and a voltage divider. The maximum attenuation depends on how well the gains are matched between the LNA gain and the AC injection divider, but with most voltage references it doesn't take much attenuation before you reach the noise floor of the low-pass filter, so you don't need a lot. This circuit is definitely a bit over-the-top, and I admit I have no idea if it will have any practical applications, but I'm working on it anyways!Anyways, I haven't yet powered it on and tested with it, but I'll provide updates when I do!I'm also well stocked with LNA's to measure the input vs output noise!

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 12, 2024, 09:51:36 am
content: Hi,thank you both for your inputs.@macaba: "Gyrator" was the first think that came to my mind.How do you calculate it? Is there more info about it?One could even use the buffer U1 to amplify 7V-->10V.@trtr6842: Good job on the LNA, Kudos.Noise-cancelation only works if you're in time to be 180° out of phase.Resistor-noise within the LP-filter: Yes these are critical.22k seems to be the upper limit with regards to noise.But, i'll definetly will scale the resistors down.Let's see what else i'll find in my stock:500* 1u 63V 15mm Pinspacing.That look's promissing. :-))Now we have to wait what the SMU says about leakage (See you in a few hours).I have got over 18.000 Mica-caps, but not ONE Teflon-/PTFE-cap! Someone wants to trade?Ciao4now

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 12, 2024, 11:49:24 am
content: Hi again,turns out, that distance is king for low-leakage.The best cap so far with some old Wima MKS-3.Fast setling in regards to my other samples (ERO 1818 & 1826 / Wima MKS).Picture shows sample-# and leakage-current, 10 seconds between samples.Could become a big pcb, but i'm a 19"-guy anyway. ;-)Ciao4nowEdit: It's an ERO (Emil Roederstein) MKC-cap.

author: trtr6842
author_rank: Regular Contributor
timestamp: July 12, 2024, 02:12:41 pm
content: Quote from: Roehrenonkel on July 12, 2024, 09:51:36 amDefinitely, which is why the high-pass input to the amplifier section has a 3.6mHz -3dB frequency, but the overall low-pass -3dB point doesn't occur until 15mHz.This topology results in a slight increase in noise between those two frequencies, but the overall the output noise is reduced.Quote from: Roehrenonkel on July 12, 2024, 11:49:24 amWhat capacitance was the part tested for leakage, 1µF?If so, even ~25pA you measured might cause significant offsets in a ≤1Hz low-pass, since 160kΩ * 25pA = 4µV. It would be 40µV for a 0.1Hz cutoff. Even if you add more parallel capacitors and reduce the resistance to keep the same cutoff frequency, the offset will still stay the same. If that offset is fixed, then its not too bad, but if isn't stable with time & temp it might cause issues.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 12, 2024, 03:26:09 pm
content: Hi trtr6842,yes 1 Microfarad, i went through my stock of 1u's to see what's best.Test was done with Keithley 237, charged the cap to 50V (or 45V for 50V-caps),and took current-samples every 10 seconds. No shield or guard.In my design there is no DC-voltage over C1 and C2a anyways.Best regards, CU

author: mawyatt
author_rank: Super Contributor
timestamp: July 12, 2024, 04:08:58 pm
content: Quote from: macaba on July 11, 2024, 07:37:26 pmAgree, this is a very practical topology with good component insensitivity. One weakness, as common with VCVS (Sallen-Key) types, is the Op-Amp output impedance limits Stop-Band attenuation. This can be dramatically improved without self generated noise penalty by splitting the input resistor with a shunt C to ground. This provides a 1st order Low-Pass with the split input R to keep higher frequency signals from entering the feedback control loop with the Op-Amp.We don't have a OPA140 model, so just used a OP07 for comparisons, see below with added 0.1uF shunt.Best,

author: trtr6842
author_rank: Regular Contributor
timestamp: July 12, 2024, 04:47:26 pm
content: You don't need much stop-band attenuation when working with voltage references. For something like a good bandgap reference with maybe 5µVpp of 0.1-10Hz noise, you only need 20dB of attenuation before you hit the 0.5Vpp noise floor of a filter using the 11.2k + 2µF values. You need even less attenuation for lower noise references.

author: DeltaSigmaD
author_rank: Contributor
timestamp: July 13, 2024, 07:34:53 am
content: Low-noise lowpass filterAn addition to the circuits here: the filter circuit shown below is calculated as Bessel 4th-order filter, but other filter characteristics are possible. The resistance values in the signal path are relatively low to reduce noise. The filter output is free of 1/f-noise, if we neglect the 1/f-noise of the resistors. The single inductor determines the lower limit for the cut-off frequency which can be realised.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 13, 2024, 08:08:14 am
content: Using an LC instead of RC filter could indeed help, but real inductors have there downside. One is that they may pick up hum from external magnetic fields. To get high inductance one would essentially need a magnetic core (e.g. use a rel. high voltage transformer) and the core material may show Barkhausen jumps from the magnetic core material. One would still get some noise in the transition region from the resistance needed for dampening resonance.For a low leakage capacitor I would look at polypropylene types. One may even get away with PP type motor run capacitors that are available to some 100 µF, though a bit bulky. Testing the capacitor leakage is not easy as there is also dielectric absorption (especially with polyster types, PP is about 10 x better, but still not perfect).Especially the electrolytic capacitor used in the PARF circuit can also react to temperature fluctuations and the dielectric absorbtion can take a long time (e.g. days) to settle.

author: DeltaSigmaD
author_rank: Contributor
timestamp: July 13, 2024, 09:42:17 am
content: @Kleinstein: You are right, the Barkhausen noise is a problem. This was the reason for using huge air coil inductors (ca. 50 cm height and 20 cm diameter) to filter the input power of a furnace required for a key comparision noise-vs-radiation thermometry. The results were excellent.

author: macaba
author_rank: Regular Contributor
timestamp: July 13, 2024, 09:54:30 am
content: mawyatt - Good suggestion, I was thinking about voltage reference filtering here, I do the split input on the first stage of PWM DAC filters.DeltaSigmaD - Neat extension of the principle. I use a simple response in my circuit because I usually prioritise "settling time to 1uV" (say, around -1udB from nominal fully settled gain) characteristic.

author: mawyatt
author_rank: Super Contributor
timestamp: July 13, 2024, 12:25:09 pm
content: Quote from: macaba on July 11, 2024, 07:37:26 pmBTW a BPF rendition of this topology is shown here, concept credited to Bob Pease.https://www.eevblog.com/forum/projects/edn-published-active-bpf-in-error/msg5563865/#msg5563865Best,

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 14, 2024, 12:09:07 pm
content: Hi DeltaSigmaD,thank you for your input.Besel is very good (Q=0,5) - for audio.Here i want a steep slope, critical damping (Q=0,7).What about the leakege of the first cap?As @Kleinstein pointed out inductors are haunted by parasitics and other ill-effects.Is there even a core-material for such low frequencies?I'm useing lots of inductors/tranfos in my tube-amps: Input, Anode-choke, Drive- & Output-transfos.And they (Lundahl/Reinhöfer) work really well in their frequency-band.My biggest choke is a 5Hy 600mA that has 32.5 Ohms (and over 6kg) and will have a very low Qat thouse frequencies.In theory fine, but not practical.Best regards

author: Kleinstein
author_rank: Super Contributor
timestamp: July 14, 2024, 08:08:23 pm
content: The inductors get better (less resistance) the larger they get. The chokes are usually for a certain DC current, for the ref. filter one could use a core without a gap as there is essentially no DC current. A mains transformer primary or an evenen higher voltage winding could be a first start. No need for really special material for low frequency, but a hi µ material (e.g. grain oriented iron in a toroid or Nonoperm, maybe metglass) can help to get high inductance.

author: CurtisSeizert
author_rank: Regular Contributor
timestamp: July 19, 2024, 01:39:23 pm
content: Quote from: Roehrenonkel on July 12, 2024, 11:49:24 amWhen I was working on an LNA with a high input impedance I tested a number of caps for leakage current and DA. For film caps, the major mechanism of apparent leakage is DA based on a reference cited in AOE3 that measured over a year. My measurement setup just used two points spaced a couple hours apart (the ones here were 2.4 to 4 h except the PTFE, which was 8.75 h) and assumed linearity because I was measuring a lot of samples, the FOM I was looking at was self discharge time constant, which I recorded in ks. Here are a couple highlights:C0G 470nF (Murata): 162 ksPPS 470nF: 203 ksPE 6.8uF (WIMA MKS2): 246 ksPTFE 470nF (Soviet surplus): 39400 ksPP 1uF (Panasonic): 3840 ksPP 4.7uF (Panasonic): 6850 ksPP 12uF (Kemet): 14400 ksI did not do a ton of measurements with PE because PP was so much better, but with PP, the time between points and whether a sample was pre-charged for 24 hours prior to the measurement were big factors, which is consistent with most of the apparent leakage being a result of DA.That said, I have successfully used bootstrapped SMD tantalum caps for voltage reference filtering down to a breakpoint of 2.7 Hz (single pole). In the case I tested most thoroughly with an ADR1000 reference, the concern was more TC than absolute accuracy, and settling time was not a major concern. The higher voltage cap was biased with about 10 mV because the R of the filter was part of a divider, and I wanted a bit of margin on the bootstrap node to ensure there was a positive bias on that cap. The ratio TC I got before and after the filter was negligible, i.e., <10 ppb/K. I have generally used this in preference to RC LP filters using Rubycon multilayer film caps, which you can get in fairly large sizes as SMD components (up to 22 uF I think) because it's cheaper and the solution size is less than a single 2220 cap. I like using fairly small (i.e., 3k3 or lower) R values because of the current noise characteristics of my preferred buffer amp, the OPA2205. Also, if I am going to this trouble, I use the standard RC filtered buffer amp topology at the output with something like 10R/47uF and appropriate feedback R and C values to maintain stability. I attached a screenshot of a schematic where I implemented this sort of thing for providing a 5V reference to an AD4030-24.

author: huababua
author_rank: Contributor
timestamp: October 11, 2024, 06:58:37 pm
content: @ trtr6842 any update on you circuit mentioned above?

author: trtr6842
author_rank: Regular Contributor
timestamp: November 10, 2024, 01:48:46 am
content: I started testing a unit with a single supply and injected noise from a signal generator and an 80dB(1/1000)(1/10000) attenuator, but that wasn't working. With a single supply the circuit actually relies on there being some electrolytic leakage current to keep the 1st gain stage output above 0V. I noticed this quickly, so I switched to a bipolar supply. To do that I used two external PSU's, but apparently the power-up sequencing coupled with the opamps trying to saturate their outputs on startup caused a problem, and now one of the opamps is running very hot and probably broken. Then other work took over, so I haven't made any more progress.I will try again, probably with some schottky power supply clamp diodes to ground to make sure the internal power diodes on the opamps never start conducting on power up. I may get some time to re-visit this soon.

author: mawyatt
author_rank: Super Contributor
timestamp: November 10, 2024, 02:15:47 am
content: Just a note , 1/1000 is 60dB not 80dB.Power supply sequence can create a problem with some circuits, with long settling times. SBD can help, also a FET clamp in the right place can also help during start up sequencing. Hope you find a good solution.Best

author: trtr6842
author_rank: Regular Contributor
timestamp: November 10, 2024, 02:35:17 am
content: Good catch on the typo, It was a 1/10000 attenuator, the same on I use for testing my LNA's. Its great for injecting really small signals using a regular signal generator.I think one of the start up issues is that my circuit relies on using multiple opamps in parallel to keep the noise low. I optimistically used 1Ω series resistors on each of their outputs in order to combine them, which should be fine in steady state. But at startup I think they are fighting each other enough to prevent them from entering normal operation. I think the input buffer stage should be ok with fairly low resistances since the gain is just unity, but I probably need much higher resistance in the noise amplifier section in order to prevent issues. At least there high resistance can't cause DC errors, and since its after the initial gain it shouldn't increase noise much.

author: ivo
author_rank: Regular Contributor
timestamp: November 11, 2024, 05:46:56 am
content: CurtisSeizert's topology looks similar to one explained by Walt Jung, see attachment. If one "stacks" two (or more?) 'leaky' ECs on top of each other, the bottom one that 'intentionally' leaks will create only a tiny voltage differential for the cap at the top (the one that interacts directly in the signal path) to leak away the PPMs (since I=V/R, if we intentionally minimise V across the cap, it can't generate any large leakage I). That allows you to have a big RC filter but not suffer the Johnson noise of a large resistor. Note I can follow the logic and have implemented it, but don't have the ability to empirically confirm the results.

author: iMo
author_rank: Super Contributor
timestamp: November 11, 2024, 08:49:53 am
content: FYI - some sims with the OPA140 and its competitor AD4620..

author: EC8010
author_rank: Frequent Contributor
timestamp: November 11, 2024, 09:04:09 am
content: I have a family of low-pass filters I use for measuring noise of voltage references. All are simple LC. 600 turns of 0.2mm enamelled copper wire on an RM5 former gets you 36mH when the cores are added; that's the series element. 470nF shunt capacitor determines high frequency cut-off, and its resonance is damped by the 430R series resistor. Series 14uF polypropylene capacitor blocks DC and sets 11mHz high-pass, allowing lots of gain to be added. All the other components are measured parasitics of the real components. You don't have to use the 14uF capacitor, but you can't use lots of gain if you don't. I use the filter in conjunction with a 1M input resistance LNA feeding an oscilloscope set to FFT. The 1kHz high frequency limit allows me to clearly distinguish between white and 1/f noise. Another variant uses 120uF series capacitor (polypropylene, but quite large) for a 1mHz high-pass filter. Both variants have grounding switches after the input coupling capacitor to reduce the wait for stable operation.

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2024, 09:16:40 am
content: nice filter its a breath of fresh air compared to all the op amps. I always wondered what a nice one would be like.If you try to actually figure it out with a program its not so easy.I have a big polypropylene capacitor.Do you mind posting the 1 mHz circuit? This looks like some fun projects to build into a tinI have the active circuits built (5Hz) from a long time ago, having the passive ones would be nice too,.

author: iMo
author_rank: Super Contributor
timestamp: November 11, 2024, 12:21:10 pm
content: ..fyi..

author: EC8010
author_rank: Frequent Contributor
timestamp: November 11, 2024, 02:13:50 pm
content: Quote from: coppercone2 on November 11, 2024, 09:16:40 amHere it is. As you can see, I shifted everything down by one decade, so it's now a 100Hz low-pass rather than 1kHz. It needed quite a large (Marks & Spencer toffee) tin. The choke was a commercial one found at a junk shop in Toronto. Be warned that large chokes are usually made for power supply smoothing and are magnetically leaky, so they easily pick up hum. If you have the choice between an EI or toroidal choke (of the same value), the toroid will pick up less hum. Of course, you could just fit your larger capacitor in the 1kHz filter in place of the 14uF - it won't change the other required values.The reason I reduced the low-pass frequency by a decade was that if you genuinely want to look down to 1mHz, you need a sweep speed of 10,000s because the resulting FFT will go down to 0.1mHz but you need to discard the first ten frequency bins because they're unreliable. If you then note that you need to average 100 FFTs together to get a smooth noise spectrum, that's 100 x 10,000s = 11.6 days. That's an awful lot of data, so you need to be able to reduce sample rate, hence reducing the filter high frequency response to 100Hz. You also need to compensate in the spreadsheet (into which you imported the FFT data) for the LF loss of the filter. Thus, my 1mHz filter is actually only used down to 10mHz, requiring a mere 28 hour collection time and no equalisation is needed.

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2024, 09:21:09 pm
content: also, what frequency did you measure those parasitics at? and is it the same one for both filters?I was thinking of not using a RM5 former and instead winding a torroid. its alot of work but so is making a good box so you might as well. Do you have a material suggestion for it?The 1mHz filter is interesting but I think its getting to the realm of 'annoying' to measure based on how you described it, its really a lot. 28 hour data log.. someone would need to pay me to try that one. I don't have the patience.

author: EC8010
author_rank: Frequent Contributor
timestamp: November 12, 2024, 09:04:23 am
content: Quote from: coppercone2 on November 11, 2024, 09:21:09 pmParasitics were not measured at a single frequency but by measuring impedance at 68 different frequencies between 20Hz and 200kHz, then least-squares fitting a model, so those values are valid for the entire working frequency range of the filters.Winding a toroid will indeed be a lot of work but you should get a very good result.The requirement for the box is to be electrically conductive with no gaps. Anything that is capable of keeping food fresh is good for gaps. We're coming up to the time of year when shortbread and chocolates appear in fancy tins. They make very good enclosures for electronic testing. Look for tins having a large overlap on their lids. Tobacco tins are good, too. Use BNCs fitted to the tin to make external connections via screened cables. You can improve screening by adding self-adhesive copper tape (as sold to gardeners for dealing with slugs) over the join between lid and tin. A glass fibre pencil will take paint off the outside of tin/lid allowing solder to make a definite connection between tape and tin. Beware that modern tins have an epoxy coating inside, so the lid is insulated and will cause hum. Glass fibre pencil inside and an earth strap between lid and tin solves that problem. The tin is too thin to provide electromagnetic screening <200kHz; you just have to experiment to find a position that suffers least hum.Little patience is need for a 28 hour noise log - you just set it running and come back much later. What is a problem is ensuring that it doesn't get disturbed.

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 12, 2024, 09:38:14 am
content: HelloWhy not use a analog filter as in use on the Adret 104 ?RegardsOS

author: iMo
author_rank: Super Contributor
timestamp: November 12, 2024, 11:08:44 am
content: The above filter from EC8010 is an AC filter, not a "DC-accurate LP filter"..If you try with the above "Adret 104" patented filter - see below 1Hz cutoff for say 10k i/o impedance.You may scale it easily - for 0.1Hz cutoff simply multiply both L and C values 10x, and so on..

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2024, 09:47:24 pm
content: Quote from: EC8010 on November 12, 2024, 09:04:23 amOh no I meant magnetic material.And IMO, those food tins are almost not good enough for connectors because they are too thin. They buckle when you plug stuff in. For isolated connectors, you can put a backer plate behind it, but if it needs to be RF grounded, that one is a bit harder. I guess you can still put a backing plate with a big hole for the back nut of the connector.I am good with sheet metal chassis stuff, not so good at magnetic coresIf you wanna take it to the next level, copper plate it, then kool-amp silver plate the interior, then varnish it after masking the ground points.For altoids tin projects, that have isolated connector (i.e. battery power supply), I glue aluminum in there and then drill it out, its WAY stronger when you plug it in. I caught my BNC connector on a larger shallow tin 'gapping' after awhile because the thin metal got distorted! When the cable was plugged in, sometimes it could get enough force to open a gap. I did not notice anything electrically, but to me it means it needs reinforcement. The wall buckled in and there was a gap between the lid and the interior

author: EC8010
author_rank: Frequent Contributor
timestamp: November 13, 2024, 09:16:11 am
content: 3H1 A100.Yes, tin plate tins are bendy and a bit marginal on strength, but they screen well. A quick glance around spotted twelve tin plate tins ranging from shortbread to 1 oz. baccy tins fitted with BNCs and small switches; I haven't had problems. I put a large washer behind BNCs and switches on the inside of the tin to spread the stress. I mark the BNC holes with dividers and scriber, drill a 6mm hole (about as big as I dare in something that thin) then file the D-shaped hole with needle files to be a close fit. Having a close fit is key, otherwise the connector can move and cause damage. Altoids and 1 oz. baccy tins are more robust than large cake tins. If you fit XLRs, you definitely need reinforcement behind the connector. I made a spanner that locates on BNC lugs to allow BNCs to be done up tightly using appropriate spanner on nut. If you fold your own box, self-adhesive copper tape over the folds will obscure apertures that let hum and RF in; essential if an electrometer is inside.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 13, 2024, 09:48:25 am
content: An LC type filter could work, but one should not have a resistive load at the output for a DC accurate filter. The damping part would be with a resistor parallel to the inductor and / or an RC series element at the output. For the start and comparison with the other filter circuits one should use a lower order. Already just 1 inductor gives 2nd order.A problem with an LC fitler is that one needs rather large inductance and capacitance. The kH range would be somewhing like a higher voltage transfromer winding. The magnetic core can also add noise (Barkhausen noise) and pick up hum.One may consider a supercapacitor, though these have a rather limit voltage range. In combination with a relative low resistance inductor the leakage current may not be that bad. However the loading to the source could be an issue.

author: EC8010
author_rank: Frequent Contributor
timestamp: November 13, 2024, 01:14:27 pm
content: It's not a case ofcouldwork; theydowork. And very well; those component values are practical. The circuits shown are full circuits including measured parasitics of what was actually built and is used. The original poster wanted to investigate noise in voltage references, and that's exactly what these filters were made for. You don't actually want to pass DC because that limits the gain you can apply to look for noise. Obviously, the polypropylene coupling capacitor sets a low frequency limit, but so does your measurement time - something nobody has explicitly discussed.Barkhausen noise in an inductor is quite difficult to observe. Conversely, DC references are noisy; 100nV/root Hz is not unusual. Agreed, hum pick-up is always an issue with wound components, but if you capture a spectrum, it's not a problem to remove it in the spreadsheet. With care, the hum is only 20dB or so above the noise floor - at least, that's what I see.What I have posted has been built, tested, and found to be fit for purpose; it is not a theoretical musing living only in SPICE.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2024, 01:18:58 am
content: Quote from: EC8010 on November 11, 2024, 09:04:09 amwhat do you think of nanocrystaline as a core material for this application?

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: December 28, 2024, 08:51:06 am
content: Hi EC8010,Quote from: EC8010 on November 13, 2024, 01:14:27 pmNo, exactly the opposite. I want a DC-accurate Low-pass-filter so i have a clean +10V without the noise.Ciao4now

author: EC8010
author_rank: Frequent Contributor
timestamp: December 28, 2024, 06:53:41 pm
content: Quote from: Roehrenonkel on December 28, 2024, 08:51:06 amWhoops! My mistake. You can filter high frequency noise away from a voltage source/reference but (by definition) you can't filter the lowest frequencies; you need a source that is itself low noise. I've used my filters in the search for quiet voltage references.No idea what nanocrystalline core material would be like, I'm afraid. But I had some 36mH inductors wound on RM5 cores, so I used them. The 100Hz low-pass filter used some large inductors made by UTC. My measurement filter really isn't critical, so almost anything will do the job.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2024, 09:35:28 pm
content: Well I experimented how to get 36 mH on my nanocrystaline core.It looks like about 18 turns of wire is needed on a 60uH Al core to get 36mH at 120Hz and 1KHz. Drops at 10KHz and you get 4mH at 100KHz. It's not what I expected but I made sure the measurement is repeatable and verified with similar value manufactured components (I got a large amount of different shielded axial through hole chokes for a pretty penny).I think that is alot nicer then trying to do 600 turns ! Especially because its a torroidal core.I am thinking to follow up with a different filter at say 100KHz to supplement the NC core that drops off there.Also, with fine gauge manganin wire, it looks like that winding should give me between 400 to 600 ohms of resistance if wound on the same coil, making it possible to combine the resistor and the inductor on the torroidal core, but working with the sub 1/20th of an mm manganin is a little frustrating (first time), it is easy to kink that tiny wire. My plan is to use dots of UV adhesive to kinda attach it to the core while working on the torroid.Comes out to like 0.7 meters of wire to get the correct turn count. I used the DE5000 meter, but I can follow up on a HP 1KHz mohm meter possibly, it does measure L, but not C.The manganin should be +-10ppm, making it comparable to a pretty good resistor.What do you think?And any thoughts on a second filter stage to follow up the first?For the construction I plan to wind the wire on the core using dots of UV adhesive, then mount it to a PCB by lashing the core to some holes using nylon lace over a pool of epoxy that is then drizzled with more epoxy to make a robust connection between the plastic core and a garolite board, and terminate the windings to turret terminals (2 stage).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2024, 11:13:17 pm
content: I would prefer the inductor to be low resistance and thus use copper wire. This way one can adjust the damping if needed and also consider a parallel resistor in stead of the series one. For DC accuracy low series resistance would help and allow more leaky capacitors, possibly even electrolytic.I would not worry so much about the higher frequencies like 10 KHz or 100KHz. There the filter would still attenuate enough from the capacitor even if the inductance goes down somewhat.For a low pass filter to suppress ref. noise one may want a really low cross over and may want more than only 36 mH. AFAIR some 100 mH could be available as ready made CM chokes and using both coils in series could give an extra factor of 4. They are often just ferrite, but could still be wirth a first try.

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2024, 11:35:23 pm
content: Lol my first attempt at thinking it was linear the NC torroid got 130mH from like 25 turns or somethingthese cores can give you absurd inductance with not too many turns. I imagine getting it into the Henry range would not be very time consumingI imagine what would happen if you really fill up one of the larger torroids, it might become a 'magnetic blocker' more then anything else lol

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2024, 01:08:12 am
content: I really want the built in resistor though. If its too much you can put a splice to make a hybrid coil with two wires, if its too small then there seems to be no good way to get rid of the resistor.

author: EC8010
author_rank: Frequent Contributor
timestamp: December 29, 2024, 11:05:29 pm
content: Quote from: coppercone2 on December 28, 2024, 09:35:28 pmThat's very few turns, so the effective permeability of the core must be enormous. Of course, a toroid isn't gapped, so that will help. However... The nice thing about an air gap is that it stabilises the inductance. I would expect the (gapped) RM5 core to have a much more stable value of inductance than an ungapped toroid. 600 turns isn't nearly as much of a problem as you'd think provided you have a coil winder with a counter. Such things are now cheaply available. What might surprise you is that the winder's crucial quality is smooth rotation - it's jerkiness that breaks fine wire. My cheap and cheerful winder had an awful rotating plastic sleeve as a handle, so I made a closer fitting wooden rotating sleeve that allows smoother rotation.I've never felt any need for a second stage of analogue filtering. When measuring noise, an accuracy of +/-1dB is perfectly acceptable, so provided your sample frequency is 5x the filter's cut-off, its 12dB/octave slope is entirely adequate to keep aliases under control.

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2024, 11:18:29 pm
content: you could get a gapped core that will probobly have a large inductance too. the distributed gapped version (not with a cut, looks solid)I think I saw NC Distributed gap cores somewhere. The gap is leakage I think, so it also has a downsideOr the powder core is kind of gapped. I see a few exotic powder cores.Anyway, its been years since I put anything into the thermal chamber, maybe I can put this in there.But, are you sure that it still needs a gap, because the core I believe is wound, so I think it ends up being kind of gapped?https://www.transmart.net/a-news-effect-of-temperature-on-nanocrystalline-core-performanceNanocrystalline cores are known for their good thermal stability compared to conventional ferromagnetic materials. This is primarily due to their fine grain structure, which limits the movement of domain walls and reduces the overall eddy current and hysteresis losses. However, even nanocrystalline cores have a limit to their thermal stability, beyond which their magnetic properties can significantly degrade. It is important for designers and engineers to consider the thermal stability of nanocrystalline cores when selecting materials for specific applications to ensure reliable and efficient performance over the expected operating temperature range.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2024, 08:39:56 am
content: I have another option too, a 39mH shielded axial inductor. Its not a torroid but I don't have to wind anything. Perhaps its good enough. This one is ferrite.I wonder how it compares to your R5 core. And I could give it a wrap in iron, hy-mu or NC foil.From my not so junk box16uF polyproyplene foil axial 10% 16.1470 ohm ceramic composition 10% 477.539mH shielded axial ferrite inductor 10% 40.1470nF C0G ceramic 5% - measures 46539I wonder how it will do. It looks to be over dampened because the inductor is 120 ohm and the resistor is like 460.The RC HP calculation is 9.88mHz

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2024, 09:32:57 am
content: What exactly will the result of having a over dampened filter be ?In a scope your rising edge would be like slow. If you look at the average under the wave, I guess you lose data, for a square wave.how does the error look like in measurement fornoisein real life?If you put a giant value like 5k, the bode plot looks like it basically is a much lower pass filterDid you ever quantify the amount of error you get from over/under dampened filters?I never thought about the actual amount of error you can get from a over or under dampened RLC filter in a RMS measurement before.THe other part of it seems also to be a voltage divider between Rseries and the 1MWould you bother tuning that series resistor? I kind of wonder how it would be if you put a potentiometer there. Like a 1K pot

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2024, 10:24:12 am
content: An underdampend filter will show a resonance and thus peaking at the transition. An overdampend filter will be more smooth, a bit more like 2 x 1st order filters and not a proper 2nd order filter. There is anyway no perfect value for the damping part. There are a few classic values (Tchebychev, Bessel, Butterworth and Linkwitz) for the damping depending on what aspect one is aiming for. Damping values in between are possible too.One may want to tune the damping, if one needs a special phase response, especially linear phase, which acts like a delay. Other uses are likely less critical and a resistor from the E24 series would be good enough.For the LF noise measurement (the classic 0.1 to 10 Hz band) the more logical way to make the low pass filter for the 10 Hz limit is anyway some form of digial filtering. An active 10 Hz RC based filter is also not problem and the filter in not so noise critical as it can be behind quite some amplification. The question may be if one could implement the input AC coupling (high pass) as an LC filter to reduce the noise there. This however would be at 0.1 Hz or lower, so not so easy. There is also the good alternative to have the AC coupling at the input at a frequency lower than 0.1 Hz and have a separate high pass filter for the 0.1 Hz later - possibly also in the digital domain. This avoids the RC filter noise in the transition region from the often relatively large resistor. For a LF noise measurement there is little use for an LC based fitler.As a ready made high high value inductor with relatively low resistance one could consider a CM choke: 100 mH are available of the shelf and the 2 coils in series would give 4 x the inductance. A closed core is a big plus, as it gives high inductance (though early saturation) and it also keeps the hum pick-up small.For a DC accurate LC filter I would avoid intentional resistive loading to ground. So only the capacitor leakage to compete with the coil resistance.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2024, 10:40:44 am
content: but what does it end up looking like for RMS measurement? Usually noise is RMS

author: EC8010
author_rank: Frequent Contributor
timestamp: December 30, 2024, 12:12:04 pm
content: It all depends on how you measure your noise. The traditional method used quite steep high-pass and low-pass filters to define bandwidth, then used a meter preceded by an RMS rectifier to measure the total noise. The implicit assumption being made was that the noise was white (unchanging voltage with frequency).But we can do much better now. A much more useful measurement is made via the FFT of an oscilloscope. The FFT breaks the frequency range into bins, each defined by a mathematical high-pass and low-pass filter and assumes that the noise between the filters is white. If the bins are narrow (perhaps only 1Hz wide), then the assumption of unchanging amplitude with frequency within the measurement bandwidth remains appropriate even for 1/f noise (-10dB/decade slope). Thus, the FFT is the ideal way to characterise noise.But! Oscilloscopes rarely contain an anti-alias filter. Worse, with 20MHz of minimum bandwidth, they see a lot of noise, whereas we are often only concerned with 10kHz and below. So my filters are designed to remove the high frequency noise we're not interested in so that we can wind in oscilloscope gain and use the FFT to measure what we are interested in without overloading the front end.As pointed out, although an LC filter has a slope of 40dB/decade, it is has a resonance before entering that final slope that must be correctly damped. We want Q = 0.7 for a maximally flat response. And that's the only critical thing, but it's easily set by adding the right resistance to the inductor. That's why my filters have an additional series resistance.What isn't immediately obvious is that the inductor has shunt capacitance and your deliberate capacitor has series inductance. Both of those parasitic components mean that the 40dB/decade slope does not continue for ever. Fortunately, you have to try quite hard to make a 36mH inductor that has enough shunt capacitance to be a problem. Likewise, you'd need extremely long wires on your deliberate capacitor to cause a problem. If you do a SPICE analysis of my circuits (which include the measured parasitic compoonents) you'll see that they're (deliberately) quite well behaved.Once we've added our input filter, we no longer need to worry about overloading the oscilloscope. But oscilloscopes sell by bandwidth and small low capacitance devices have plenty of 1/f noise. So we need a low noise pre-amplifier having at least x100 gain to ensure that any 1/f noise we measure is actually from the device we're trying to measure, not the oscilloscope itself. I use an OPA1641 configured as a non-inverting amplifier having A = 100 after the LC filter. That does for most work, but it is perfectly possible to do better using discrete FETs connected in parallel and driving an NE5532A to get noise down to 1nV/root Hz.Finally, once we've got an FFT on the oscilloscope, it is ragged. We need to average FFTs together to smooth the noise spectrum. Averaging FFTs together seems to be reserved for the more expensive oscilloscopes. If you average 100 FFTs together, the deviations from a fitted curve will be about 0.6dB, which is why I earlier commented about 1dB for noise. Having averaged 100 FFTs together, we export the FFT data as a csv file to a spreadsheet and plot it again (logarithmic frequency axis). Having the raw data available, we can now fit a line to the white noise and characterise that. We can fit a -10dB/decade slope to the 1/f noise and characterise that. Usually, I find that there's also -20dB/decade noise due to thermal dependencies, so that's also needed. It all sounds complicated (and it is), but having fitted in this way and defined what the noise is, you can try reducing it and see whether you have actually made a difference. The -20dB/decade thermal dependency noise is the easiest to tackle and is usually reduced by adding thermal insulation around semiconductors and shielding them from draughts (just like the voltage reference data sheets tell you to do).As Lord Kelvin observed, once you have some proper numbers, you have proper knowledge and you can advance. The old-fashioned single figure noise measurements aren't really much use. Here's an example of what I measured via one of my filters:

author: EC8010
author_rank: Frequent Contributor
timestamp: December 30, 2024, 12:23:15 pm
content: @coppercone2: Your junk box bits should do just fine, just set Q = 0.7.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 30, 2024, 12:39:58 pm
content: Today one would ideally implement the 0.1-10 Hz filter to get the standard low frequency noise digitally. So the analog part with amplifier and ADC would have a somewhat larger BW (e.g. 0.01 to 1000 Hz) and the actual limits are set in the digital domain. This could also to an FFT or similar analysis. Even with a full analog filter it would help to have the accurate filter later, after some gain. The analog filters usually have some extra noise in the transition region that can be reduced this way. Normally 0.1 to 10 Hz is in a range where one would not use inductors, but active RC based filters.It is only for ultimate DC precision where an inductor may get interesting again, as such a filter could handle the capacitor leakage better, possibly work with electrolytic capacitors. This would not be for noise measurement, but more ref. filtering (known to be difficult) or a PWM DAC.For the scope preamplifier it depends one the scope that is used. The usual configuartion is to have the higher ranges with a divider in front and one should aim for the highest range that does no use the divider - which range this is depends on the model. The lowest range that uses the divider may not be useful and have relatively high noise again.

author: EC8010
author_rank: Frequent Contributor
timestamp: December 30, 2024, 12:57:08 pm
content: Quote from: Kleinstein on December 30, 2024, 12:39:58 pmThat's what I initially thought, but it turned out that if you want to measure the noise of a bench power supply, there are narrow (but high amplitude) spikes of digital clatter at high frequencies that can overload your amplifier, so for practical measurements it's worth having the filterbeforethe amplifier.In the end, it's all about dynamic range and trying to reduce the real world's dynamic range to fit within the very limited dynamic range of the oscilloscope's ADC (usually about 50dB, no matter what claims are made for 16 bit sampling). So although doing the filtering in digits is attractive, it leaves the ADC exposed to a very large bandwidth of noise and therefore possible overload. Of course, if you had a dedicated low frequency digitiser, you might well have much more dynamic range than 50dB and could afford to do the filtering in digits.I've only measured using a Tek MSO54, but I don't imagine there will be a huge difference between oscilloscopes at that level. I measured the 'scope's self-noise, then calculated what external gain was needed to make its noise negligible compared to that of the amplified DUT. For a lot of work, x100 is adequate, but for really quiet noise sources, x1000 is needed. As you point out, you really want to operate the oscilloscope at 50mV/div rather than 1mV/div.The "standard" 0.1Hz - 10Hz filter was done because it was practical and FFT measurements at the time were impractical. But that's no longer the case and we don't need to be constrained by such a crude method. A noise spectrum is far more informative than a single digit measurement via 0.1Hz -10Hz filter.By the way, all of my measurements started as a means of measuring and taming voltage reference noise.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2024, 05:25:25 pm
content: Right I forgot about my little dynamic signal analyzer with 50KHz bandwidth, I was thinking about how much it could effect RMS (I also have that thermal RMS meter). I think that thing could be set to like 2KHz, and it went down to some 1Hz ?That DSA one DOES have some nice filters.You can turn off its AA filter but that makes it do some crazy aliasing on the display .I need a cloneI might throw in some other ceramic res into a digikey cart, I only bought 1 per decade for looking at it with the impedance analyzer last year because there was no good information but alot of information on carbon composition.E24 ceramic resistor might be worthy to buyI was visually looking at the impulse response visually and forgot about Q, if you do Q it does seem to dial in on 0.7 at ~417 ohms. But it does still have a "bump" on impulse when the Q is 0.7. I guess there is no right answer. At 510 ohms, there is no bump/ringing, but the Q is 0.57For your values I got 0.62 Q factor BTW. But I am not taking into consideration the parasitic of the componentsI do have the jim williams 100KHz active (analog) filter circuit that i built too, but I think that one has a HP of 5Hz built into it with the sallen key filters. HOWEVER I may have added a design change that I can just use the LT1028 gain stage without the filter on a auxillary output BNC, its possible that I might be able to use that box for amplifying this signal. It is in a steel enclosure, but it is hard wired with a 330uF polymer capacitor input. I suppose I should just build another amplifier, its fun.About where to put inductor, EC8010I got something to stop malfunctioning for a customer before by doing exactly that, kick the inductor out side the circuit. Its like a dog. It gets rid of stuff the actual electronics don't like. Industrial system stopped misbehaving. It was in a larger chemical facility that was having a issue with some data card. I thought its like putting a guard dog outside in the yard instead of having it in the living room. It would probobly be better if they let me have two inductors, aka leave the old one alone and add a new one, but it was a real cheap place, so thankfully I could just move it and it stopped doing whatever bad thing it was doing. I never heard about any measurements, but its for sure a valid strategy that the inductor might serve a better purpose on the input before the amplifier, because the complaint went away, and those people loved to complain. Very fortunate that someone put a inductor in the circuit, because if I could not just move it, then I can't have it on the circuit, because its a new expensive part! I don't know what kind of insanity that could have turned into if I was not fortunate to have that inductor there on the circuit board

author: EC8010
author_rank: Frequent Contributor
timestamp: December 30, 2024, 10:17:02 pm
content: Quote from: coppercone2 on December 30, 2024, 05:25:25 pmYes, Q = 0.57 = Bessel response, optimised for impulse. But the LC filter before pre-amplifier/oscilloscope is just there to protect the two from overload. Its impulse response doesn't really matter. What matters (slightly) is the in-band response before the roll-off. But it's easy to measure that and correct for it in the spreadsheet.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2024, 11:29:40 pm
content: Ok I windowed it nice and I guess I can see how from a FFT prospective something around 400 ohm seems the best for flatness but also not cutting off early but also not having a big bump. At the bessel point it looks alot different.

author: coppercone2
author_rank: Super Contributor
timestamp: December 31, 2024, 09:59:09 am
content: Ok, I put it together. I have two seperate boxes, one for the capacitor and the other one for the RLC filter, boxes with BNC, because I realized I already had cap couplers built up. I thought I might roll up a tube of 'iron' around the filter inside of the box, because its made out of aluminum.I think this might be the first 'air wired' inductor I ever soldered. Very satisfying.I do have a very large cap coupler built up already too, the thing on my mind is if I can get 4H out of one of those NC torroids. Then I can construct the lower value filter.But I also wonder, why do you use a 470nF for 100Hz? Would a 400mH + 4.7uF be more reasonable? Why go huge on the inductor?I found a inductor that is 1.4H @ 120Hz and 400mH @ 1KHz. It is tiny, 1cm cubehow does your 4H inductor look like frequency?

author: EC8010
author_rank: Frequent Contributor
timestamp: December 31, 2024, 06:29:57 pm
content: Beware the large coupling capacitor. If you connect it to (say) a 200V supply and switch on, couple a 200V transient to your oscilloscope or pre-amplifier. Thus, you need a shorting switch at the output so that the capacitor can charge without destroying subsequent electronics. Once charged, you open the switch. Doubtless, there are more sophisticated ways of dealing with this problem, but it works for me.I went huge on the inductor because that's what I had in stock... But it's nothing to the 120uF 400V polypropylene also in the box.The inductor is good enough and a SPICE simulation of the circuit will reveal the inductor's self-resonance (I've forgotten where it is).

author: coppercone2
author_rank: Super Contributor
timestamp: January 01, 2025, 01:35:10 am
content: I want more filters with inductors in them to buildMaybe a 60Hz notch filter ?I wrapped the filter I made with a sheet of "pure iron" inside of the pomona bnc box, with maybe 3/4 inch of overlap. hard to form, but it seems to have 'seated' itself on those 4 solder points inside of the box when I bent it into place.I bet if you really work hard you could make a full thing, but I think just putting it into a tube shape will help alot and be 0 work. I will tie it together with lace because I have it around and it will bearly fit with the cover up.there must be some other filter to build for noise measurements in a box. Maybe a high pass to only look at the HF rejecttion capabiltiies?I do have a few wave tek filter amps, but these little ones are very interesting

author: EC8010
author_rank: Frequent Contributor
timestamp: January 01, 2025, 09:23:05 am
content: I've found simple LC filters to be very useful and have made them in pairs to that I can put one before and one after the DUT, or tested two DUTs at a time to see which works best. I've made low-pass of 1kHz, 100Hz, and 10Hz, mostly in tobacco tins. A mains notch filter probably won't be too good because inductors invariably pick up a bit of magnetic hum from the mains transformers in power supplies, although rotating the inductor can often help considerably.

author: coppercone2
author_rank: Super Contributor
timestamp: January 08, 2025, 03:10:57 am
content: I connected the LC filter between the multifunction synth (FM mod) and the 3054 oscilloscope and it showed me a satisfying bode plot on X/Y mode.it looks like RLC filters do infact workwith how much people hate low frequency signal inductors, I thought it might explode or at least have a inappropriate response.a 36mH axial inductor, does actually work as theory describes

author: EC8010
author_rank: Frequent Contributor
timestamp: January 08, 2025, 10:29:30 am
content: Whoever would have thought it?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 08, 2025, 01:04:30 pm
content: The main problem with reasonably large inductive components is the susceptibility to external AC magnetic fields.CheersAlex

author: coppercone2
author_rank: Super Contributor
timestamp: January 08, 2025, 02:34:33 pm
content: Quote from: EC8010 on January 08, 2025, 10:29:30 am

author: coppercone2
author_rank: Super Contributor
timestamp: January 10, 2025, 07:59:01 am
content: look at this interesting parthttps://www.ebay.com/itm/335599282240?gQT=1

author: EC8010
author_rank: Frequent Contributor
timestamp: January 10, 2025, 10:41:09 am
content: That's how 1970s wah-wah pedals were tuned! I forget the frequency, but there was a pair of ferrite cores very similar to the RM series with a brass bolt through the middle. Careful adjustment of tension would put the frequency in the right place and a tiny amount of overtightening would crack the (expensive) core. Beware that an EI inductor is leaky and will pick up mains hum and one with a deliberate gap like the picture will be terrible; I tried an EI lamination 2H inductor in a filter and it picked up too much hum to be usable. The people who have commented about coils and hum are quite correct; you have to choose your inductors carefully. The RM series is very good and toroids ought to be even better. I haven't seen them for years, but Vinkor ought to be good as well.