Oscilloscope as a DMM

[zorthgo]

@Andrejako, Thanks for all the great tips! I did a calibration on my scope, hope it helps. But, I do have a question. You said your Vk (cycrms) was close to the multimeter, is that what I should be looking at? Do you know what the cycrms stands for?

@ptricks, I was looking at the numbers you provided but I have to be honest, I don't know what they mean!


Thanks guys!

[zorthgo]

@AndrejaKo, I think you found what the problem was with my scope! It was the lever clamp holding the probes (ie.. Me). I was reading Vmax, but If I look at the Cycle RMS (Vk) I get precisely what my DMM is reading (off by 1.4 mV). I guess these things aren't idiot proof!   


Thanks guys for all the help!

[AndrejaKo]

Well it seems you got it. cycrms is indeed the Cycle RMS voltage.

One thing I noticed from your multimeter thread is that perhaps you rely too much on the money and too little on yourself. You can't fix inexperience by just getting a more expensive instrument. In fact it's the opposite. Using a more expensive instrument properly can be more difficult than using a cheaper one.

You got one stage right by posting questions here.

Here are some more mysterious numbers, this time from the manual of your scope:

DC accuracy  ±3%
DC accuracy(average) Average?16: ±(3% rdg + 0.05 div) for  V

and

Interval( T ? ) accuracy (DC?100MHz)
Single?±(1 interval time+100ppm×reading+0.6ns);
Average>16?±(1 interval time +100ppm×reading+0.4ns)

Try thinking a little bit about them and do your best to decode them. This isn't very difficult and experience should be useful to you. Don' be scared of abbreviations. Remember to use the Google!

Also about the thumb-drives, here's what manual say:
Install the USB disk: Insert the USB disk into the "1. USB Host port" of
"Figure 4-3 Right side panel". If an icon  appears on the top right of the
screen, the USB disk is installed successfully. The supported format of the USB
disk: FAT32 file system, cluster size cannot exceed 4K. Once the USB disk
cannot be recognized, you could format it into the supported format and try again.

So you need a drive which is formatted in FAT32 and has cluster size of 4096 bytes or lower. If you use Windows, just right-click on an empty drive (or at least one having no important data) and use Format... option to format it using setting explained here.

Another two things I'd recommend you read, if you already haven't, are free electronic books from Tektronix called ABC of Probes and XYZ of Scopes. They do focus a bit on the high-end of equipment, but they contain lots of useful information which can be applied to pretty much all scopes available today. There are also explanations for a lot of old terminology that came from the previous era of scopes and is still being used .

[Lawsen]

It is a good question.  Oscilloscope for DC signal will be horizontal line straight across.  As mentioned by others replying, an oscilloscope is mainly for viewing the signal, you will see both the DC and AC components of the signal if any are present, but it will not be accurate +/- 2% to 4%, depending on the calibration and oscilloscope specification.  Radio Shack Micronta brand multimeters are not bad, but again Like I like the Fluke brands like Fluke 187, 8050A bench, and HP (now Agilent) brands.  You need another different brand of multimeter to compare it with your Radio Shack multimeter.  An oscilloscope cannot replace a multimeter and a multimeter cannot replace an oscilloscope.  I think there are test and measurement products available that combines the two, an oscilloscope and multimeter as one instrument, but still, a separate function, not the same.  Fluke Scope Meter, Agilent hand held oscilloscope, Tektronix older analog has a model with a separate digital multimeter included into the same case, and Hantek.  These are still spearate instruments combined into the same case by separate circuit or separate software function.

[zorthgo]

@AndrejaKo, Dude you're awesome! I partitioned my thumb drive to 1.5GB (the lowest I had was a 32GB drive) and it worked fine. Now I know why! The whole semester of Electronics I, I kept taking pictures of the lab's o-scope screen with my cellphone to use on my lab reports because none of our thumb drives worked.

By the way, I can take a chance with the first set of numbers you gave me. I think it means that the scope is + or - 3% off when measuring. But the second set of numbers I have no idea!

[zorthgo]

@Lawsen, that's what I was trying to do, compare the measurements I got from the o-scope with that of the DMM. But when the numbers don't match, I don't know which one is right. I thought the o-scope might have been the one with the faulty measurements because the DMM was much closer to my calculated values than the o-scope. But that is why I am look to get another DMM, then I will have 3 devices to compare values with! 

[w2aew]

@Lawsen, that's what I was trying to do, compare the measurements I got from the o-scope with that of the DMM. But when the numbers don't match, I don't know which one is right. I thought the o-scope might have been the one with the faulty measurements because the DMM was much closer to my calculated values than the o-scope. But that is why I am look to get another DMM, then I will have 3 devices to compare values with! 

One other important thing to remember is that the scope input is always ground referenced, so you can only make measurements with respect to ground (unless you use a differential probe, or ch1-ch2 math).  With a DMM, you can make voltage measurements between any two arbitrary nodes in a circuit.

[zorthgo]

@w2aew, I  remember that in Electronics I, one of my labs talked about that. We had to isolate the ground connector on the o-scope and do our measurements. I think it was called a "floating something". But you actually brought that lab into practical use. Thanks! Let me see if I got this right, the DMM can measure the voltage difference between two point in a circuit (such as the voltage drop across a resistor), while the o-scope can only measure the difference between ground and the specific point in the circuit. Is that right?

[w2aew]

@w2aew, I  remember that in Electronics I, one of my labs talked about that. We had to isolate the ground connector on the o-scope and do our measurements. I think it was called a "floating something". But you actually brought that lab into actual practical use. Thanks! Let me see if I got this right, the DMM can measure the voltage difference between two point in a circuit (such as the voltage drop across a resistor), while the DMM  scope can only measure the difference between ground and the specific point in the circuit. Is that right?

Oh my, I can't believe that they let you isolate the ground to float the scope, especially in a class environment!!!  You should NEVER do that, it can be very dangerous - to the equipment and YOU!

Yes, the DMM is naturally isolated, and can be used to measure the voltage between any two points, like the voltage across a resistor or other component - no problem.  With the scope, you're measuring voltage from any point to ground (unless you have one of the scopes with isolated inputs, which isn't the norm).

[c4757p]

Oh my, I can't believe that they let you isolate the ground to float the scope, especially in a class environment!!!

As a current student: I can.

[zorthgo]

My bad, I meant o-scope not DMM. Regarding the floating scope, I do remember the teacher warning us that we needed to be very careful or we might electrocute ourselves. Those labs only helped us get familiar with the equipment. I don't think we actually learned anything new about electronics in the labs. Most of the time we looked like this ->  ! lol!

[c4757p]

If you float anything, float the DUT, not the scope. The front panel has no electrical insulation because it is expected to be grounded, very dangerous if it's at an elevated potential and you go to twiddle a knob.

The "correct" answer is a differential probe, but we don't all have one of those. (Though you can set the scope to display Ch 1 - Ch 2, which can work as a diff probe.)

And above all, don't modify anything safety- or power-related unless you understand what it is, why it is designed like that, and what the implications of your change are.

[eKretz]

Let me see if I got this right, the DMM can measure the voltage difference between two point in a circuit (such as the voltage drop across a resistor), while the o-scope can only measure the difference between ground and the specific point in the circuit. Is that right?

Actually, most 'scopes can do this too, using differential measurement of 2 channels and the math function. I.E. Ch.1-Ch.2 with Ch.1 at 5VDC and Ch.2 probe at ground will give you 5v-0v, or 5v, with no ground connection, for the same measurement a DMM will give you.

[zorthgo]

Awesome, I think I got it. Channel 1 would be the voltage at the place I am measuring, and channel 2 would be the ground/reference point. I was definitely not doing that when I was trying to use the scope as a DMM. I will try that when I get home from work today.


Thanks guys! I've learned quite a bit from this thread. 

[w2aew]

Awesome, I think I got it. Channel 1 would be the voltage at the place I am measuring, and channel 2 would be the ground/reference point. I was definitely not doing that when I was trying to use the scope as a DMM. I will try that when I get home from work today.


Thanks guys! I've learned quite a bit from this thread. 

I did a video a while ago to show how we used to do this on the older analog scopes.  Of course, today's digital scopes simply do this as a simple mathematical operation on the sampled signals.  Here's the video on the analog scopes:

[alm]

While this method works, expect to see reduced accuracy and fidelity compared to single-ended operation or a real differential probe. If you're in an EE program, then low common-mode rejection ratio (CMRR) might ring a bell. No reason to avoid it, but if you see something odd like something superimposed on your signal, be sure to check for common mode signals by connecting both probe tips to the same point before suspecting a problem with your circuit.