Measuring low voltage noise on SDS1104X-E - Page 1
Forum section: electronics / beginners

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 05:31:44 am
content: I'm trying to confirm that a PCB I've made is working as expected, so I'm probing the input to see what noise is coming into the board (via USB) and the output after my voltage regulator (LT3042). The problem is that I'm not seeing the ripple/noise that I'd expect, and there are probably a number of issues causing this. But before I start coming to conclusions I want to make sure that I'm probing correctly, but I'd also like clarification on improving the probes I'm using.Currently I'm using a single probe with a short clip attached to ground on the end to avoid the default ground wires loop. I'm probing right after the last filter cap before the load. I'm seeing higher noise than I'd expect, so I'm beginning to think that I'm picking up ambient noise. I plan to try doing a differential across two probes and using MATH to subtract the results tomorrow to see if that has any improvement.However, I noticed in Dave's videohttps://youtu.be/Edel3eduRj4?t=1927"How To Measure Power Supply Ripple & Noise" where he mentions making a set of probes at the end. I'd like to build a set of those probes, but the issue is that my scope has 1Mohm input, not 50Ohm. After looking into this I believe that it can still be done, especially after reading this snippet found athttps://community.element14.com/technologies/test-and-measurement/b/blog/posts/building-solderable-in-circuit-oscilloscope-probesQuoteRight now I'm confused as to what I'd use to terminate the length of coax. I've seen terminators like these being recommendedhttps://www.amazon.com/gp/product/B07G566JC7which are great because they're cheap, but I've also seen people mention thesehttps://www.tequipment.net/Rohde-&-Schwarz/HZ22/Jacks-/Adapters-/Plugs-/Clips-/Terminators/Coaxial/which are much more costly considering I'd need a pair, but I'm unsure what the difference is other than brand and general quality. Im thinking it might be related to the last statement from the blog post mentioned above, but I'm not completely sure. I'm hoping someone can shed some light on this and hopefully have a lower cost solution that $50 for each terminator.Thank you! Please don't hesitate to ask for any clarification.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 15, 2023, 05:57:35 am
content: QuoteNo insult intended, but did you do a simple sanity check by probing the PCB ground plane in the area of interest?ie: If you short your "single probe with a short clip attached to ground" across the PCB's ground plane and still see the same noise level, then yes you are picking up ambient noise. But if most of the noise disappears, then your probe method is probably OK.

author: TomKatt
author_rank: Frequent Contributor
timestamp: February 15, 2023, 12:48:41 pm
content: You didn't mention what kind of probe you're using and hopefully others smarter than I will hopefully comment on this, but for low voltage low impedance measurements like 5v power rails, I think you're likely to get better results using a 1X probe than a 10X probe.Probe Considerations for Low Voltage Measurements such as RippleQuote from: Tektronix

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 05:01:01 pm
content: @TomKattI am using the 1X setting on the probe, but thank you for the heads up!@Kim ChristensenI did not test that! I just tried and I'm seeing similar noise levels shorting the probe and clip to ground on my board.I also switched to make the probe AC coupled which I believe is the correct way to dig into these issues. The benefit I'm seeing from that is that the voltage center is near 0 now instead of offset 1.8V (the output of my regulator), so I don't have to hunt for it when I adjust the zoom level. Although I would've assumed the trace should be centered around 0 but it's constantly off-center a little bit.I also tried differential probing, but it's showing far worse results and I'm unsure why. I even created little pigtail wires like mentioned by w2aew here(which were much harder to solder to the board than anticipated!) and stabbed each probe into each pigtail on either side of the last filter cap.Note: The noise I'm seeing on the scope is around the 300 uV level, but I believe my circuit should be outputting sub 10uV.Has anyone created coax cables for a lower end scope like mine? I'm still thinking I'm picking up ambient noise. If I wiggle the wires around a bit even with lighting in the room off (LEDs) I can get the trace to move around a bit.Any ideas?

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 15, 2023, 05:26:29 pm
content: QuoteIt might be very low frequency ripple that you are seeing. You'd have to slow the sweep rate down to see it properly.QuoteThat's a pretty low level you're trying to measure. I'm curious how much noise is coming from the scope itself.ie: If you switch your input coupling from AC to GND on the scope, how much noise do you see?

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2023, 05:51:37 pm
content: Quote from: jesse86 on February 15, 2023, 05:01:01 pmYou aren't going to be able to see 10uV noise on that scope, or likely any other scope without significant bandwidth limits. You don't need any special or custom probes for this, or any probe in fact. The easiest way is to just use a BNC cable cut off and the wires soldered directly to the board. Or get one of those BNC-to-test hook cables. To determine what is background noise from all sources, first do the measurement with your power supply off, then turn it on and observe any difference. This works best with AC coupling since you don't have to do any adjustments, just wait a few seconds when you turn the PSU on for it to come back into range.The slight DC offset you see in AC coupling is internal to the scope and is normal--see the specs. You didn't say whether the noise was ~300uV RMS or P-P, but if you set your scope to take measurements you'll have a better way to compare to other's results. If you select PK-PK, MEAN and StDEV, BW=20M and 10ms/div, for example, you can compare to mine which gives ~440uV, 80uV (this will vary) and ~50uV with nothing on the input. So 440uVp-pand 50uVrms. Clearly not good enough to look at something where you expect 10uV. You'd need a low-noise limited BW preamplifier to see that.

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 06:12:38 pm
content: @Kim ChristensenIf I switch to GND coupling, I see 37uV Pk-Pk and 13.8uV RMS (taking the mean of each).If I switch to AC coupling and turn off the device under test, I see 1.9mV Pk-Pk and 370uV RMS (taking the mean of each).@bdunham7I'll follow up in a bit. My wife and I are just heading out now to take our kid for a walk.

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 07:11:06 pm
content: @bdunham7So a cable like thishttps://www.amazon.com/gp/product/B07SZ4SH2Vwith no 50ohm resistor or 50ohm terminator would be fine? I'd much prefer to solder the "probe" directly to the board. Even with pigtails on there it seems a bit finicky. I might pick up some BNC to test hooks as those seem like they'll come in handy in the future, but this board is all SMD components with no leads so a direct solder connection would be easier.I'll try probing off/on like you said in a bit to try and discover my scopes limits.Re: differential probing -I've found that differential probing using 1X won't work as the voltages go off the scales. Setting to 10X works, but I'm seeing heavy oscillations at 60hz. Is there a way to cancel those out in-scope since it seems to be being picked up on each cable equally? I have to keep the y scaling large in order to keep each probe's trace in view which I need to be able to take the difference measurement. Having such a large probe scaling seems to be making noise readings impossible to pick up.

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2023, 07:27:04 pm
content: The core of an RG58 cable might be stiffer than you'd like, but you can solder two short thin wires onto the board and then attach to those. No termination or resistors needed to probe the power rail, as the input capacitance of the combined cable and scope is trivial. That might not be the case testing a different point in the circuit.Faux-differential probing on a setup like this will be difficult and noisy. It is a workable method in other cases, but I don't see it working very well here. You need something like this:https://www.alphalabinc.com/product/lna10/

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 08:08:38 pm
content: @bdunham7Not sure if you just meant mean and stddev for pk-pk or the actual "mean" and "stddev" measurements, so I'm including bothNo power, AC coupling, BW Limit 20M, 10ms/div, (probing across last filter cap if that matters):Pk-Pk:mean: 660uVstddev: 40uVMean:mean: 750uVstddev: 44uVStddev:mean: 61uVstddev: 1.9uVRMS:mean: 760uVstddev: 44uVPowered, AC coupling, BW Limit 20M, 10ms/div, (probing across last filter cap):Pk-Pk:mean: 5mVstddev: 580uVMean:mean: 1mVstddev: 62uVStddev:mean: 187uVstddev: 17uVRMS:mean: 1mVstddev: 55uVFor a more general purpose probe (beyond power measuring), would it need to be built with an additional 50ohm terminator and 50ohm resistor in series? I don't mind taking the time to modify them and buying a couple 50ohm terminators in addition to the cables if they'll have more utility in the future. Either way I'll pick up some cables to give this a go.

author: donlisms
author_rank: Frequent Contributor
timestamp: February 15, 2023, 08:10:36 pm
content: As a thought experiment... ignoring details that only make things worse... let's say my oscilloscope has a minimum setting of 5mV per division, and there are 10 divisions vertical (50mV), and it's an 8 bit A/D, so that vertical range is divided into 256 counts (195uV / count). In other words, my scope can't actually distinguish between two values unless they're 195uV or more apart. That would assume perfection and noiselessness and so on, perfection, for the rest of the system, which in the real world I'm pretty sure it's not!So... I think I should keep in mind the characteristics of my instruments while I'm thinking about how to get the best measurements from them. The more accuracy I want, the more I need to consider... everything!

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 08:26:52 pm
content: @donlismsGood points!Working this out for my scope (referencinghttps://siglentna.com/USA_website_2014/Documents/DataSheet/SDS1000X-E_DataSheet_DS0101E-E03A.pdf):"Vertical Scale (Probe 1X) 500 μV/div - 10 V/div" - so 500uV/div using 1X probe is the best it can doVertical Resolution 8-bit - 8 bit ADCSo I should see in ideal circumstances ~19.5uV resolution at best. Pretty good! It seems that the 660uV Pk-Pk I'm currently seeing with the PS disconnected means there's room for improvement in my probing setup/technique. Maybe @bdunham7's BNC cable technique (with a much shorter cable) might help get me closer.

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2023, 08:36:32 pm
content: Your power off measurements are close enough to my no-input readings (660uVp-pand 61uVrmsvs 440uVp-pand 50uVrms) that I'd consider them fairly normal. The probe cables will pick up some noise as well, but no matter how well you manage that, you will only get a 20% or so baseline noise reduction. Your power-on readings indicate that the PSU noise is significantly more than your scope noise floor, and certainly much greater than the 10uV you were hoping for. So the scope is certainly able to display and measure your power rail noise as it currently is. The next thing to do is look and see what that 5mVp-plooks like at different timebases. Is there a triggerable waveform?I wouldn't worry about custom probes at the moment, they're something you deal with when the need arises because you don't know ahead of time what that need will be. The usefulness of low-Z probes is quite limited.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 15, 2023, 09:12:14 pm
content: Quote from: jesse86 on February 15, 2023, 07:11:06 pmIf you want to solder the end of a coax onto a board with limited space, you might want to consider using a smaller coax like RG178. This is assuming that you're good at soldering and fiddling with small wires.Get one like this or longerand just cut the unwanted end off.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 15, 2023, 09:27:34 pm
content: i know the LT3042 quite well--I developed the circuits and methods to be able to measure noise on the output of this part and wrote the application note on it:https://www.analog.com/media/en/technical-documentation/app-notes/an-159.pdf. I can tell you right now that if the design of your board is right, you won't be able to measure anything worthwhile on your oscilloscope. The signals are just too small.With that said, it's hard to tell what you have going on here in terms of noise that you are seeing on your board. Measuring ultra-low noise levels requires a lot of work to ensure you aren't getting something corrupting your measurement. The evaluation boards that were developed for the LT3042 use BNC connectors to avoid loops that can pick up signals from any other equipment. Powering the regulator as well leaves you open to a lot of potential concerns. Ground connections between the oscilloscope and power supply via the earth ground can feed noise into the circuit, ground loops are always a possible concern. We used batteries as power supplies for our noise measurements.From there, the noise floor of the oscilloscope isn't just determined by the smallest range and ADC number of bits, but instead is controlled by the gain of the oscilloscope front end and noise levels. You may not get anywhere better than a few hundred uV of noise because the front end gain of the oscilloscope has to be high at the 500uV/DIV level and it has its own input referred noise.There are a lot of questions that come to mind here. What is powering the LT3042? Where are sources of potential noise coming from? Is it rejection issues on the part of the LT3042 (be aware that it does not have infinite supply rejection, especially as you get above a few MHz). What can you truly get with your scope in terms of measurement levels?

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 09:52:41 pm
content: @bdunham7Attaching some pictures so you can see the waveform when powered on. Also showing the way I'm probing.@Kim ChristensenThanks for the link to an easier to use cable!@ AnalogToddWow, the perfect person to have helping out! To answer some of your questions:When testing, I'm powering through this Anker usb wall charger. This one in particularhttps://www.amazon.com/gp/product/B00VH8ZW02When running my design, it gets powered via my laptop (mac air).The final design will get its power over USB C, so it can be connected to any power source (and I need to try and filter assuming it's connected to a potato).The current prototype board currently is powered via a knockoff arduino nano (usb -> nano 5v -> input to my board) but I intend to design around an STM32 in the final version, so no wires jumping around. I can share the current board design in a bit, but I can say that I've got an LC pi filter before the LT3042 to do some prefiltering, and the design is built around an AS7341 color sensor which states that it draws 300uA maxhttps://ams.com/en/as7341. The prototype board was actually designed to test several different voltage regulators to see what's needed to reduce noise/errors on the sensor.The spec sheet for my oscilloscope says "DC Gain Accuracy: ≤±4.0%: ≤2 mV/div".FYI: I'm no EE, I'm just trying to build a board that gets the cleanest data possible out of that sensor. In doing so I believe that voltage regulation is an issue causing measurements to fluctuate more than the datasheet specifies. However my oscilloscope skills are minimal and I'm pretty sure my probing technique is probably rubbish, but I'm trying my best to track down what I can with the tools I have.

author: bdunham7
author_rank: Super Contributor
timestamp: February 15, 2023, 10:06:22 pm
content: You don't need to use that complicated method for putting a screenshot on your USB and it just obscures the channel information. Just press the PRINT button and a screenshot will show up on your USB drive.To improve your oscilloscope skills, change the timebase to 1ms/div and then zoom in on those spikes to see what they are comprised of. I'm guessing you'll see some much higher frequency switching noise. That's pretty noisy if you are expecting a very clean output from an analog regulator. You might want to scope the power rail before the regulator and share your board layout.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 15, 2023, 10:28:16 pm
content: Yeah, that USB charger is going to give you all sorts of weird signals on the 5V output as it is a switching supply in there that is going to give some ripple. Cleanest possible solution would be a battery.From there, I think the ripple you're seeing isn't ripple on the output of the LT3042. One easy way to check that is by measuring directly across the LT3042 output capacitor (next to C8 on your board). I'd hazard a guess you'll get better results at that point. The LC pi filter will do well in filtering some ripple, but depending on the values used for the L and the C it may not work as well as you think; inductors turn capacitive at high frequency (due to turn-to-turn capacitance) and capacitors have parasitic ESL that makes them look inductive above their resonant frequency. This is why you see the high frequency edges coming through onto the output signal.My bet right now is that you're actually getting line frequency signal being coupled into your circuit. Look at the period of the main ripple component--you're right about 16.67msec, which is 60Hz. It's possibly through earth ground, or it may be related to the rectifier circuit inside the USB charger. The LT3042 has great supply rejection down at those frequencies, so don't be surprised if you still see the signal on C8--it's likely coming in and corrupting your ground that you're using for the scope.

author: jesse86
author_rank: Contributor
timestamp: February 15, 2023, 10:34:31 pm
content: I'll jump back on the scope after dinner, but here are some screenshots from the schematic / pcb layout that might help. Note: this is my first pcb, and I *am* open to criticism if it'll help improve the design.Also, the PCB is 4 layers, both middle are full ground pours.

author: tautech
author_rank: Super Contributor
timestamp: February 15, 2023, 11:42:48 pm
content: Quote from: jesse86 on February 15, 2023, 09:52:41 pmOh FFS get rid of that POS PSU and get a low noise one which until you do you'll be chasing your tail around and around !These DSO'a are incredibly sensitive to any local noise and SMPS supplies of any sort. It may not even be the one you are using to power a project but one in the next room such is the EMI/RFI they can emit.Too many times have customers contacted us insisting one of these SDS1104X-E was faulty but when convinced to start turning OFF some possible local noise sources it becomes evident noise was coming from somewhere else rather than the DUT.Best advice is to go back to basics and not overlook anything.

author: jesse86
author_rank: Contributor
timestamp: February 16, 2023, 12:03:06 am
content: @tautechYou're probably right, but how do I test that the circuit is properly filtering and regulating the input if the input is already clean from a linear supply? I figured it made sense to test in the conditions that I'd expect to see in real life.

author: tautech
author_rank: Super Contributor
timestamp: February 16, 2023, 12:19:51 am
content: Quote from: jesse86 on February 16, 2023, 12:03:06 amIsolate any possible external interference first, always.QuoteSure but what you see might not influence the way the DUT actually operates but certainly will confuse the fuck out of you.Take a step back and examine your procedures and any that might introduce things thatwillmatter and those that don't. Properly analyse what you see on the display and where it comes from.When testing any sensitive DUT one really needs be on their game.When it comes to DIY projects so much can matter and IMO the supply rail is where it all begins then final layout to protect the integrity of mission critical signals.Who said any of this is easy, mostly it is until it isn't.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2023, 12:34:37 am
content: Quote from: jesse86 on February 16, 2023, 12:03:06 amGiving it something that noisy as a supply is quite a challenge even if you eventually want it to deal with that much interference. Clearly it isn't coping with it very well, so you might start by analyzing the noise for spectrum, etc. To test your design a bit more methodically, you can introduce your own signals into the input and see how much they show up in the output. As mentioned previously, the linear regulator isn't going to be able to filter out 1MHz+ stuff. Even more challenging, that sort of HF noise can be transmitted in all sorts of ways other than through the regulator. Try physically removing the regulator altogether and then do the same test with the scope. I suspect your noise may remain.

author: David Hess
author_rank: Super Contributor
timestamp: February 16, 2023, 03:29:07 am
content: Quote from: jesse86 on February 15, 2023, 07:11:06 pmDifferential probing with a pair of x1 probes works well to remove common mode noise, but typical oscilloscope inputs have a very limited input range making them very easy to overload. Oscilloscopes intended for this type of measurement have inputs with 1000 times the input range to make them useful for differential measurements. On a normal oscilloscope if the input sensitivity is lowered to make up for this, then this lowers the differential sensitivity as well.Old oscilloscopes can use AC coupling to get around this, but modern instruments use a 2 path amplifier so are always effectively DC coupled as far as overload limits. External AC coupling capacitors may be a solution to this.

author: jesse86
author_rank: Contributor
timestamp: February 16, 2023, 03:41:06 am
content: I'll have to jump back on the scope tomorrow as the kid is asleep and it's in the room above.In order to test this properly, when you refer to generating my own noise are you referring to picking up a signal generator and using that? I'm still trying to figure out what all would be needed to tease this noise apart and have control over it. I've never used one but if it helps I could look into it. Cheap FG for $130https://www.amazon.com/Koolertron-Generator-Precision-Dual-channel-Arbitrary/dp/B071WLXLQ5vs $350 for name brandhttps://www.amazon.com/Siglent-Technologies-SDG1032X-Arbitrary-Waveform/dp/B01LECZU98.Looks like I can pick up a cheap linear supply on amazon for about $100https://www.amazon.com/KORAD-KD3005D-Precision-Adjustable-Regulated/dp/B00FPU6G4E,but I'm wondering if they're any good or if their noise will still be above what I'm trying to measure. Name brand for $400https://www.amazon.com/Siglent-Technologies-SPD3303X-Triple-Output/dp/B01410O424.I'm open to spending money to properly debug this. It seems like there's some low hanging fruit based on the fact I'm seeing noise already above what my oscilloscope can handle (which I'll probe more tomorrow), but after cleaning some of that up it seems like I'll need either the supply, the FG, or both to dig into the lower noise.Does that all sound right?

author: tautech
author_rank: Super Contributor
timestamp: February 16, 2023, 08:23:12 am
content: Quote from: jesse86 on February 16, 2023, 03:41:06 amNo, the scope can handle most anything it's just you don't understand what you see.Forget the fundamental signal and look past it into the background noise and determine where it comes from.Drive the scope to do this by taking charge of it, zooming in way past the fundamental to find the frequency of the garbage. If in the 10-40kHz range I'll lay money it's SMPS noise from outside the DUT.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 16, 2023, 05:56:25 pm
content: The idea of looking at your signal and understanding what you are seeing is a huge way to understand better what is happening.Look at the scope shot you provided and let's analyze things a bit. First, you have a regular waveform occurring here. Noise is random, not periodic like this. Therefore, this signal must be coming from somewhere else. Look at the frequency of the signal and the shape of it. Up and down, but not sinusoidal. Repeating timeframe is 16.67msec--60Hz. That's power line frequency, and the variations are in line with rectified AC line power. The major noisy spikes you have are twice in that periodic signal. Those match up with the spot that rectifier diodes would be turning on in the USB charger--one in the positive half of the cycle, the other in the negative.The rest of the signal is just a few hundred uV of random noise. That could very likely be the limits of the oscilloscope and the analog front end on it. There is no chance your scope will get anywhere near measuring the real noise on the output of the LT3042. I designed the circuits to measure the noise on that part and it took quite a bit to get signals that were trustworthy.As said, there may be some SMPS noise feeding into your circuit, but first you may have some other issues that need cleared as well. Looking at your probe connections, you probably want to take the wound wire that connects to the probe ground and solder that directly to the ground side of the capacitor; don't use another piece of wire to connect it. That just gives you impedance and a questionable connection. From there, make sure you understand the grounding of your setup. Do you have only one probe and ground connected to your board? If you have multiple probes with grounds connected to the board, you've just introduced loops via the scope ground and earth ground that can give you small signal variations in your measurements. Only connect one scope ground to the board and make it the one you want the clean signal on. Last, make sure there is only ONE point of your setup connecting to earth ground. The oscilloscope regularly has probe grounds connecting to earth ground, if you have the PSU or other equipment with earth ground connections hooked up to your board, you WILL get ground loops and feed that 60Hz signal in.

author: jesse86
author_rank: Contributor
timestamp: February 17, 2023, 04:16:14 am
content: @AnalogToddDo you mean I should solder the clip on the probe to ground? Then the probe tip might still be causing a flaky connection right? The quality of the connection is the reason I've been particularly keen on creating a custom probe whose signal and ground wires I can solder directly to the board (so that I don't have to ruin the probes that came with the scope).I actually already ordered the cables I mentioned before (https://www.amazon.com/gp/product/B07SZ4SH2V) in hopes that I can figure out how to use them in place of the existing scope probes, although I still need to figure out if a 50ohm series resistor is needed to make them work with my scope.The connections to the board are giving me a bit of anxiety because they've been finicky; depending on how snug I get the clip and the probe tip into the pigtails can make the signal vary anywhere from what I shared before (2.7mV) up to 2 or 3 times that. I know everyone keeps asking me to analyze the noise within the existing signal, but I honestly don't trust the signal as it stands because I don't trust the probe connections as they are. If I can make solid connections I can go back and dig into the signal like everyone has mentioned.TLDR; I need help figuring out how to create soldered probe connections to eliminate the possibility of a questionable connection. Any clarity on this would be a helpful step toward tracking down the source of the noise / periodic signal.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 17, 2023, 09:36:36 pm
content: Quote from: jesse86 on February 17, 2023, 04:16:14 amYou don't want to solder your clip to the board. Go back and look at the picture you posted where you were probing on the board; you have a coil of wire wrapped around the probe tip, and another coil of wire that goes around the ground of the probe just above it. I can't say if the coil of wire around the ground connection is one that came with your scope or not, making another one can be done simply with a bit of single strand wire around the same size as what you have there--that's quick and cheap. The video you posted is something I've done for years for good connections. The issue is that you have the coil wire from the ground going into another small coil of wire leading to questionable connections. Those coils of wire that go around the probe tip and ground should be soldered directly to your board. They're cheap and disposable.Another option is to buy actual oscilloscope probe sockets. Those give you a way to solder things to your board the same way.If you are getting large signal variations as a function of how well of a connection you're making with the probe, then that says that you are more than likely running into grounding issues on the board. You should be able to just give a little bit of force to hold the probe against the two wire coils to make good connection and maintain it while looking at your signal.Again, it appears your whole issue is with grounding. Go through and check EVERYTHING you have connected to the board while making your measurements. Ideally, you want only one item that is plugged into line voltage to have an earth ground connection. Even better would be to only have one piece of equipment plugged in to the line during your measurement. If you look at the noise measurement picture in the application note I wrote, the only thing plugged in while we did our measurement was the oscilloscope and it only used one channel.

author: MathWizard
author_rank: Super Contributor
timestamp: February 18, 2023, 08:22:25 pm
content: How much noise gets onto the screen, from the DSO's own SMPS, and how much noise comes from the outside, enters from the power cable, and how much is just EM waves in the air, hitting conductors like a ground planeAll day yesterday, I had a 112Hz signal showing up fairly strongly in my AM RX type frontend, and I'm guessing it's some distorted 60Hz harmonic, IDK, I should see if it's there today.And so if you get a big cookie can, and put a few holes in it for wires, how much cleaner woul a typical little breadboard circuit look ?I love an excuse to get a tin of cookies.

author: jesse86
author_rank: Contributor
timestamp: February 23, 2023, 12:49:01 am
content: Sorry for the delay. I went back and set up soldered probe connections like at the end of w2aew's video (https://youtu.be/-4q8geE5ef8?t=117). I'm attaching images of what I saw and the FFTs corresponding to them. I tried 3 different setups, where in all of them the probe was connected to the board: 1 no power, 2 powered by the SMPS (anker), 3 powered by a 9v battery. The 9v battery is connected to the vin and gnd of the nano, so the entire circuit is being powered.The periodic signal seems to have gone away. Maybe the larger probe loop before was picking up something nearby. I'm only seeing noise now.Based on the FFTs I'm noticing a lot more noise overall below 15Mhz. I'm not entirely certain that the dbV is scaled correctly, or if it is it seems that the noise is low, yet I'm still picking something up based on the trace between the images. Also noteworthy is that the battery and the SMPS seem to be performing the same.A few qs remain:1. Do the FFTs seem reasonable? I'd think that if the noise is <95db I shouldn't see the noise that I do. Maybe the scaling is off somehow?2. Where is that noise coming from? I would've thought the 9v battery would lead to a super clean signal, so does that mean there's noise being generated on the board itself somehow?3. The sensor outputs data via I2C. Since the sensor works on 1.8V and the arduino works on 5V, I have some N-channel mosfets bumping the voltage up, and those use the 1.8V coming out of the regulator on the source-side of the FET. Is it possible that the I2C lines are pushing noise back into the circuit somehow? I'll follow up with a post containing the circuit diagrams / layouts in a bit.

author: jesse86
author_rank: Contributor
timestamp: February 23, 2023, 12:59:46 am
content: Here's some screenshots of the PCB layout and the level shifting I mentioned in the last post. If anything stands out please do let me know. Note that the arduino is connecting up to this board using the pins at the bottom, delivering power and I2C comms.Some help with the flow of the board:1. power/I2c in on bottom2. ESD at D1, then PI filter at C1 FB1 C2 C3 above the VIN pin on the bottom left3. branches power out to each regulator from there4. LT3042 on the left5. Sensor filtering at C5 and C46. Sensor at U17. Level shifting below the sensor at Q1, Q2, and Q3It is a weird board to be honest. I wanted to demo several regulators to see the impact on the sensor's resolution / noise.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 23, 2023, 02:44:02 pm
content: Quote from: jesse86 on February 23, 2023, 12:49:01 amYou have achieved the one thing I was advocating: getting rid of the line noise creeping into your measurement. Good job!Now, with that said, it looks as if your FFT is being done on your oscilloscope. As I pointed out before, you will NOT be able to accurately measure the noise of the LT3042 without specialized equipment. Your oscilloscope has an analog front end (AFE) whose job it is to either attenuate or gain signals to be viewed on the screen. It is NOT a noise-less circuit. On the lowest V/div setting is where you will see the effect of this noise the most. Remember that the noise is additive in RMS fashion: Vout = sqrt( Vin^2 + Vafe^2 ) for the noise voltages. If the noise of the AFE is higher than that of the LT3042, all you will see is the noise of the AFE.The numbers that you are seeing on the FFT actually make sense for the AFE of the scope. A -100dBV measurement is 10uV of signal. The frequencies you are seeing may very well be noise coupling in from circuits inside the oscilloscope. First peak is ~100kHz? Likely a switcher somewhere in the scope being used to power something. Higher frequencies could be the clock for logic in there.You are at the extent of what can be seen with a simple oscilloscope to measure the noise here. Trust me, I spent months trying to figure out how to get an accurate noise measurement on the output of this regulator. It requires creating multiple low noise gain circuits that are then paralleled to further reduce their noise contribution in the measurement. The measurements you get are showing that you are at the limits of your equipment by the simple fact that there is no significant difference between when the circuit is on or off.

author: jesse86
author_rank: Contributor
timestamp: February 23, 2023, 04:20:48 pm
content: Thank you for the follow up @AnalogTodd! I believe I understand what you're saying, but what I don't understand is why the noise coupled in from within the scope jumps up a few hundred uV after powering up the circuit, but it's not there when powered off?I'm glad you kept up with me on this. I had a feeling my probing technique was an issue, and in the end it turns out I was right! Now I just need to figure out why this darned sensor isn't performing to spec... at least now I've ruled out the voltage regulation on board. Next up: the sensor may in fact be reporting correct values, and it's instead the LED stability (thermal drift) of my laptop's monitor that is being detected.

author: amaschas
author_rank: Regular Contributor
timestamp: February 23, 2023, 05:25:45 pm
content: Out of curiosity, what sensor are you working with, and what is the variation you are seeing on the measurement?

author: jesse86
author_rank: Contributor
timestamp: February 23, 2023, 06:16:49 pm
content: I'm using the AS7341 (https://ams.com/en/as7341) which statesQuotewhere (3) states "ADC noise is calculated as the standard deviation of 1000 data samples divided by full scale".Given a 0.005% noise level and an input voltage of 1.8V, I assume that I needed a max voltage ripple of 90uV (.005% * 1.8V = 90uV). Anything higher and the ripple noise will be stronger than the ADC noise, right?Also, for those 1000 data samples taken at an integration time of 10ms, the minimum span of time over which they're sampled can be no faster than 20s: there are two sensor reads per full set of values (ADC is multiplexed reading 6 of the 11 sensor values at one time). So they're talking about .005% full scale noise for sensor readings over a span of 20s. There should be minimal thermal drift over a period of 20s, so that seems pretty ideal.At best I'm seeing sensor errors near 0.05%, and that's for 10 samples over 10s; an order of magnitude worse, and using two orders of magnitude fewer samples (my integration time was higher using my monitor since it's brightness can only go so high). My next tests will be against a much brighter source with identical gain and integration times.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 23, 2023, 07:50:00 pm
content: Quote from: jesse86 on February 23, 2023, 04:20:48 pmThe signals being shown in the faint green when powered all look to be fairly high frequency. It is quite possible that the I2C data output is putting small transient loads on the output of the regulator at a frequency too high for it to respond to. The issue you will run into if you want to get rid of all of that is the parasitic side of the components and board; you need a low impedance bypassing the supply at those frequencies at your sensor. The regulator is going to be too slow and have too much parasitic trace impedance between it and the sensor to clean things up.A 10uF ceramic capacitor has enough parasitic ESL that it has a resonant frequency of roughly 1MHz. Above that point, the impedance begins to climb. What you actually want to have is smaller values of ceramic capacitors in parallel with the 10uF capacitor. Because of their smaller capacitance, the resonant frequency increases and they will be lower impedance at higher frequencies. It can be a pretty easy simulation to run; model a capacitor as 10uF in series with 5 milliohm and 0.7nH and do an AC sweep where you have an AC voltage on it. Look at the AC voltage divided by the AC current and you'll see the impedance change. Change it to 0.1uF and see the results.