#74: Basics of Differential Probes for Oscilloscopes and their applications: a tutorial
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- čas přidán 12. 01. 2013
- This video gives the basics of differential oscilloscope probes. It defines what they are, and discusses why and how they are typically used. A few examples of typical applications for diff-probes are given.
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Nearly 10 years later and this video is still being used! Super handy and clear
Cool. Your hand-drawn diagrams are super clear! I've never been able to write schematics anywhere near as decent.
Amen to that. My diagrams suck because I suck at drawing in general. Ideally I'd stick to SPICE, but I'm still waiting for the pen and paper port of that. =P
all your videos are amazing and your teaching style is 5 stars, thanx Sir.
There is always a touch of class in your explanation.
Very informative...You simplified a range of topics that have been borderline for me. Quality troubleshooting techniques and bench equipment operation are hard to find on CZcams. Keep them coming as much as you can. Signal tracing & oscilloscope operation is always in high demand!!! THANK YOU! Centralia College Electronics Graduate.
Your explanation about "Balanced audio" is ALMOST right. "Balanced" means that every line have equal impedance to ground (receiver) and nothing more. Having signal in reverse polarity on one line is called "symmetrical audio" and it have other purpose. You can have symmetrical but unbalanced interface, or asymmetrical and balanced same time. These are two different thing for different purposes. Balancing is for noise cancellation, symmetry is for getting bigger output signal from the same voltage (less crosstalk and slightly better SNR).
Great video!
Hi! This was an interesting video! I'm an electronics technician who works on vintage and modern vacuum tube equipment...Guitar/Hi-Fi amplifiers, tube radios, tube TV's, phonos and pro audio. as you may know many of the earlier tube radios/phonos didn't use a power transformer for the series wired filament string, and one side of the AC mains was connect to the chassis which can make the outside of the set hot (depending which way the plug is inserted) Using a scope on equipment like this can be deadly since the ground clip is referenced to the electrical system ground. Using an isolation transformer while working on this type of equipment is mandatory for safety...however, you still must be careful on how you connect your scope probe to the circuit under test. I've been quite careful working with my CRT scopes. Some techs float their scope as well as the DUT. Not a good idea either. Since the differential probe is basically isolated, this makes things a lot safer for working on a lot of vintage electronics. Equipment that use power transformers are much safer because there's built in isolation. Interesting videos! Keep 'em coming :) Frank Ferraro--Audio Craft Electronics.
Excellent - clear and straightforward as usual. thank you.
BenKrasnow , you have done some brilliant presentations on your channel also - have enjoyed both your material a lot.
Keep up the superb work in 2013 both of you.
Another excellent tutorial, Alan. Thank you.
Many thanks to Alen for an excellent explanation as always.
That clarification was cool
Thank you for covering this important topic!
Thank you for a great introduction into differential probing! :)
Thank you! You are an excellent teacher.
Your videos are clear and to the point. You have a way of taking otherwise complicated material and making it easy to understand. Thanks for sharing your vast electronics knowledge
especially as it relates to getting the most use out of "scopes". 73's n5kab
I appreciate you making this lesson. Thank you!
Experimenting with Pulse Width Modulator for a class E transmitter. The PWM switching FETs work against a floating ground. Voltage measurements here are differential. It's the following LPF that delivers the modulated DC voltage which is common mode (against chassis ground). Great explanation as usual.
Great video and excellent instruction. Thank you for putting up this video.
The gift of knowledge 🎓 Thank you 👍😁
Another good video - always an education.
Thank you so much for sharing your knowledge with us. Blessings
Great explanation!
Yes, I would appreciate this as well. Thanks very much!
Excellent presentation as usual.
What is your opinion on using an isolation transformer on the D.U.T., say a switching power supply, for instance, and an oscilloscope for measurments?
Thank you for all the great videos, great video on differential probes. Please add the written note to videos in PDF form.
Very good and useful information, thank you..
Thanks very much for the helpful post.
Very educational. Thanks!
Great explanation, thanks.
Good explanation and Thanks!
THANKS VERY MUCH. I ENJOY YOUR VIDEOS .
Very informative! Thanks
thanks for a great/informative video!
Thank you for your clear and informative videos.
These probes can be quite pricey, putting them out of reach for a lot of folks like me. A quick Google search for the retail price of the Tektronix TDP1000 you displayed yielded prices over $4K (EBay prices aren't much better.)
Perhaps a video highlighting recommendations and trade-offs of less-expensive probes and possible work-arounds to address their deficiencies might help the budget-strapped among us.
Thanks again.
Thx for sharing , this help me, regards.
Thanks w2aew, great video, please keep up the great work. Would you consider a follow up on some passive probes like a 7A13 or 7A22 and show all the neat things you can do with a precision offset voltage?
Thanks again, ROBRENZ
Excellent
The "invert and add trick" only works for small offsets from GND if DC coupling is used. Using AC coupling it has always been good enough for me.
If the difference is small compared to the signals you do need a differential probe.
***** Excellent
Thanks!
You should be just fine when measuring arbitrary points (but only 2 points, with single channel!) in electronic circuit with your scope, if you isolate the device you are testing from the ground (it can be if it is battery operated, have a power supply with galvanic isolation, most linear and switching power supplies will do, or you use isolation transformer). Still, using differential probes, is much more safer (especially in high voltage applications), and allows you to use multiple channels without woring about grounds. Good video.
Great video as always! Question: Can I measure a transmitter antenna using the 2 probe method?
If/when you do get your hands on one. I am unclear about when in the infinite impedance mode having the signal source provide a dc path for the FET gate current.
Thanks, ROBRENZ
Hi, Alan, What are the suggested values/types for the capacitor and transistor in your test circuit, at 48 seconds? Thanks!
cool !!!
Hello, thank you for this great description! Why there is a "Ground" socket on differential probes?
+Raymond Koosha Some diff probes can measured common-mode, which needs a ground reference. It is also used to ensure that the two signals stay within the common-mode range of the probe.
Is the Tektronix P6046 probe and amp a good probe? They're on ebay and every once in a while a good deal comes along.
A much appreciated video. 73 de GI8WFA.
So what would be the difference between isolating the DUT and using differential probes? I've been pricing out differential probes and they can be pretty expensive so I'm assuming there must be some benefit over simply using an isolation transformer for the DUT. Great vid, thx!
+Mike O'Brien It really depends on what the signal and circuit look like. Simple circuits and low speed signals - this would work fine. However, problems can arise with high speed signals or high-impedance circuits because of coupling from the isolated supply and true ground.
+w2aew wow, thx for the quick response!
+Mike O'Brien You're welcome - you happen to catch me online, answering several viewer comments and questions (I get a lot of them!).
Would you please make an episode about different interfaces of different probes and different scopes. For instance I don’t know which probe can be directly used or converted for use with DPO2024.
Thanks,
Another option not mentioned in the video for taking a differential measurement is floating the scope. Of course this has safety implications and should not be attempted without understanding the dangers, but is done routinely in EMC labs. However I have also heard that aside from the safety issue that this is not good for the scope. I can only presume this has to do with input protection circuitry but is only a guess. Can you confirm my suspicion?
Thanks
As far I understand it, differential probes require power from the scope for the amplifier in the "head". Can you add external power if you are using it on the old analogue scope or DSO without the special connectors? Or you are stuck with the two passive probe solution only?
How about a ADA400A? All the principles would be the same.
Thanks again, ROBRENZ
RS-485 send take difference between signals as well.
So as long the circuit is not connected to mains earth I can use one probe right?
In general, yes...
Nicely explained! Alan Lowne CEO Saelig Co. Inc.
Thank you!
Are there any gotchas with using a high voltage differential probe (e.g. 700V) for probing low voltage (e.g. 12V max)? E.g., would a high voltage differential probe be appropriate in your recent V-I curve tracer video (197, another great video)? There is a significant price difference between the high and low voltage differential probes. Thank you
The only real gotcha is that the HV differential probes generally have a higher attenuation factor, making it difficult to observe the small signal across the DUT or current shunt resistor in that case. In general, you also have to be careful of the input capacitance, although that wouldn't matter in the VI tracer case since a very low frequency was used.
+w2aew And their price ;)
What happens above the "common/differential mode voltage range" but below the "maximum nondestructive voltage range" that I see on Tek's low voltage diff probe datasheets? Is this region merely where the output becomes "untrustworthy" (CMRR, attenuation, etc might not live up to the in-range spec) or does the intermediate region present a danger to the probe's amplifier, while the region above the destructive threshold poses an especially severe danger (probe turns into a short or something)? PS Thanks so much for making these videos! The educational quality is fantastic!
Generally, exceeding the common mode range will result in not meeting specifications - which could mean anything from distortion of the signal, incorrect voltage measurements, degradation of CMRR, etc. No damage to the probe as long as the maximum non-destructive voltage isn't exceeded.
Thanks! It was going to be a real challenge to treat the 850mV diff mode maximum on the P6248's 1x range as a safety limit. Very pleased to learn that Tek has done the hard work for me. The 25V nondestructive max gives very comfortable headroom in my application.
Thanks Alan! Can you improve the CMRR of the 2 probe Invert and Add method by twisting the leads of the two probes together? Intuitively it seems like it would work but I'm not sure how much benefit we'd get out of it. Hmmm!
Since the probe leads are shielded, I would expect that any improvement in CMRR would be small.
Hi Alan:
The derating curves that I've seen for most high voltage differential probes are pretty steep, so they look like they would be inadequate for high voltage measurements at high frequencies, say in the range of 30 MHz. Do you know of any high voltage differential probes that are still good at higher frequencies? Thanks.
It all depends on what combination of voltage and frequency that you need. Tek has a large variety of HV differential probes with different ratings.
www.tek.com/probes-and-accessories/high-voltage-differential-probes
I looked at the specs for the Tek probes; perhaps I missed something. It seems that for high voltage, they're only good up to about 1 MHz, and then there's a sharp decline. Say I wanted to measure 1000 volts RMS at 30 MHz? Are there any probes that can do that? Thanks.
The only probe system that I know of that can do this is the new TIVH series of IsoVu probes from Tektronix - up to +/-2500V differential with 120dB of CMRR.
Hello, thank you for a great video. I wanted to ask one question. Can a differential probe of one company be used with the oscilloscope of other company. What are the factors important for such. For example can I use techtonix probe with picoscope oscilloscope.
Often the differential probes are powered up via the probe interface that is specific to the manufacturer - making it impossible to use it with other manufacturer's scopes unless you have a dedicated probe power supply.
Hello, can you tell me which model of differential probes are using in your video?
tnks
The probes shown in the video were the Tektronix TDP1000 and THDP0200.
When using two probes to make a differential measurement (like in this video or with a DSO with math function), is it best to clip the two probe grounds to circuit ground or can/should the ground leads be clipped together and left floating wrt the circuit ground?
It's usually best to clip them together. If there is a lot of stray common mode, then you additionally clip them to the circuit ground.
w2aew - Thanks for the reply! I love your videos. Please keep up the great work!
Are these isolated or simply have resistive dividers that bring the voltage into the supply range?
Differential probes are generally not isolated - they'll have a specified permissible common-mode voltage range. There are (very expensive) isolated differential probes that typically use fiber optics for galvanic isolation.
In your video the Gate to source is neither +180V or -180V. 180V and -180V is on the Drain to Source? The turn ON gate voltage is much less than 180V right? Please clarify
The common-mode voltage of the gain and source voltages can be 180, the gate to source differential voltage is much less of course.
Thanks for the details.
My Tek 7403 came with a Differential Amplifier plug-in. I have not used it for anything yet as I am still having trouble keeping the 'scope HV supply running but presumably, it could be used with two probes to function in this manner. Is my thinking correct?
Yes.
@@w2aew Thanks Alan.
The common-mode rating is with respect to which signal? I can only think about the BNC shell, correct?
The common mode signal is one that appears identically on both 'sides' of the differential signal with respect to ground.
I just bought a TDS754C to replace my old Hitachi V-680 that I've had since the '90s. Can you advise on what probe to get for it on a budget? Love your video's by the way. 73's
The recommended 10x passive probe for this model was the P6139A probe. It is a 500MHz 10x probe. Generally, any 10x probe whose compensation range includes 10pF should work.
@@w2aew Thanks for your advise. I will look into getting one.
Very informative. How does using the differential probe differ from using standard probes and the differential plug in module on my 545b scope? Thanks
For one thing, you won't have to match the probes, and the CMRR will likely be better.
I have a set of matched P6006 probes for the 545 differential amp, and for what I am doing, I thnk I will have to stick with that setup for now. I am really enjoying getting back to the hobby again.
if I power the osilloscope with isolation transformer, what would still be the differences between single ended probe and differential probe measurement?
Powering the scope from an isolation transformer is not recommended - it can potentially put dangerous voltages on the chassis (and BNC connectors) of the scope. There will likely still be differences because the circuit would have to "drag" the scope chassis to follow the signal on the probe's ground lead. Bad idea.
Thank you for this!
Vy 73 OZ7TDA
Hi,
Do you have any idea how to probe a high freq tig welding machine in its pulse fase in ac mode and dc mode there about 250 amps and about 500 hz and shifting in the puls more positief or less postief in a pulse.
How do you measure that
Regards
Cees
That might be best probed with a non-contact method, using a simple coil of wire as an H-Field probe.
***** ok thanks
is that easy t make? or is it a simple coil attached to the BNC connector or do i have to watch out for several other things? that could interfere
Cees
Great video. I have looked at these differential probes and wow they are expensive!!! You can get a really fine scope for the money, one single differential probe costs.
Do you know why that is so?
A combination of supply and demand, as well as the amount of engineering that goes into a well designed active probe.
I got a set off Amazon for about $120. Micsig brand. Good enough for me and they work fine.
@@tractortherapy42 Yes Kiss Analog made a great review of them, and they sounded to be good. But if you hope to get any probe that goes as high as many cheap scopes do, then do you have to pay BIG money!! :-)
@@friedmule5403 Yeah - the high end ones are crazy expensive for what they are. I wont be buying a probe that is priced 10-20x what my scope was anytime soon - I'm a hobbyist and don't need.
Thank you very much. how about a battery oscilloscope, should one use a differential probe with a battery scope or it does not need one.
For a battery operated scope, you might not need a diff probe, as long as the scope isn't grounded. But you will have to take great care since the scope's local ground will be elevated to your circuit's voltage.
@@w2aew thank you very much for your advice I would have to let that settle in to get a good grasp and understanding of, I mean, what you said about my local ground will be the circuit ground. Thanks again.
@@w2aew can I ask you about 2 oscilloscope I am thinking about purchasing 1 or the other, tektronix TDS5054 and TDS3054. Which one would be more useful for circuit repair lab?
At moment I am trying to use and learn on a portable fluke 123B, and a 100 MHz Hameg rohde & Schwartz HM1000.
@@AB-yu2tj Both of them are 4 channel 500MHz scopes, but there are significant differences. The TDS5054 is physically much larger. It has a much larger screen, much deeper memory. But, it also includes a PC (very old now) that is likely running Windows 2000 (and can't be upgraded). The TDS3054 is much smaller, has less sample memory, but is still a very capable scope. Since it doesn't have a PC embedded in it, there's less to go wrong. For a circuit repair lab, I would go with the 3054.
@@w2aew thank you very much 3054 it is
excellent video, thank you. your drawings are very similar to Forrest Mims.
Hi das the DM in the tek analog scop is like the Differential Probes Thanks
Using 2 channels with the ADD and INVERT functions, and matching probes, is similar to a differential probe.
no i min the tektronix 2465A DM build in multi meter thes it function like Differential Probes
The inputs of the digital multimeter option are isolated from ground, so they can make a differential measurement across a component, etc. It is basically a 4-1/2 digit digital multimeter with true RMS ACV from 20Hz to 100kHz, and true RMS ACI from 20Hz to 10kHz. It has 10uV resolution on DCV, a continuity beeper, input for temperature probe, and convenience features like smoothing, display hold, min/max, dBV and dBm measurements. It displays numeric results, NOT waveforms like you get with a differential probe on the scope.
really good information thanks for your time
oh.. that probe is built for the scope in form..
Heh I jut read a Quora post the other day where the guy was saying he disables the ground on his meter to leave it floting so he can take measurements from nongrounded references. Naughty boy.
fine job describing common mode noise.
for an actual procedure to adjust a diff'l amp circuit to minimize the common mode noise see p. 6 of this article. its both easy to follow and to do. all you need is an
audio signal generator and a VOM.
www.n5dux.com/ham/files/pdf/Build%20A%20Brain%20Wave%20Feedback%20Monitor.pdf
So... i guess since most differential probes are "active probes" the would need to be "matching" the scope vendor ?? so i cannot use a tek diff-probe on a HAMEG DMO or vice versa. That may also explain why those probes are expensive like hell! Like for the HAMEG they are 350+ EUR for diff probe. Jesus christ!!!
Axel Werner It depends, some probes are universal. The newer probes are less universal as they add their own connections on top of the bnc connection. So if it is a simple BNC connector and the power is provided by internal batteries or a separate power input - then it can be used universally. There are some lower cost diff probes out there. ;)
4500€for a probe! hobbyists are left out! :( I know... there's "passive" differential probe!
Thanks!