Oscilloscope as a DMM

[zorthgo]

Hi Guys,

      I've got a question for you. I was playing around with some voltage division circuits (to refresh my memory from circuits I), and I was trying to measure the voltages with my oscilloscope (because I only have a cheap $20 radio shack multimeter I thought it would be more accurate). Well, the problem is that when I did all my calculations, the %error was pretty high @ 52%. So, I took out the radio shack multimeter and I measured everything all over again, and now the %error between my calculated values and my measured values went down to 17%. Does this mean that I should not rely on my oscilloscope for voltage measuring? Which would mean that a DMM is essential, and I should pick on up?

Thanks for the help!

[c4757p]

How large were the resistors? The scope has only one tenth the input impedance of the multimeter, so the readings will drop out with resistors roughly a tenth the size. Try using a 10x probe.

BTW, oscilloscopes are for seeing wave shapes, not making accurate measurements. Don't trust it for that - though it should be better than this.

[alm]

A scope should do a decent job as a voltmeter, but will be less accurate than a DMM unless it's way off. Expect the accuracy to be about 2%. They should be in the same ballpark, however, unless you're using a 1x probe/simple wire and the 1 MOhm input impedance becomes an issue, as c4757p already suggested. If your circuit was ground referenced, than clipping the ground clip to the circuit may have shorted parts of the circuit, and affected your measurements. Also when measuring AC, pay attention to the difference between peak, peak-to-peak and RMS voltage. If it is a digital scope, then it should have the RMS voltage as one of the automated measurements. Even then the DMM and scope will disagree if the signal is not sinusoidal and the DMM is not true RMS.

A DMM is essential, however. A scope is quite inaccurate as a voltmeter, and will not do current, resistance or continuity without external components. It's also ground referenced, while a DMM can easily measure between any two points in the circuit. A decent DMM will also have a wider range without switching probes, auto-ranging, better input protection and a bunch of other features.

A DMM is by far the most used test instrument for most electronics work. I would definitely buy a decent DMM before buying a scope.

[Paul Price]

My older Tek 475B analog oscilloscope was an excellent voltmeter that offered with 10x probe isolation very quick and reliable DC and AC voltage and current readings straight from the graticule that just about matched the accuracy and beat the settling time of  my 1.999 Full Scale count Fluke DVM. The 10x probes assured me that I wasn't upsetting the circuit I was attempting to measure, unlike the unshielded probes of the DVM which did.

In my opinion, a well functioning analog oscilloscope is the ideal universal measurement device and should give precise, repeatable and accurate DC readings and also accurate readings of measurements of changing voltage waveforms versus time.

A digital scope adds the  the possibility of recording capability, but my analog oscilloscope would never be just thought of as some occasionally used piece of equipment considered to be inaccurate and just needed to view "wave shapes."

My Fluke DVM offered portability, best for safe mains voltage isolation and a good  way to measure low-impedance floating voltages and is an essential tool to measure resistance and even test semiconductors and circuits for short-circuits.

A DVM and an analog scope are the two most essential electronic test bench and circuit development measuring and testing tools.

[zorthgo]

@c4757p, the resistors weren't very high. One was a 2K and the other was a 100k. Very simple circuit!

@alm, I was using a 10X probe, and had configured the oscilloscope accordingly.

Should I have been getting better readings from the oscilloscope or is this normal?

Thanks guys!

[c4757p]

Yes, if it has a history of accuracy. My Tek 2445A is calibrated in both time and voltage to within the limits of my eye to read from it, including both graticule and digital cursors. But I calibrated it myself, I've used it for a long time, and I have reason to believe that it's accurate. If all you can compare it against is a $20 Rat Shack multimeter, you don't know how accurate it is.

Should I have been getting better readings from the oscilloscope or is this normal?

You should have read much closer. Check the VAR / vernier knobs - they may be out of the set position, and a lot of times people don't think to check them.

[zorthgo]

@Paul Price, at school we used tektronix oscilloscope and they were fairly accurate, but the one I have is an Owon DS8102V so I don't know if it is as accurate as my school's!

[c4757p]

A new DSO should be quite accurate, even though an Owon's no Tek. Check all your settings and check your probes.

When I warned you not to trust the voltage reading, I didn't know whether you had a brand new DSO or an old clunker analog CRO. You should absolutely be able to trust the voltage and time readings from a new digital scope. (Just remember that the ADC is only 8-bit, meaning it's only a "256-count" voltmeter. Your $20 meter is undoubtedly a 2000 count meter. So it's accurate, but not precise.)

[Paul Price]

Zorthgo, in one word, yes!

Your readings should not have been anywhere so far out of the ballpark.

[zorthgo]

I might be reading the wrong value on the oscilloscope. There are quite a bit of different voltage values. I was reading the pk-pk voltage. But the Vmin seams to give me a closer value to what the DMM is giving me. Should I be looking at Vmin for a simple DC voltage division circuit? 

[c4757p]

Yes. Peak-peak reads from the lowest value to the highest. For pure DC that should be zero. In reality you'll pick up some noise and it will measure that. Use Vaverage.

[zorthgo]

I don't have V-average, the only one I have are: V-pk-pk, Vmean, Vmax, Vmin, Vtop, Vbase, Vamp. Vmin seams to be closer to what the DMM is giving me. But at school, we usually looked at Vmax. But here, Vmax is also off by quite a bit.   

[c4757p]

Average = Mean.

 

If Vmax is significantly out from Vmin, you've got a good amount of signal noise that you're picking up. Some of the Owon scopes are infamous for it. Look for an averaging function (for the waveform, not the measurement) if it bothers you. Also, consider what I said before about the 8-bit ADC. This means that if you're set to 0.5V per division, with 10 divisions (so 5V across the screen), the smallest voltage difference that can be resolved is 5/256 = 0.02V. Don't expect to get any closer than that.

[zorthgo]

The Vmax is reading 400mV while the Vmin is reading 240mV which is exactly what the calculated voltage is. The oscilloscope is set to 2V per division. Is there a way to eliminate the signal noise?

Thanks!

[c4757p]

Disconnect your probe and zoom in as far as you can to whatever you see on the screen, and post a screenshot. Do it with both channels. That is a lot of noise, and there's always the chance that it's faulty.

It could also just be a limitation of using measurement functions that expect a substantial AC waveform with DC, but it's probably best to make sure.

[zorthgo]

@c4757p, none of my thumb drives works with this oscilloscope so I had to take a picture with my phone.

Both leads are measuring the same thing. But giving me different readings!

[c4757p]

Ch2 is a bit yucky. You did disconnect the probes completely, right? I'd say Ch1 is perfectly normal, though. There are a few threads here about either this scope or a related one, why don't you go read up on it? I seem to recall noise issues.

[IanB]

The Vmax is reading 400mV while the Vmin is reading 240mV which is exactly what the calculated voltage is. The oscilloscope is set to 2V per division.

Wait a minute, what? 

Elementary instrument theory says that for maximum accuracy you should measure as close to the full range of the instrument as possible.

You can't measure 240 mV with the scope set to 2 V per division. That would be what, 16 V or something full scale deflection? Set the scope to a scale compatible with your measurement range.

[c4757p]

Nice catch, I missed that. I still think the point about the noise stands, though - look at that picture.

[zorthgo]

@c4757p. My bad, That was a picture of the circuit I was talking about. This is a picture of the oscilloscope without any probes connected to it.

[zorthgo]

@IanB, I don't quite know what "full scale deflection" means! Are you saying that I should go up or down in voltage per division?

[IanB]

An oscilloscope has a beam that is deflected by an input signal to produce a trace on the screen. In a traditional analog scope this is a real electron beam deflected by high voltage plates in the CRT, but in a digital scope the beam is emulated using digital signal processing.

Nevertheless, it helps to think of the basic operation of a digital scope in the same way as an analog scope.

So the beam with no signal has a baseline on the screen. When there is an input signal the beam is deflected up or down to produce a signal trace.

To get the maximum resolution with a positive going signal (i.e. upwards), you would position the baseline near the bottom of the screen and then adjust the vertical scale so the maximum upward deflection is near the top of the screen.

[zorthgo]

Thanks guys, for the help! I guess I am going to be buying a better DMM so that I can compare measurements with the oscilloscope (at least in the beginning while I am getting the hand of how to use the o-scope).


Thanks!

[AndrejaKo]

I have the 100 MHz model of that scope, and here are a few tips I hope you will find useful:

If you want to do averaging on that scope, press acquire button, H1 for acquire mode, F3 for average and use the multipurpose know to set how many samples you want to average.  The more samples you use, the more time it takes for the values to settle down, so this will keep noise out. Use acquire, H1, F1 to go back to sample mode. If you're looking for quickly changing signals, you can use H1, F2 to go to peak-detect mode. With it, the scope will display the most extreme values it captures on the screen.

Also keep in mind that you can have up to 8 measurements at the same time, although it looks as if only 4 will fit the screen.

Another thing to keep in mind is that your scope does all measurements and cursor calculations on the displayed data and not on the captured data. So what does that in practice mean? Well it means that if you can't see it nice, it can't be measured nice. For example period measurements and rise times will provide incorrect results at too low sec/div values, voltages will be incorrect if volts/div is set too high and so on. Try to have the actual wave fill as much of the screen as possible.

To get rid of that "1: None is saved" screen at the scope, press the save button, then H3 button to go to Object and show and finally F2 button to set Show to off.


About precision:
Keep in mind that the scope has 3 measurement ranges which it can use. At 10X settings their borders are at 2 V and 20 V. Try to measure your signal with the lowest range that will fit on screen.

There's one more thing that you can do to increase the accuracy and that's self-calibration. Turn on the scope and keep it running for at least 30 minutes, so that it warms up. Then disconnect the probes and press utility button and then H1. Use the multipurpose knob to go to adjust function then press H2, pause for a second (a text screen should pop out now) and then press it again. You should get a screen with a progress bar. Wait for it to finish. Also keep in mind that you need to do this whenever the room temperature changes significantly, since measurements are affected by temperature. If you haven't done this in a while, make sure you do it before you continue to use scope voltage measurements.

My scope's Vk (cycrms in the measurement menu) usually very close to the reading I get from multimeter for DC and low frequency signals.

[ptricks]

I have a Tektronix 2246 and the voltage accuracy is listed at:
+- (.5% of reading + 2% of the SEC/DIV division + 250 uv)

[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.