My ESD mat hums - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: May 13, 2025, 04:00:54 pm
content: I just noticed that my scope picks up a 100mV 60 cycle hum from my ESD mat which IS grounded. When I move the probe away the amplitude decreases.100 mV seems like a lot. I turned off lamps, usb stuff, etc, but it remains.What did I do wrong? Can I get this reduced (eliminated)?THANKS!Dewey

author: BennoG
author_rank: Frequent Contributor
timestamp: May 13, 2025, 05:46:18 pm
content: ground the scope.Or your pc / laptop if you use a USB type of scope.Benno

author: Stray Electron
author_rank: Super Contributor
timestamp: May 13, 2025, 05:47:03 pm
content: It sounds to me like your mat isn't WELL grounded. What do you have it grounded to? If it's connected to the ground in your AC wiring then it sounds like you may have a poor neutral connection somewhere and that there is current flowing through your ground line.

author: richnormand
author_rank: Unknown
timestamp: May 13, 2025, 09:18:14 pm
content: A few possibilities.In addition to the previous post get yourself a socket tester to verify the status of the ground and neutral and GFI status. They are inexpensive.If you have access under the bench do the scope test again. Does the signal increase getting close to the mat again or does it get bigger toward the floor?If the bench is made of metal does the signal increase when near. Is the bench grounded?ESD mats of good quality will have a dissipative top and a more conducting layer under. The surface resistance will be quite high (measure with megger).The cheaper single layers will have a low resistance (can be measured with a DVM).Some cheap stuff (silicone mats in particular) may claim to be ESD mats but are not at all.First test measure the ground point at the mat connection and your scope ground at the probe probe. repeat between the probe ground and mat ground. Should read near zero. Some will introduce a 1meg for safety but that should be for wrist ESD bracelets only. Make sure you have the direct ground.If good, put an aluminium foil(~30x30cm) on the mat and ground it to your probe. Test for the behaviour again for E field.Second make a coil (~10-15cm diag) with about 5-10 turns and hook that to your scope probe. This should detect if there is a magnetic field generated from a close wire (unbalanced current) that could be inducing current on your bench.Good luck with it. I can send you photos if you want more info.Cheers.

author: watchmaker
author_rank: Unknown
timestamp: May 14, 2025, 10:02:15 am
content: Thank you all. I have been up since 3AM working on this; you have me motivated.I must admit, my bench is like my first iteration of my watch shop. I KNOW you have no idea what that means. In my first shop things were added on and much was “make do” until I decided I really had to tear it all out and start over. The culmination is the watch shop you see in my general chat thread “Pics of my watch shop”.I am probably reaching that point in my learning lab. I do have a corner I can use giving me about 120sq ft I can enclose with a separate circuit (I have a sub panel). But I dread the thought of moving a 1500 lb vertical mill. Will wait till winter.First a fun fact. I get 125mv AC p-p with the hook attachment on the probe. W/O it I go down to about 60mV AC p-p as measured on the scope. I have the original probes and LeCroy dedicated 10 x. Same thing. Correction in followup.To the issue at hand. I always subscribed to the fact that an attorney can help only if you make a full confession.So, what I currently have is two Amazon switched power strips plugged (via a 10 ft extension cord) into what I thought was an independent utility outlet straight from the main panel. Not.That is issue no. one. Easily solved by installing a ckt in the subpanel with a 4 gang outlet box.Then I have a switched outlet box for my USB stuff. When I remove all USB stuff I still get over 90mV AC p-p. When I turn that box off, I get down to 55 mV AC p-p. Second issue found.Turning off the overhead LED troffers does nothing.Measuring the voltage between the ground of the utility outlet and the scope, I get less than 1 ohm but 72mV AC p-p as measured by the only meter I have that provided p-p; the Phillips 2525. Regardless of the above configurations.I get 1Mohm between the mat coupling connector to the end of the coupler ground wire. So that seems to be OK.All these measurements were made with the LAN cable disconnected.The panels have been installed/wired by only professional electricians to this point. Revision Energy when they installed the solar, and the company that installed our Kohler whole house generator. Of course both were inspected by our town inspector who has a rep for being very particular about electrical inspections.So, this AM I am going to tear everything apart, plug things in one at a time beginning with the sccope.Then I will install a dedicated ckT from the subpanel for the bench; with the USB stuff connected to the “utility”outlet in the main panel. Of course there will be a GCFI breaker, 20 A. While there, I will check the resistance of the ground to neutral; but I know we have a residential ground rod. Stranger things though.And RichNormand, I will do your tests after I get this squared away. I think this issue may have been a source of my frustration and I have to get a handle on it. THANK YOU ALL!!

author: watchmaker
author_rank: Unknown
timestamp: May 14, 2025, 11:04:50 am
content: I just stripped out everything with only the scope remaining BTW, my mat is a Benton. When I connect the ground alligator clip to the mat,, the ground from the probe to the mat ground, the mat connected to the ground on the scope, I get down to about 40mV AC p-p. Around 1/2 of what I saw earlier this AM and 2/3 of when I started yesterday. And yesterday was with the probe tip NOT touching the mat.When the probe is just resting on the mat, I get less than 4mV AC p-p. Both results occur with the hook attachment and without. No idea why the results differed, as struck out above.

author: watchmaker
author_rank: Unknown
timestamp: May 14, 2025, 02:17:25 pm
content: The hum seems fixable now as you can see by the attached screenshots. I will install a dedicated ckt. It ws primarily the USB hubs. I also attached the grounding lead from the mat connector directly to the housing of the Amazon power strips. All TE ground points to ground are less than 1 ohm.But there is something I do not understand about how the Siglent handles 1x/10x attenuations at low amplitudes.I used CH2 and CH 3 as controls: both are fixed 10x Lecroys.Ch 1 is the switchable 1x/10x PP510. There were two conditions. Ch1 attentuation (and the probe switch at 10x) at which settings all three probes agree at around 21mVAC p-p.Condition 2 is when Ch 1 attenatuation (and the probe switch) are set to 1x.In this second condition, Ch1 reports 2.3 mVACp-p while CH2/Ch3 continue to report 23mVACp-p.Why is not Ch1 reporting 23 mVAC p-p regardless of attenuation settings? What do I not understand?

author: electr_peter
author_rank: Unknown
timestamp: May 14, 2025, 02:59:39 pm
content: First, X1 probes have much lower bandwidth than X10. Check your probe datasheet. Lower BW -> lower amplitudes on wide BW noise.Second, can you describe what exactly are you probing? If probe tip is just sitting in the air, it will pick all sorts of crap. Place tip on scope's ground or some circuit, then most of noise will disappear.

author: watchmaker
author_rank: Unknown
timestamp: May 14, 2025, 04:24:23 pm
content: I initially was shocked (pun) that sitting on my mat my 10x probe was reporting 120mVACp-p. That led me to track down why. After tearing everything out, moving the USB hubs, attaching the mat ground lead directly to the Amazon housing, I got it down to the 20mV range. Tautech also dinged me about the loading of the 1X probe. And I Thank you also. (I posted a link to my last post in TE/Siglent 804).I think this issue (the 120mV noise) was giving me grief I could not understand. At any rate, I should have the bench on its own circuit and this winter I may build a real room with three walls of counter/bench and a place for materials I will use on current projects. No LEDs if I can help it but the LED overhead trays do not seem to create noise for me (on/off test). I am even thinking of putting grounded chicken wire behind the drywall, although one wall will be 8 ft below grade.I was thrown by the fact that the 1x was almost precisely 1/10th of the other readings. One of the things that makes me a decent watchmaker is that I am bothered by discrepancies. But those are at least physical things you can touch, hear and measure directly. With scopes, I am finding you have to learn which are artifacts that must be ignored.At least I got my bench cleaned.Regards,Dewey

author: electr_peter
author_rank: Unknown
timestamp: May 14, 2025, 06:00:03 pm
content: With regards to X1/X10 to make sure you have setup everything correctly. Setup one probe to X1, setup other to X10 and place both probes on scope's calibration output. If everything is correct with setup, measurements should be almost identical (on a known low BW signal) for those two channels.Scope tip in the air acts as a poor antenna, "radio reception" is on scope's screen. Your ideas to lower noise levels are conceptually similar to anechoic (EMC) chamber construction for RF work. IIRC there are some excellent EMC chamber build topics on EEVblog forum.Electricity in general is almost invisible to us, but with tools and knowledge one can peak behind the curtain. Learn bit by bit and it will be more visible.Have a nice day.

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2025, 11:03:56 am
content: The ESD mat doesn't need to be and indeed shouldn't be well grounded. If I remember rightly, it should be connected to earth via a 1M resistor, to limit the current to a safe level, should it make contact with the mains. In reality the resistance to earth at any point of the mat will be much higher than that, as it'll be made of carbon loaded, rubber/PVC which is a poor conductor.The ESD mat is picking up mains hum and the resistance to earth is far too high to adequately couple it to ground. I doubt it'll be an issue, unless you're probing a circuit with a high impedance and in that case your probe will cause significant loading and you should consistor using a differential probe, which will include a built-in, high impedance amplifier.

author: watchmaker
author_rank: Unknown
timestamp: May 15, 2025, 11:56:16 pm
content: Yes, the mat does go thru a 1Mresistor to my earth ground.For those following, the issue was a poorly grounded mat lead that is now screwed to the metal cover of the Amazon power strip. That took my initial 120 Vac p-p down to around 50 mV.Then I turned off the outlet box that powers my USB hub and devices. That got me down to under20 mVac all traces show the noise is 60Hz.These measurements are with the probes unloaded and sitting on the ESD mat. My concern was the noisy data are impediments to precision measurement. And 120mV seemed awfully high when working with 1 V signals.Now however, the question is what IS an acceptable unloaded noise level. I literally have no idea what the expectations are.I wound up setting up a math averaging channel and then doing an FFT on that a couple days ago, but would rather fix the issue if possible, instead of compensating for it (which I know is required in the real world).Below are screenshots of my unloaded probes resting on the mat, one with USB ON, and one with USB OFF.THANKS!

author: thm_w
author_rank: Super Contributor
timestamp: May 16, 2025, 12:20:02 am
content: Quote from: watchmaker on May 15, 2025, 11:56:16 pmThere are none, don't use it as a reference for anything.Measure an actual circuit with it (say a power supply rail) and then shut the power off and measure again, and report back.If you did have some ultra low noise product you were working on, you could construct aground planewith a large metal sheet, or a metal box, as explained in the posts above. But its unlikely you are working on something that precise.If you want to measure magnetic field, you can short the probe tip out in a loop:https://emcesd.com/tt070199.htmUnless you are very close to a transformer you shouldn't measure much of anything.

author: ME
author_rank: Frequent Contributor
timestamp: May 19, 2025, 05:04:46 am
content: Most things hum when they forget the words.

author: Sorama
author_rank: Frequent Contributor
timestamp: May 19, 2025, 05:22:02 am
content: Your scope inputs are set to AC coupling.Can you measure/make screenshots when using DC coupling ?

author: JustMeHere
author_rank: Frequent Contributor
timestamp: May 19, 2025, 08:56:52 am
content: Make sure your mat and scope are on the same ground. Same power strip if possible. You're describing the symptoms of a ground loop.

author: watchmaker
author_rank: Unknown
timestamp: May 22, 2025, 12:34:41 pm
content: As a follow up.Went back tostudyDave's videos and Keysight on probing low voltage signals.I was not using the intensity control properlyI was not using the Eres and averaging properlyFor low amplitudes, a 1x is better than a 10x probe.These are all things that were I in an "apprenticeship" situation (school, work environment, etc) my overseer would have corrected.I especially gained a lot on Dave's videos on "Why DSOs "appear" noisy"!! They are revealing much more info than is possible on an analog scope.Then I watched his videos on Bypass capacitors. Much of what I remember from 50 years ago is useless! Which is why it is taking me so long to learn the Siglent Scope. Interference from old learning.