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I have been thinking about this for awhile. I have played with it a bit and it seems to function somewhat ok.
Opinions?
Nielsentelecom
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Hi Nielsen,
It is sort of a poorman's differential plugin.
1) It works OK at low to medium frequencies as long as both probes have been adjusted for their proper and identical risetime.
2) It works fine as long as either of your signals doesn't go off screen. When one of them is off screen their voltage is clamped and if it is added to the other one you will get an incorrect addition.
Dennis Tillman W7PF
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-----Original Message-----
From: nielsentelecom@..., Sent: Wednesday, May 16, 2018 8:16 AM
I have been thinking about this for awhile. I have played with it a bit and
it seems to function somewhat ok.
Opinions?
Nielsentelecom -- Dennis Tillman W7PF TekScopes Moderator
|
Floating anything begets you capacitance and radiation and possibly a shock hazard. If you know the limitations and precautions fine, but if your scope uses plugins they are out there and are much better. Also a small circuit could be built to do it.
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Hi Jeff,
I don't believe anything was said about floating in the original post. There is nothing about adding two channels together that automatically involves floating unless I'm missing something.
Both channels are grounded to the scope through the BNC connectors by attaching both signals to the inputs of a 7A18, 7A25, 7A26 which is what I thought was the type of plugin he was thinking about.
After all if you are thinking about using ADD to combine two signals differentially then we aren't talking about a 7A13 or a 7A22 since they do it naturally. That only leaves the 7A18, 7A25, 7A26.
Perhaps I should have asked which plugin he was thinking about before answering.
The other thing you would want to do a decent differential measurement is a set of differential probes like P6046.
Dennis Tillman W7PF
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Show quoted text
-----Original Message-----
From: Jeff Urban Sent: Wednesday, May 16, 2018 5:33 PM
Floating anything begets you capacitance and radiation and possibly a shock
hazard. If you know the limitations and precautions fine, but if your scope
uses plugins they are out there and are much better. Also a small circuit
could be built to do it. -- Dennis Tillman W7PF TekScopes Moderator
|
At this point we don't even know what model he has.
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On Wed, May 16, 2018 at 07:04 pm, Dennis Tillman W7PF wrote:
I don't believe anything was said about floating in the original post.
It was mentioned in the title. At least he is talking floating the DUT rather than the scope. At times, rare, but when working on an SMPS with opto coupled feedback I have, on isolation of course, connected hot and cold grounds together. I wanted to see exactly what was happening in the regulation loop for some reason. It worked fine and was no danger when it was on isolation. It could have been floated without isolation but that is of course a shock hazard. I wonder really how much better of a reading one actually gets floating the DUT. The dual trace scopes I've seen, even the not so great ones seem to have pretty well matched channel;s and a fairly decent maximum input level before it causes a problem. How does that stack up against adding a bunch of capacitance to the - input ? Of course it depends on the bandwidth needed but even knowing that, the effect would be quite unpredictable. Until the OPer gets back... (but it has only been one day)
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Hi Dennis / and Nielsen (Telecom).
@Dennis... Indeed the reference to floating is in the very subject. In fact, Nielsen wrote more about his question on the subject than in the body of the message.
@Nielsen... By "vs floating DUT" I assume you refer to float the DUT with the sole purpose of allowing the use of a single channel, non differential scope input, to take a differential measurement on a circuit (a measurement of the voltage between 2 different nodes on a circuit in which none of them is ground on the DUT).
What you suggest, using 2 channels of the scope, in ADD mode with Channel 2 inverted is, by all means, a much better approach to taking a differential measurement than floating the DUT and is the very reason why most 2 channel scopes have the option to invert channel 2... To allow for differential measurements.
But, when you float the DUT, although dreadful and although inspiring all the caution that Jeff underlined and more (and wait... even more), you accomplish more than just allowing a differential measurement to be taken so, you must be careful in assuming that you can use the differential approach (CH1 + CH2, CH2 inverted) in every case where someone would float a DUT.
For example, the 2 nodes from which you intend to take the differential measure are both, at a DC voltage level that's beyond the scope inputs' maximum (say 500V).
The difference signal that you intend to measure is small so, you won't be able to handle it using 2 x 100:1 probe. In that case you can't actually solve this problem by using alone CH1+(-CH2), because you are supposed to connect each channel's ground lead to the DUT's ground, and that will subject each of the scope's inputs to a voltage that exceeds their maximum.
And no, by no means you can think of not connecting the ground leads and assume the scope would be protected, because if you didn't float the DUT (which is the proposal of the topic to start with), then it's referred to earth's ground as much as the oscilloscope itself is, and even if you don't connect the ground leads, each input will still see 500V.
In that case, sometimes, floating (as dreadful as it is and as caution inspiring as it is) is the only solution *at hand* and using CH1+(-CH2) can't solve it.
I said "at hand" because there are sometimes other *technically-sound and yet safe* alternatives, such as galvanically isolated probes.
There are many other examples of situations that an differential input setup can't accommodate one (or more) of the reasons behind the need to float a DUT.
So, answering to your consideration that "it seems ok", the right answer is: It depends...
It's OK whenever the technical limitation of your differential setup can be respected, given the circumstances of the DUT, be it (CH1 + (-CH2)), or scope with differential input stage, or differential input probe, or galvanically isolated probes.
It's not OK whenever the circumstances of the DUT vs signal being measured would disrespect any of the input's limitations then, sometimes, Floating ***may*** be the only solution at hand.
Rgrds,
Fabio
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Show quoted text
On Wed, May 16, 2018 at 07:04 pm, Dennis Tillman W7PF wrote:
Hi Jeff,
I don't believe anything was said about floating in the original post. There
is nothing about adding two channels together that automatically involves
floating unless I'm missing something.
Both channels are grounded to the scope through the BNC connectors by
attaching both signals to the inputs of a 7A18, 7A25, 7A26 which is what I
thought was the type of plugin he was thinking about.
After all if you are thinking about using ADD to combine two signals
differentially then we aren't talking about a 7A13 or a 7A22 since they do it
naturally. That only leaves the 7A18, 7A25, 7A26.
Perhaps I should have asked which plugin he was thinking about before
answering.
The other thing you would want to do a decent differential measurement is a
set of differential probes like P6046.
Dennis Tillman W7PF
-----Original Message-----
From: Jeff Urban Sent: Wednesday, May 16, 2018 5:33 PM
Floating anything begets you capacitance and radiation and possibly a shock
hazard. If you know the limitations and precautions fine, but if your scope
uses plugins they are out there and are much better. Also a small circuit
could be built to do it.
--
Dennis Tillman W7PF
TekScopes Moderator
|
Hi everybody,
I missed the reference to "floating" because I got as far as the "vs" and stopped reading because I started explaining the problems you might encounter when using ADD if you were not careful.
Dennis Tillman W7PF
toggle quoted message
Show quoted text
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Fabio
Trevisan
Sent: Thursday, May 17, 2018 7:42 AM
To: [email protected]
Subject: Re: [TekScopes] using scope channel 1 and channel 2 invert and add
functions vs floating DUT
Hi Dennis / and Nielsen (Telecom).
@Dennis... Indeed the reference to floating is in the very subject. In
fact, Nielsen wrote more about his question on the subject than in the body
of the message.
@Nielsen... By "vs floating DUT" I assume you refer to float the DUT with
the sole purpose of allowing the use of a single channel, non differential
scope input, to take a differential measurement on a circuit (a measurement
of the voltage between 2 different nodes on a circuit in which none of them
is ground on the DUT).
What you suggest, using 2 channels of the scope, in ADD mode with Channel 2
inverted is, by all means, a much better approach to taking a differential
measurement than floating the DUT and is the very reason why most 2 channel
scopes have the option to invert channel 2... To allow for differential
measurements.
But, when you float the DUT, although dreadful and although inspiring all
the caution that Jeff underlined and more (and wait... even more), you
accomplish more than just allowing a differential measurement to be taken
so, you must be careful in assuming that you can use the differential
approach (CH1 + CH2, CH2 inverted) in every case where someone would float
a DUT.
For example, the 2 nodes from which you intend to take the differential
measure are both, at a DC voltage level that's beyond the scope inputs'
maximum (say 500V).
The difference signal that you intend to measure is small so, you won't be
able to handle it using 2 x 100:1 probe. In that case you can't actually
solve this problem by using alone CH1+(-CH2), because you are supposed to
connect each channel's ground lead to the DUT's ground, and that will
subject each of the scope's inputs to a voltage that exceeds their maximum.
And no, by no means you can think of not connecting the ground leads and
assume the scope would be protected, because if you didn't float the DUT
(which is the proposal of the topic to start with), then it's referred to
earth's ground as much as the oscilloscope itself is, and even if you don't
connect the ground leads, each input will still see 500V.
In that case, sometimes, floating (as dreadful as it is and as caution
inspiring as it is) is the only solution *at hand* and using CH1+(-CH2)
can't solve it.
I said "at hand" because there are sometimes other *technically-sound and
yet safe* alternatives, such as galvanically isolated probes.
There are many other examples of situations that an differential input
setup can't accommodate one (or more) of the reasons behind the need to
float a DUT.
So, answering to your consideration that "it seems ok", the right answer
is: It depends...
It's OK whenever the technical limitation of your differential setup can be
respected, given the circumstances of the DUT, be it (CH1 + (-CH2)), or
scope with differential input stage, or differential input probe, or
galvanically isolated probes.
It's not OK whenever the circumstances of the DUT vs signal being measured
would disrespect any of the input's limitations then, sometimes, Floating
***may*** be the only solution at hand.
Rgrds,
Fabio
On Wed, May 16, 2018 at 07:04 pm, Dennis Tillman W7PF wrote:
Hi Jeff,
I don't believe anything was said about floating in the original post.
There is nothing about adding two channels together that automatically
involves floating unless I'm missing something.
Both channels are grounded to the scope through the BNC connectors by
attaching both signals to the inputs of a 7A18, 7A25, 7A26 which is
what I thought was the type of plugin he was thinking about.
After all if you are thinking about using ADD to combine two signals
differentially then we aren't talking about a 7A13 or a 7A22 since
they do it naturally. That only leaves the 7A18, 7A25, 7A26.
Perhaps I should have asked which plugin he was thinking about before
answering.
The other thing you would want to do a decent differential measurement
is a set of differential probes like P6046.
Dennis Tillman W7PF
-----Original Message-----
From: Jeff Urban Sent: Wednesday, May 16, 2018 5:33 PM
Floating anything begets you capacitance and radiation and possibly
a shock hazard. If you know the limitations and precautions fine,
but if your scope uses plugins they are out there and are much
better. Also a small circuit could be built to do it.
--
Dennis Tillman W7PF
TekScopes Moderator
-- Dennis Tillman W7PF TekScopes Moderator
|
I've also missed things, and had to come back and explain something that was totally wrong. It happens.
|
I see that I need to clarify further. I thought the topic would be enough, but I should have been more specific, from the replies this topic has received. I wasn't aware what scope I was referring to would be important. I thought it was a generic enough topic.
I have a Tek 2246A and a Hitachi VC-6025 hybrid analog/digital older scope. They both have the channel 1 and channel 2 invert and add functions, and I just have basic 1x-10x probes for them. I have in the past floated the DUT, and that topic has been beat here in the past I am aware. I really don't fully understand what the add function and invert function can do for me. I have played with them, but probably don't know how to apply the functions, and I thought add/invert could possibly be an alternative to floating the DUT, but with some unknown limitations without a fair amount of experimentation.
So I come here looking for knowledge/experience with add/invert functions, and the application of it that can't be met by other methods.
NielsenTelecom
|
Without rehashing this whole thing, just change your thought picture terminology from "add/invert" to "subtract" - that's what a differential amplifier does. The issue is, how well that subtraction is done, in terms of accuracy and AC characteristics. Using the scope channels this way just gives you an option that may be sufficient. You can experiment with the probes both hooked to the same signal source, and twiddle the scope settings to see how it does.
Ed
|
One more thing - the add and invert functions have been included in many scopes over a very long time. You should read the manual - it likely has instructions and examples of making simple differential measurements. If this generation of scopes is skimpy on application details in the manual, then look at the manual of almost any other older scope (that has the capability) - they all work pretty much the same. There should be plenty of info available.
Ed
|
Hi Nielsen,
Just to clarify my experiences when using "CH1 + CH2 Inverted" I was never floating anything. I would consider that to be dangerous and not a valid or approved use of the ADD function of a plugin or mainframe. The proper way to perform a differential measurement is with a P6046 differential probe (which has matched probes and its own differential amplifier) or with a 7A13 Differential Plugin which is a remarkable plugin everyone should own for taking differential measurements, or very small measurements on top of a very large DC offset.
In case you haven't noticed all of the valid responses you have received (ignore my first one when I missed the reference to floating) have been cautionary or advising you this is a bad idea. I hope you listen to the advice of those who speak from experience and professionalism when the urge you not to do this.
One more thing: scope mainframes that are powered from the power line have breakdown voltages you must not exceed. They are listed in the specifications in the front of the manuals. The purpose is to warn you not to ever apply more voltage to the instrument that the insulation of the power transformer was designed to withstand. Otherwise the power transformer can break down and you will lose the benefits of transformer isolation.
The only solution I know of, if you want to measure 1000 volts for example, is to use a battery powered scope. For the few times I need to work around 440V 3-phase I keep a Model 214 dual trace 500KHz calibrated battery operated storage scope. There are two others as well in the same product line (212 and 213) that are worth their weight in gold in situations where you are dealing with a high voltage. If you really want a great portable scope the take a look at the 222 & 222A which are a battery-operated portable 10 MHz dual-channel digital storage scope with independently isolated inputs. Those are really nice.
Dennis Tillman W7PF
toggle quoted message
Show quoted text
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Dennis
Tillman W7PF
Sent: Thursday, May 17, 2018 8:43 AM
To: [email protected]
Subject: Re: [TekScopes] using scope channel 1 and channel 2 invert and add
functions vs floating DUT
Hi everybody,
I missed the reference to "floating" because I got as far as the "vs" and
stopped reading because I started explaining the problems you might
encounter when using ADD if you were not careful.
Dennis Tillman W7PF
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Fabio Trevisan
Sent: Thursday, May 17, 2018 7:42 AM
To: [email protected]
Subject: Re: [TekScopes] using scope channel 1 and channel 2 invert
and add functions vs floating DUT
Hi Dennis / and Nielsen (Telecom).
@Dennis... Indeed the reference to floating is in the very subject. In
fact, Nielsen wrote more about his question on the subject than in the
body of the message.
@Nielsen... By "vs floating DUT" I assume you refer to float the DUT
with the sole purpose of allowing the use of a single channel, non
differential scope input, to take a differential measurement on a
circuit (a measurement of the voltage between 2 different nodes on a
circuit in which none of them is ground on the DUT).
What you suggest, using 2 channels of the scope, in ADD mode with
Channel 2 inverted is, by all means, a much better approach to taking
a differential measurement than floating the DUT and is the very
reason why most 2 channel scopes have the option to invert channel
2... To allow for differential measurements.
But, when you float the DUT, although dreadful and although inspiring
all the caution that Jeff underlined and more (and wait... even more),
you accomplish more than just allowing a differential measurement to
be taken so, you must be careful in assuming that you can use the
differential approach (CH1 + CH2, CH2 inverted) in every case where
someone would float a DUT.
For example, the 2 nodes from which you intend to take the
differential measure are both, at a DC voltage level that's beyond the scope inputs'
maximum (say 500V).
The difference signal that you intend to measure is small so, you
won't be able to handle it using 2 x 100:1 probe. In that case you
can't actually solve this problem by using alone CH1+(-CH2), because
you are supposed to connect each channel's ground lead to the DUT's
ground, and that will subject each of the scope's inputs to a voltage that exceeds their maximum.
And no, by no means you can think of not connecting the ground leads
and assume the scope would be protected, because if you didn't float
the DUT (which is the proposal of the topic to start with), then it's
referred to earth's ground as much as the oscilloscope itself is, and
even if you don't connect the ground leads, each input will still see 500V.
In that case, sometimes, floating (as dreadful as it is and as caution
inspiring as it is) is the only solution *at hand* and using
CH1+(-CH2) can't solve it.
I said "at hand" because there are sometimes other *technically-sound
and yet safe* alternatives, such as galvanically isolated probes.
There are many other examples of situations that an differential input
setup can't accommodate one (or more) of the reasons behind the need
to float a DUT.
So, answering to your consideration that "it seems ok", the right
answer
is: It depends...
It's OK whenever the technical limitation of your differential setup
can be respected, given the circumstances of the DUT, be it (CH1 +
(-CH2)), or scope with differential input stage, or differential input
probe, or galvanically isolated probes.
It's not OK whenever the circumstances of the DUT vs signal being
measured would disrespect any of the input's limitations then,
sometimes, Floating
***may*** be the only solution at hand.
Rgrds,
Fabio
On Wed, May 16, 2018 at 07:04 pm, Dennis Tillman W7PF wrote:
Hi Jeff,
I don't believe anything was said about floating in the original post.
There is nothing about adding two channels together that
automatically involves floating unless I'm missing something.
Both channels are grounded to the scope through the BNC connectors
by attaching both signals to the inputs of a 7A18, 7A25, 7A26 which
is what I thought was the type of plugin he was thinking about.
After all if you are thinking about using ADD to combine two signals
differentially then we aren't talking about a 7A13 or a 7A22 since
they do it naturally. That only leaves the 7A18, 7A25, 7A26.
Perhaps I should have asked which plugin he was thinking about
before answering.
The other thing you would want to do a decent differential
measurement is a set of differential probes like P6046.
Dennis Tillman W7PF
-----Original Message-----
From: Jeff Urban Sent: Wednesday, May 16, 2018 5:33 PM
Floating anything begets you capacitance and radiation and
possibly a shock hazard. If you know the limitations and
precautions fine, but if your scope uses plugins they are out
there and are much better. Also a small circuit could be built to do
it.
--
Dennis Tillman W7PF
TekScopes Moderator
--
Dennis Tillman W7PF
TekScopes Moderator
-- Dennis Tillman W7PF TekScopes Moderator
|
On Thu, 17 May 2018 16:29:13 -0700, you wrote: I see that I need to clarify further. I thought the topic would be enough, but I should have been more specific, from the replies this topic has received. I wasn't aware what scope I was referring to would be important. I thought it was a generic enough topic.
I have a Tek 2246A and a Hitachi VC-6025 hybrid analog/digital older scope. They both have the channel 1 and channel 2 invert and add functions, and I just have basic 1x-10x probes for them. I have in the past floated the DUT, and that topic has been beat here in the past I am aware. I really don't fully understand what the add function and invert function can do for me. I have played with them, but probably don't know how to apply the functions, and I thought add/invert could possibly be an alternative to floating the DUT, but with some unknown limitations without a fair amount of experimentation. OK, going back to basics, the scope has two channels, which may be added together or run separately. Imagine a circuit with an LED, a pulse generator sufficient to run the LED, and a series resistor. You are running the LED in pulse mode, which means that you are putting MORE current through it than it can tolerate steady state, but not putting that current through all of the time. It averages out. Please look up multiplexed LED displays for an explanation. You'd like to know what the pulse current is for the LED. Even though you could measure it with a standard DMM, you'd be getting the average current over time, not the pulse current. Your meter takes that off time and the on time, averages the current flowing through them, and gives you a reading. The problem is that even though you are averaging out to an acceptable value, the peak value may be too much. (and the average and peak currents are not the same). So, if you took an oscilloscope with one probe, and measured the voltage from one end of the resistor to ground, you'd get a reading and perhaps a waveform (depends on how the LED is hooked up). Now, you measure that and record the value. Now put the probe on the other end of the resistor, measure that value and record it. Subtract the two values to get the voltage drop across the resistor, which is the peak current, which is what you want to know. (scope probe grounds are connected to power supply ground). Your two channel scope can make that nicer and easier. Put one probe on one end of the resistor, the other probe on the other end of the resistor. Running in alternate or chopped mode (two traces), you'd see the two voltage readings at the same time. Well, it somewhat saves time. Now, if you added the two voltages (ADD mode), it wouldn't tell you too much, not in this case. But suppose you subtracted them? To subtract you add a negative value. In an oscilloscope, you select ADD (to get a single trace, which is A + B), but you throw the "invert" switch, (so you add A + (-B)) which works out to (A-B). Now the reading you see is the true voltage drop across the resistor, which can be used to tell you exactly what the current through the LED happens to be. ADD, and then INVERT. The difficulty with this is that both readings may be very slightly different, but both could be at a very high voltage, say 500 volts, and you're looking to measure that tiny 0.1 volt drop across the resistor. (it's a thought experiment, so it's not meant to be super actual). Because the probes and the front end have a voltage limit, you may not be able to make the measurement since the 500 volts is too high for the scope. Some people (and you have to be very careful with this) might put the UUT (Unit Under Test) to float, connect the ground at say, the 500 volt supply, and then the scope inputs won't see 500 volts, but the moderate difference. Unfortunately, this puts the REST of the UUT at 500 volts with respect to the scope frame, which is NOT a good idea. Barring the use of special high voltage probes, the only good way to make this kind of measurement is to use a battery powered scope, running on battery power, where all the ground parts of the scope are kept from casual contact. Tektronix did make scopes that were like that. These problems are one reason why you have to be careful with not only what you measure, but how you measure it. Tektronix did make high voltage probes, so those could be used. Some will withstand 30,000 volts or a bit more, depending. Differential measurements *could* be made with those probes, and safely, once all the safety precautions are observed. You use the UUT ground as ground, and don't float ANYTHING. However, the probes are x100 (or X1000) probes, not x10 (divide by 100), so there's a limit of how much signal you get to the scope, which can limit your measurements. Hope this helps. Harvey
So I come here looking for knowledge/experience with add/invert functions, and the application of it that can't be met by other methods.
NielsenTelecom
|
The battery powered scope is actually floating the scope, but they are designed for that. Even if not they will be quite tolerant of it.
In the case of your LED example, one could only hope that wither one side of the other is anchored fairly well to a constant voltage source. Then it is a simple matter of using AC coupling.
If not, the choices are to float the scope say, positive or to float the DUT negative, or vice versa, whichever is needed. Neither are very desirable from a safety or signal integrity standpoint.
Let's say in the case of a BTL audio power output, common in automotive audio. One amp feeds one speaker wire and the other the same signal in opposite polarity. When measuring such an output, the supply voltage will dip during the peaks, should one be measuring dynamic clipping headroom for whatever reason. This makes simply using AC coupling a poor option, you simply need enough CMRR to handle it at whatever deflection sensitivity needed. Also, one manufacturer actually did not send flat frequency response to both legs of the output so as to act like a biamped unit, reducing certain forms of distortion when at or near clipping, which such amps see on a regular basis usually. In that case you're adding 2 different signals and inverting one. Suppose you wanted to see the actual output's frequency response and feed it a 1KHz square wave, which I use commonly for that purpose. Manually taking one waveform and the other and figuring out the resulting sum is not something I would like to do.
These are not extremely important issues, for one the actual power output of the type of amp mentioned is not that important, and a circuit that elevates both ends of an LED to hundreds of volts would be quite rare, in fact it would take some time to just think where such a situation would be necessary. More likely it would be floated itself, like a panel power indicator running directly off the AC mains when the rest of the unit is isolated and Earth grounded.
This discussion is rendered mainly academic because the OPer has not revealed the purpose of the question. Perhaps it is academic for him as well. But I'm OK with it if everyone else is.
|
Jeff, thanks for the example. A question, I believe I've seen with this type amp, push-pull IIRC, where there is a common 'center' rail, where that voltage is 0, and there is a negative voltage for the lower waveform, and positive for the upper waveform, could you attach both probe's ground to that center rail and get the same waveform when displaying both channels? Assuming there is no potential between that rail and scope chassis.
Also, I'm not really thinking in terms of elevated voltages, perhaps I should be so not to limit the discussion either.
NielsenTelecom
|
Harvey, thanks for your example. I do somewhat the same with a voltmeter in respect to measuring voltage drop/gain rather than doing the math, I can get higher resolution too since the decimal moves over.
Your high voltage idea in respect to exceeding limits; would it be possible to use the 2 channels to measure a floating high voltage that exceeds the single channel to scope chassis limit of say, 500V peak? Given a 1M channel impedance, do I end up with a 2M impedance? Could this be a method to cut scope loading on the DUT in half?
And yes, this is an academic question, the examples help me to understand the testing possibilities with my scope.
NielsenTelecom
|
On Sun, 20 May 2018 10:40:54 -0700, you wrote: Harvey, thanks for your example. I do somewhat the same with a voltmeter in respect to measuring voltage drop/gain rather than doing the math, I can get higher resolution too since the decimal moves over.
Your high voltage idea in respect to exceeding limits; would it be possible to use the 2 channels to measure a floating high voltage that exceeds the single channel to scope chassis limit of say, 500V peak? Given a 1M channel impedance, do I end up with a 2M impedance? Could this be a method to cut scope loading on the DUT in half? I am not happy with that use. Say you have 800 volts, you'd need to find a 400 volt supply to "float" the scope, and measure the lower and upper end with the scope probes. While you may not exceed the input voltage maximum, the thought of a scope chassis somehow floating at 400 volts above ground, (OR your UUT having that kind of voltage extent somewhere (as in scope chassis is 400 volts, UUT chassis is -400 volts) is not a pleasant thought. Again, a battery operated scope (double insulated) *might* be able to do this, and likely would within the limits of (whatever voltage) per channel. That would be the only use that I'd even consider, and I'd go find the high voltage probes that are made to handle this kind of situation. In theory, you would get a 2 meg impedance, but even on a low voltage circuit, I just have a feeling that this is a bad idea, measurement wise. Best practice and safest practice is to avoid putting the equipment in this kind of situation. Overall, my immediate feeling is "don't do this". Harvey
And yes, this is an academic question, the examples help me to understand the testing possibilities with my scope.
NielsenTelecom
|
Hello Nielsen See my comments right after your points (upon which Harvey already answered... this is just my 2 cents). On Sun, May 20, 2018 at 10:40 am, nielsentelecom@... wrote:
Harvey, thanks for your example. I do somewhat the same with a voltmeter in
respect to measuring voltage drop/gain rather than doing the math, I can get
higher resolution too since the decimal moves over.
Your mention of using a voltmeter, to measure voltages between 2 points captures the essence of taking a differential measurement (because in a "sorf-of" way, this is what your voltmeter is doing, measuring the voltage of the red lead "in relation" to the voltage in the black lead). However, that usage you mentioned of a voltmeter is only possible because you're talking about a portable voltmeter that's isolated from the mains, isolated from earths ground and also isolated of your D.U.T. in other words, the voltmeter is floating, and then it fits exactly in the same bill as using a floated scope (i.e.: a battery operated one). Should you be using a bench voltmeter, which is powered from the mains and therefore grounded and you would need to make that same measurement between two nodes of the circuit where neither of them is ground on the D.U.T., you would face exactly the same problem as when trying to make that measurement with the scope. Your high voltage idea in respect to exceeding limits; would it be possible to
use the 2 channels to measure a floating high voltage that exceeds the single
channel to scope chassis limit of say, 500V peak? Given a 1M channel
impedance, do I end up with a 2M impedance? Could this be a method to cut
scope loading on the DUT in half?
And yes, this is an academic question, the examples help me to understand the
testing possibilities with my scope.
NielsenTelecom The high voltage idea... I suppose you're talking about this part of Harvey's answer: Harvey wrote...
Some people (and you have to be very careful with this) might put the UUT (Unit Under Test) to float,
connect the ground at say, the 500 volt supply, and then the scope inputs won't see 500 volts, but the moderate difference.
Unfortunately, this puts the REST of the UUT at 500 volts with respect to the scope frame, which is NOT a good idea. Well, in that answer, he's only referring to an alternative to floating the oscilloscope, which is to float the D.U.T. Since the signals you're measuring are hypothetically riding on top of a voltage (e.g. 500V) that would be forbidden to the scope, if you float the device and connect the scope's ground lead to that hypothetical 500V of the D.U.T. you would be bringing that 500V to the oscilloscope ground lead's potential (0V), turning any voltage on the D.U.T. that are around that ballpark of 500V closer to ground and measurable by the scope. However, since the scope's ground lead is at 0V, and now you turned the DUT's 500V voltage into 0V, all voltages on the D.U.T that would be near 0V, will be now in the -500V ballpark (read... possibly ever y exposed metal part of the D.U.T. is now probably at -500V). This "strategy is as much hazardous as "floating" a scope that's not meant to be operated floated (i.e. any non-battery operated scope, including your Tek or your Hitachi). This strategy also, doesn't have any thing to do with the impedances seen by the D.U.T. as you suggested. Indeed, what you suggest makes sense, but the rational is flawed because one normally cannot disregard the ground leads of each channel input. The scope's inputs' impedance from the tip of the channel 1 probe to the tip of the channel 2 probe would be the sum of each channel's impedance (i.e. 2MOhm) if the ground leads would remain disconnected (from the U.U.T.s), and also disconnected from ground (which would take - again - the scope to be floating because, if it isn't floating, both channels' ground leads are grounded, via the scope's grounding wire to your facility ground). So, academically... 1. if you use a floated oscilloscope (for safety's sake, a battery operated one) 2. If you don't connect EITHER of the inputs' ground leads to the D.U.T., but only connect both ground leads one to each other (I`m assuming here that, a battery operated scope can also be one where the two inputs' ground leads are isolated from each other, such as some Tek scopes meant for this kind of floated operation). Then, academically, 1. The resultant input impedance would be 2MOhms 2. Possibly (depending on how the oscilloscope is wired internally, as this is an academic reasoning), it **could** withstand twice the maximum input voltage from the tip of CH1's probe to the tip of CH2's probe. Rgrds, Fabio
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In the data acquisition world this might be called "pseudo-differential".? The DUT has to be able to tolerate the probes impeadance to ground at the measurement (probe) points.
It does allow you to directly measure the voltage "across" something such as a current sense resistor and it does allow you to measure stuff without introducing a "ground loop".
It's NOT an "isolated measurement" and it's NOT a "true differential measurement".
Both "isolated" and "pseudo-differential" have their limitations.? You generally have up to a given isolated voltage and a DC+peak AC requirement to meet.
I think it's bad to think of the measurement as subtraction.? It's (A-G)-(B-G) = A-G-B+G = A-B with limitations.
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On Monday, May 21, 2018, 3:43:08 PM EDT, Fabio Trevisan <fabio.tr3visan@...> wrote: Hello Nielsen See my comments right after your points (upon which Harvey already answered... this is just my 2 cents). On Sun, May 20, 2018 at 10:40 am, nielsentelecom@... wrote:
Harvey, thanks for your example. I do somewhat the same with a voltmeter in
respect to measuring voltage drop/gain rather than doing the math, I can get
higher resolution too since the decimal moves over.
Your mention of using a voltmeter, to measure voltages between 2 points captures the essence of taking a differential measurement (because in a "sorf-of" way, this is what your voltmeter is doing, measuring the voltage of the red lead "in relation" to the voltage in the black lead). However, that usage you mentioned of a voltmeter is only possible because you're talking about a portable voltmeter that's isolated from the mains, isolated from earths ground and also isolated of your D.U.T. in other words, the voltmeter is floating, and then it fits exactly in the same bill as using a floated scope (i.e.: a battery operated one). Should you be using a bench voltmeter, which is powered from the mains and therefore grounded and you would need to make that same measurement between two nodes of the circuit where neither of them is ground on the D.U.T., you would face exactly the same problem as when trying to make that measurement with the scope. Your high voltage idea in respect to exceeding limits; would it be possible to
use the 2 channels to measure a floating high voltage that exceeds the single
channel to scope chassis limit of say, 500V peak? Given a 1M channel
impedance,? do I end up with a 2M impedance? Could this be a method to cut
scope loading on the DUT in half?
And yes, this is an academic question, the examples help me to understand the
testing possibilities with my scope.
NielsenTelecom The high voltage idea... I suppose you're talking about this part of Harvey's answer: Harvey wrote...
Some people (and you have to be very careful with this) might put the UUT (Unit Under Test) to float,
connect the ground at say, the 500 volt supply, and then the scope inputs won't see 500 volts, but the moderate difference.
Unfortunately, this puts the REST of the UUT at 500 volts with respect to the scope frame, which is NOT a good idea. Well, in that answer, he's only referring to an alternative to floating the oscilloscope, which is to float the D.U.T. Since the signals you're measuring are hypothetically riding on top of a voltage (e.g. 500V) that would be forbidden to the scope, if you float the device and connect the scope's ground lead to that hypothetical 500V of the D.U.T. you would be bringing that 500V to the oscilloscope ground lead's potential (0V), turning any voltage on the D.U.T. that are around that ballpark of 500V closer to ground and measurable by the scope. However, since the scope's ground lead is at 0V, and now you turned the DUT's 500V voltage into 0V, all voltages on the D.U.T that would be near 0V, will be now in the -500V ballpark (read... possibly ever y exposed metal part of the D.U.T. is now probably at -500V). This "strategy is as much hazardous as "floating" a scope that's not meant to be operated floated (i.e. any non-battery operated scope, including your Tek or your Hitachi). This strategy also, doesn't have any thing to do with the impedances seen by the D.U.T. as you suggested. Indeed, what you suggest makes sense, but the rational is flawed because one normally cannot disregard the ground leads of each channel input. The scope's inputs' impedance from the tip of the channel 1 probe to the tip of the channel 2 probe would be the sum of each channel's impedance (i.e. 2MOhm) if the ground leads would remain disconnected (from the U.U.T.s), and also disconnected from ground (which would take - again - the scope to be floating because, if it isn't floating, both channels' ground leads are grounded, via the scope's grounding wire to your facility ground). So, academically... 1. if you use a floated oscilloscope (for safety's sake, a battery operated one) 2. If you don't connect EITHER of the inputs' ground leads to the D.U.T., but only connect both ground leads one to each other (I`m assuming here that, a battery operated scope can also be one where the two inputs' ground leads are isolated from each other, such as some Tek scopes meant for this kind of floated operation). Then, academically, 1. The resultant input impedance would be 2MOhms 2. Possibly (depending on how the oscilloscope is wired internally, as this is an academic reasoning), it **could** withstand twice the maximum input voltage from the tip of CH1's probe to the tip of CH2's probe. Rgrds, Fabio
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Jeff Urban
Fabio Trevisan