SDS 1104X-E I could use some help capturing DC  - Page 1
Forum section: electronics / testgear

author: slodat
author_rank: Contributor
timestamp: March 30, 2019, 08:25:29 pm
content: As the subject states, I need some help. I have a Siglent SDS 1104X-E. I’m needing to use it to capture a 0-3.3vdc output from a three servo drives. I’m new to this oscilloscope and it’s kicking my arse.For the life of me I can’t get a nice, continuous rolling dc trace. Instead I get all of the minute noise that’s most likely being induced by the mains ac going to the servo drives.I was thinking I would do a single shot when the signal is changing and then be able to change the time base and get a meaningful trace of what the signal did during that capture. I need to capture about 3-10 seconds. I have time bass at 500ms, hit single and it get this:As I “zoom in”, it’s just noise.Is there a way to filter out the ac and just show the dc?

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2019, 10:19:37 pm
content: Quote from: slodat on March 30, 2019, 08:25:29 pmSure.First, where are your probes referenced to ?Is it 0V on the servo drives ?Is this point the same as mains Ground ie. the same as the BNC's on the scope ?It reallymustbe at the same potential in order not to create a ground loop and have potentially damaging currents pass through the probe reference (GND) leads and the scope.Important^Above all ^ satisfied, let's just use a single channel to keep things simple.Check probe attenuation switch and channel settings are both 10x.Set this one channel to DC coupling and V/div to 1V. ( Ch3 in your pic is in Fine V/div mode....Select that channel and re-press the V/div button to return to Coarse V/div steps mode.)20 MHz BW limit ON is OK for this job.@ 500ms/div you're into auto Roll mode territory and you can chose to turn that OFF with a press of the Roll mode button.Trigger level needs be comfortably above 0V, if you have a waveform you can set the level to 50% with a press of the Trigger Level encoder.Try all that and give us a screenshot with your results.Screenshots to USB with an appropriate menu displayed are easy with a press of the blue Print button and then you can attach the PNG file to a post using the Attachment feature below the text entry box.If you get all out of shape with the scope settings you can get them sorted with Default.....that you can also customize Default to return to your favorite settings....investigate this when you know the scope a little better.

author: slodat
author_rank: Contributor
timestamp: March 30, 2019, 10:44:03 pm
content: I'm referencing "internal signal ground", per the drive manufacturer's instructions. Both are pins on a DB25. I added a wire for the signal, and reference to my DB25 cable. I'm clipped on to them. There's 31.49 vac second harmonic (120 Hz) / 0.070 vdc between the "internal signal ground" and the protective earth terminal on the drive. Crap... i bet that difference in potential is what the scope is showing.. Trying to decide if I can ground that "internal signal ground" and not cause any damage.This is the manual for the driveI'm working with.

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2019, 10:55:04 pm
content: Are pins 1 and 12 the same ?If not try referencing off pin 12.......with the same proviso it won't create a ground loop.From your pic I don't see any reason why the 3V DC signal you're trying to read should be obscured by AC interference. Maybe shorten your pick off leads a little but later when we know you're on the right track.

author: Rerouter
author_rank: Super Contributor
timestamp: March 30, 2019, 10:57:51 pm
content: The other thing to test if that ground would have issues with being referenced is to measure if there is any DC voltage between the 2 grounds, and again with some resistance e.g. 10K between them, if it rapidly falls to 0V its a floating internal power supply and wont care, if it maintains some difference bigger than 0.3V, its likely some current shunt offset trickery and cannot be directly referenced

author: slodat
author_rank: Contributor
timestamp: March 30, 2019, 10:59:21 pm
content: Quote from: tautech on March 30, 2019, 10:55:04 pmThe drive manufacturer told me to reference pin1 and measure on pin8.I updated my previous reply with more info.

author: slodat
author_rank: Contributor
timestamp: March 30, 2019, 11:03:44 pm
content: Quote from: Rerouter on March 30, 2019, 10:57:51 pmI get open on my Fluke 87 between pin1 and earth ground when powered off. I’m a little nervous about screwing things up. Not sure what you’re saying to do..

author: tautech
author_rank: Super Contributor
timestamp: March 30, 2019, 11:34:06 pm
content: Let's us examine this:With V/div set to 200 mV we can see ch1 0V trace reference point is below the bottom of the display so adjust the V/div to a lower sensitivity as previously suggested.Then set Trigger to some point below the max amplitude of the trace but above the 0V position.This flatline trace IS a DC value but with zero fluctuation in the time domain however it does have some noise on it but we can disregard that for now.

author: Rerouter
author_rank: Super Contributor
timestamp: March 30, 2019, 11:34:43 pm
content: sorry, set your fluke to DC voltage, between the 2 ground pins,Your multimeter is essentially a 1 Megaohm resistor, so if its a floating power supply inside the servo controller, it should quickly drop towards 0V when powered on.If it remains at a consistent DC voltage higher than about 0.3V, it may not be safe to connect your scope ground to,

author: slodat
author_rank: Contributor
timestamp: March 30, 2019, 11:40:39 pm
content: Quote from: Rerouter on March 30, 2019, 11:34:43 pmPin1 to earth ground is stable at ~0.070 vdc.

author: Rerouter
author_rank: Super Contributor
timestamp: March 30, 2019, 11:41:35 pm
content: Then you should be perfectly safe to connect scope ground there to measure relative to it.

author: StillTrying
author_rank: Super Contributor
timestamp: March 31, 2019, 12:08:35 am
content: The last 2 scope screens showing the 0V offset to -1.640V means you're viewing the servos +1.65V level.I sometimes switch probes to X1 to filter HF spikes.

author: nctnico
author_rank: Super Contributor
timestamp: March 31, 2019, 12:36:15 am
content: IIRC you have the GDS1054B as well. Use that and enable input filtering. It will get rid of the noise your probes are picking up.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 12:46:33 am
content: Quote from: tautech on March 30, 2019, 11:34:06 pmQuote from: StillTrying on March 31, 2019, 12:08:35 amI changed to 1X and this is what I have now.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 12:49:05 am
content: Quote from: nctnico on March 31, 2019, 12:36:15 amThat's the problem.. it already went back. So, I'm trying to figure out how to do this with the Siglent.

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2019, 12:50:53 am
content: Perfect but bring your trigger level down some.Then you're all ready to Single capture an event. Just remember your trigger Edge setting needs to match what you expect to capture. ie. a rising or falling event.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 12:53:11 am
content: Quote from: tautech on March 31, 2019, 12:50:53 amThere’s something I’m doing that keeps setting the trigger at the signal level. Haven’t sorted out when encoder press is doing it. I thought I dialed it down to .8v.I guess I thought the trace would be thinner... perhaps I’m seeing the ac ripple of the servo drive internal dc supply?

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2019, 01:09:11 am
content: Quote from: slodat on March 31, 2019, 12:53:11 amAs far as the scope knows the waveform is just the 1.68V trace so pressing Trigger puts the level on the trace.If you had a trace with a step then yes, a press of Trigger level would place it to 50%.Yes I know there's a lot of basic stuff to learn with any scope but now you're close to capturing an event (if there was one) you'll soon have your head around this elementary stuff.Have a play with just a 1.5V battery across the probe and then use settings to capture the step event.QuoteDon't worry about this for now it's only clouding your mind as it's a decent event is what you're looking for and how to capture it. KISS for now.

author: StillTrying
author_rank: Super Contributor
timestamp: March 31, 2019, 01:24:06 am
content: I think you're not seeing the +1.65v trace move because the +1.65V signal is not changing!Whatever the servo drives are doing you might only see changes of the +1.65V with big loading changes or at switch on.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:28:28 am
content: This isn't working the way I was hoping it would.This is a small error signal:

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:32:01 am
content: This is what I'm going for (yes, I regret returning the other scope

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:37:08 am
content: Maybe that gives an idea of what it is I need.When I connect additional probes, the signals get worse.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:39:56 am
content: Quote from: tautech on March 31, 2019, 01:09:11 amI appreciate you taking the time to help me out! Thank you!

author: genghisnico13
author_rank: Regular Contributor
timestamp: March 31, 2019, 02:47:53 am
content: the source of that noise needs to be found, but wouldn't High Resolution mode be ideal for the signals that OP is trying to capture?

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2019, 02:49:19 am
content: Quote from: slodat on March 31, 2019, 02:28:28 amNope, some user error.QuoteLook at the 2 images and in particular the trigger position in relation to the selected Trigger's waveform and trigger settings are also different.IF in both these screenshots ch1 is on the same source there is indeed a mismatch of ch1's signal in relation to ch's 2 and 3.We can examine each in more detail by going to AC coupling and/or increasing sensitivity and having the channel 0V level off the display (within datasheet limits).

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:53:20 am
content: I'm realizing how oscilloscope dumb I am.I'm not concerned with where each channel's trace is in the gridplot. I'm only looking at +/- deviation from 1.65vdc. I move the zero's up and down so I an see all three drives at one time.I measured the noise, as reported above, with my DMM as second harmonic (120hz) 30ish vac. I don't know how to get rid of it.. I'm sure this is easy, but it's kicking my ass.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:55:10 am
content: Quote from: tautech on March 31, 2019, 02:49:19 amAll three channels are on the same signals in both screenshots.

author: tautech
author_rank: Super Contributor
timestamp: March 31, 2019, 03:05:53 am
content: Quote from: slodat on March 31, 2019, 02:53:20 amNot really as diving in doing 3 channels to start with can be pretty daunting.Just using one channel takes a little bit to understand sound scope usage let alone 3.Ya doin alright.QuoteYep, when using DC channel coupling that is what you need to do.QuoteHave a play with Averaging or E-Res. Noise can be a curse but it can also indicate there's another issue that needs addressing.....when we see it on the display we then have to determine if it's a problem, something we can ignore or something to use the tools within the scope to clean up.I'd be more concerned if the scope didn't display all the noise......environmental and circuit noise.Threads like these are great resources for the DSO newbie as they give lots of hints of operation and things to try.Please carry on............BTW, get some sleep as it must be late in the US.Others will chime in and we can have another go tomorrow.

author: StillTrying
author_rank: Super Contributor
timestamp: March 31, 2019, 03:28:34 am
content: Quote from: slodat on March 31, 2019, 02:32:01 amDon't worry - yet. I think when you find out what's causing the noise the X-E display should should look at least as good as the Instek.I don't think you can use AC coupling at these very low 5Hz signal and 1s/div. speeds, but you can use the DC offsets.The noise when you connect the other channels is very strange, are you sure the GND gnd clips are alright and all connected toexactlythe same GND.As others have said you could use Hi-Res, not waveform averaging, at low sample rates some scopes use it in the background anyway, the Instek could have been, - I don't know.I suppose you could try it with the other 3 or 4 channels switched on but without the probes connected to the BNCs.

author: StillTrying
author_rank: Super Contributor
timestamp: March 31, 2019, 03:57:28 am
content: Quote from: tautech on March 31, 2019, 03:05:53 amIt's even later in the UK, and the clocks have gone +1 hr. so it's become early again.

author: nctnico
author_rank: Super Contributor
timestamp: March 31, 2019, 10:40:36 am
content: Quote from: slodat on March 31, 2019, 12:49:05 amAh... you have thrown the perfect tool for this job out. What is left is to turn on bandwidth limiting and hi-res mode to get a cleaner signal.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:49:40 pm
content: The ground clips are connected to their respective drive’s pin1. As I mentioned, there’s significant second harmonic voltage on these signals. When I connect the ground lead to earth ground all I see is the 120hz signal. I don’t think I can do this with this scope.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 02:51:48 pm
content: Quote from: nctnico on March 31, 2019, 10:40:36 amI know. I’m not impressed with my quick decision. At all.I have the bandwidth limit on and eres didn’t do anything to make the dc signal pop out any better.I don’t know what else to try on this scope. Wishing I wouldn’t have chosen between the two so quickly.

author: nctnico
author_rank: Super Contributor
timestamp: March 31, 2019, 03:01:03 pm
content: An alternative would be to try a differential probe or differential amplifier. That way the common mode noise gets suppressed.Edit: something else you can try is not to use the probes but connect coax cables directly to what you are measuring and don't strip the shielding of the coax any further than you have to.

author: rf-loop
author_rank: Super Contributor
timestamp: March 31, 2019, 04:00:51 pm
content: Quote from: slodat on March 31, 2019, 02:51:48 pmOf course there is different scopes and different methods to filter out higher frequencies. Some scopes, like example GW have user settable low pass, band pass and high pass filters and quite useful in some situations but not all. Also some older Siglent have these. But then it need also note that these are made in digital side (DSP). In some cases, my opinion is that even they works ok many times but - as told they are digital side. If example low pass filter freq is set for 100kHz how it know if signal is true signal or false signal produced internally before this digital filter.So or so, I do not want start infinite unnecessary debate about this.If you really have there high noise level in signals, in signal itself and more due to noisy environment and noisy ground and ground loops.You can - if you have available - differential probe.But if there is still in signal line itself high noise...If you have some materials and tools... and can do some experimental things.You can filter noise out _before_ scope input.Because I do not know enough details about your signals under tesdt and what you need look, exmple what kind of risetimes/frequencies you need look and what are these what can filter out.Because I do not know your signal true level related to ground I can not recommend next.Set probe to 1x (and scope). Take 1kohm resistor. Other end to probe tip and other end to signal. Then connect 0.1uF capacitor from probe tip to probe ground (just this ground what is around 5mm after tip. (take this hook out first). Then look how it looks now. After then, depending result, can think more.(this is 1kHz 1st order LPF what give 20dB attenuation for decade. It is not lot but it may give some idea what to next. You can do same with cheap 50ohm or 75 ohm coaxial.Because, what ever skope yuu take if there is very heavy noise and it go to scope front end and to ADC it may still rotten your result. So it is some cases quite wise use filter before scope input.Your connection to signal in one image looks really poor. It is like "please come in all noise..please come all in... "Also your probe... (in image I can see setup with 10x) it is full band in 10xScope input 20M BW is also something like 1st order filter sxo even it do not attenuate even 40 - 100MHz very well.But this same probe in 1x mode, it acts also like 1st order LPF around 5 - 8MHz, together with 20M BW you have bit bnetter attenuation but 0 - several MHz come in without attenuation. Do you need look 10MHz or 35ns risetimes. I think not...It do not cost lot if you make some trial with simple external RC (but also this, do it so that it also attenuate and is not like antenna for collecting all noise.I think whole problem is "how to connect this signal to scope" (and how to filter it before scope input)There is no scope what have internal analog side low frequency low pass settable filters. Least I do not know any - and over 50 years have not seen any. But very often have used external filters before oscilloscope inout and in some cases they are really easy to do (exept if need more steep that 2nd order)You can start with one resistor and one capacitor. After thenifit looks anyhow useful you can think how to do it bit more better and usable. Test do not cost nearly anything. Just think what are highest frequencies/fastest risetimes what you need see reliable without attenuation... this is starting point.

author: StillTrying
author_rank: Super Contributor
timestamp: March 31, 2019, 05:08:37 pm
content: I've read the thread and the servo manual, all I can say is that this seems to prove that it's the connecting of just 2 or more grounds that causes the noise.

author: slodat
author_rank: Contributor
timestamp: March 31, 2019, 05:32:47 pm
content: I think so, too. But, I don't know what to do about it. It appears each drive's internal power supply is floating..

author: Martin72
author_rank: Super Contributor
timestamp: March 31, 2019, 05:46:12 pm
content: As written here before, try the Enhanced Resolution mode:Normal Mode siglent:Hi-Res Mode (+3bits) :Or, try to connect a little cap (1n..10n)on the bnc input of the scope ( by using a bnc t-adapter)

author: rf-loop
author_rank: Super Contributor
timestamp: April 08, 2019, 03:19:37 pm
content: Quote from: slodat on March 30, 2019, 08:25:29 pmBecause some times also other peoples may play with same kind of situation.I will show one "poor man's" solution. Not rocket science but extremely simple (and poor) but some times still usable and may solve some problems.Method is extremely simple and most can do it (if have BNC T and some capacitors.Just like this:With this I've done some simple experimental measurements.First, PP215 1x (1:1) frequency response (source 50ohm)Measured using SDS1104X-E and one Siglent SDG function generator. (Not SAG1021)SFRA dB scale is just so that signal trace left can think as "0dB" reference. (least now this FW version there can not se "offset" so that can set 0dB to my reference level,)With this probe I measured 1.5V DC line (with extremely heavy noise)Before filter:You can see V/div is 20mV and offset 1.5V and heavy noise over screen but level roughly 1.5V.Ok, now I can not shut off this somewhere coming random noise. What to do, I want see this "DC" as OP want.In this case I want still see DC level fluctuation and possible some 100Hz ripple and other things up to near 1kHz so that these things are still roughly level accurate. Then attenuate higher frequencies.I made RC LPF using this BNC T method and look its frequency response.Frequency response: Probe 1x, and capacitor (sum of 10nF+10nF+470nF parallel capacitors =490nF)We can see corner is around 850Hz (Im lazy and not want anymore solder or remove)And how this 1.5V rail now looks.After filtering. Compare image before filtering. Signal is exatly same only added this BNC T between scope input and probe.Now I can see this 100Hz 20mV ripple and also of course if this "DC" is other ways drifting or fluctuate.(do not care right top corner frequency meter, it is nearly nonsense - noise and partially also due to method how I make this GIF animation)Also you may note I have dropped samplerate. No problem because scope can not see so much >fNyquist frequencies due to filter before scope. But reducing samplerate (memory length) I also can get more acquistions to history buffer (in this case around last 15 seconds what is nonsense for just show this filter method.Of course with this method can use with different capacitors and also it can use when probe is in 10x (1:10). Example under 1Hz... or what ever.Here 40kHz LPF (probe 1x capacitor in BNC T 10nF)It is not good to stand hands in pocket if there is a problem that needs to be solved. Quite often, some help can be found by very simply. Simple is beautiful. Or should I say the old way can sometimes be better than whole bag of new ones.

author: Martin72
author_rank: Super Contributor
timestamp: April 08, 2019, 07:06:30 pm
content: QuoteAs I said before:QuoteThanks for the pics !Martin

author: rf-loop
author_rank: Super Contributor
timestamp: April 08, 2019, 07:39:15 pm
content: Quote from: Martin72 on April 08, 2019, 07:06:30 pmYes...and as short wires as possible...etc things specially if noise have HF components. (also there can see in my BodePlots, if want good, need also select what kind of capacitors... in this 800Hz LPF they are bit "bad" as can see.)And of course 10x probe need _very_ different capacitance what 1x not at all like tenth of value due to very different R before C and after C.Old simple trick - used tens of years.Other old trick for other purposes is using scope DC coupled but do AC coupling external for more low freg or more high freg.Siglent AC coupling corner freq is bit over 1Hz (very low if compare many other scopes)

author: Martin72
author_rank: Super Contributor
timestamp: April 08, 2019, 08:31:26 pm
content: The "trick" with the LPF we must use recently for measuring the output-ripple of a dc/dc converter.In this case it´s not enough to set the coupling to AC, you´ll see everything but not the "true" ripple.After using a 1n cap directly to the input, the noise disappear and you see the switching frequency of the converter, that´s the ripple and nothing else.