SDS 1104X-E I could use some help capturing DC

[slodat]

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?

[tautech]

Is there a way to filter out the ac and just show the dc?

Sure.

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 really must be 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. 

[slodat]

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 drive I'm working with.

[tautech]

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. 

[Rerouter]

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

[slodat]

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. 

The drive manufacturer told me to reference pin1 and measure on pin8.

I updated my previous reply with more info.

[slodat]

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

I 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..

[tautech]

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.

[Rerouter]

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,

[slodat]

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,

Pin1 to earth ground is stable at ~0.070 vdc.

[Rerouter]

Then you should be perfectly safe to connect scope ground there to measure relative to it.

[StillTrying]

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.

[nctnico]

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.

[slodat]

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.

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.

I changed to 1X and this is what I have now.

[slodat]

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.

That's the problem.. it already went back. So, I'm trying to figure out how to do this with the Siglent.

[tautech]

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. 

[slodat]

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. 

There’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?

[tautech]

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. 

There’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.

As 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.

I guess I thought the trace would be thinner... perhaps I’m seeing the ac ripple of the servo drive internal dc supply ?

Don'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. 

[StillTrying]

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.

[slodat]

This isn't working the way I was hoping it would.

This is a small error signal:

[slodat]

This is what I'm going for (yes, I regret returning the other scope

[slodat]

Maybe that gives an idea of what it is I need.

When I connect additional probes, the signals get worse.

[slodat]

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. 

I appreciate you taking the time to help me out! Thank you!

[genghisnico13]

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?

[tautech]

This isn't working the way I was hoping it would.

This is a small error signal:

Nope, some user error.

Look 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).