Floating scopes and AC coupling ground - Page 1
Forum section: electronics / testgear

author: new299
author_rank: Regular Contributor
timestamp: April 17, 2015, 09:51:08 pm
content: I was watching one of Dave's old videos on a portable scope here:http://www.eevblog.com/2013/02/27/eevblog-430-fluke-91-scopemeter-teardown/About 20mins in he talks about how the ground on the BNC is AC coupled to the instrument ground.For some reason I have a hard time getting my head around this. What is the purpose of AC coupling the ground? I assume because the scope is "floating" that even if the grounds were connected you'd still be getting a differential reading?

author: tautech
author_rank: Super Contributor
timestamp: April 17, 2015, 10:12:48 pm
content: Unless the Fluke has some weird AC coupling regime, scopes normally AC couple by introducing a series (coupling) cap into the signal path.But if this is an isolated channel unit the standard circuitry used may not be used in the Fluke.

author: tggzzz
author_rank: Super Contributor
timestamp: April 17, 2015, 11:12:38 pm
content: For the avoidance of doubt, "AC coupling" and "floating scope" are twoentirelydifferent things.The former is safe normal practice for a signal lead, and is safe practice for a ground lead.The latter isdangerousas noted inhttp://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/andmanyother threads.

author: Smokey
author_rank: Super Contributor
timestamp: April 18, 2015, 01:42:02 am
content: Quote from: tggzzz on April 17, 2015, 11:12:38 pmActually floating the scope is only POTENTIALLY dangerous, and only if you don't know what you are doing and aren't careful. Everything is potentially dangerous. A 9V battery is POTENTIALLY dangerous if you stick two of them together back to back for example. The solution to that problem is understanding in what situations something is dangerous and being careful.Floating the scope can actually PREVENT you from blowing up your scope in the way that was explained in that video.Now am I recommending everyone float their scope all the time? No. But that's different than saying the world will end if you do it.

author: tggzzz
author_rank: Super Contributor
timestamp: April 18, 2015, 08:00:35 am
content: Quote from: Smokey on April 18, 2015, 01:42:02 amWe are in danger of splitting hairs here, and really ought to bring in the concepts of "risk" and "hazard".But more importantly... my message was for the OP who is self-avowedly inexperienced. Hence he (she?)doesn't know what they are doing; fortunately they are asking sensible questions.And, of course, how many people (and ex-people) incorrectlythoughtthey knew what they were doing!

author: new299
author_rank: Regular Contributor
timestamp: April 18, 2015, 10:50:39 am
content: Quote from: tggzzz on April 17, 2015, 11:12:38 pmFloating the scope can be dangerous, I wouldn't do it. But isn't a battery powered scope always floating? (Unless you add your own ground connection)What I'm unclear on is AC coupling the ground though. As the scope is isolated anyway what is the purpose of AC coupling the ground (rather than, or as well as the signal)?

author: tautech
author_rank: Super Contributor
timestamp: April 18, 2015, 11:14:12 am
content: Do you know what an isolated channel scope is?It's when the Reference (whats convienently called GND) is NOT common to all channels.Depending on circuit topography, the reference must be coupled in some way to the scope and it follows that capacitive coupling would be the most likely.http://www.siglentamerica.com/pdxx.aspx?id=93&T=2&tid=2Remember most scopes have a commoned reference, and for mains powered units this is mains GND.

author: new299
author_rank: Regular Contributor
timestamp: April 18, 2015, 12:56:24 pm
content: Quote from: tautech on April 18, 2015, 11:14:12 amI guess what's possibly confusing to me is that at DC the ground is effectively unconnected? So at DC isn't it just like the ground is floating? I understand that's probably not so, but I don't understand why.Also, and am correct in thinking these are effectively differential inputs?

author: MarkL
author_rank: Unknown
timestamp: April 18, 2015, 01:15:22 pm
content: So, assuming the question is: What is a capacitor doing between the BNC and internal ground? Here's a possible explanation.The user manual says:QuoteThe main takeaway here is that the black banana and the BNC shields are common to each other. Meaning that they cannot be floated individually.Dave, in the video, measured 515ohms from the BNC shield to the chassis common and black banana. This also says they are not isolated from each other, but also not directly connected either. I think it was a misnomer to call it "AC coupled" in a scope context since that implies no DC coupling (or infinite resistance), so that may be the confusion.But still, why the capacitor?The manual says that there is some "protection" between the commons, and the 515ohms is probably part of an overload detection circuit that opens up if the user happens to put too much voltage across the commons.The big 0.22uF capacitor looks like an AC mains safety film capacitor to me. It would play in the circuit when the protection circuit opens up since it's in parallel with the protection circuit. It's purpose is probably to shunt away some or all of the fault energy from the protection circuit, but I'd like to see a schematic.If anyone has the 91 service manual and can post a schematic of the BNC input section that would interesting to look at. I can't find one on-line and the service manual Dave has linked in is for the 93.That's my guess, anyway, FWIW.

author: new299
author_rank: Regular Contributor
timestamp: April 18, 2015, 01:50:54 pm
content: Quote from: MarkL on April 18, 2015, 01:15:22 pmThanks! Yes, that helped a lot. I'd also really like to see a schematic too now. I hadn't realized there was a protection circuit in parallel with this, and being able to review the schematic would help a lot. I'll see if I can hunt one down.

author: JDubU
author_rank: Frequent Contributor
timestamp: April 20, 2015, 05:18:14 am
content: Here is a link to the Scopemeter 93/95/97 service manual that includes schematics:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Philips/pm93_95_97_fluke_93_95_97.pdf

author: dom0
author_rank: Super Contributor
timestamp: April 20, 2015, 09:54:19 am
content: Quote from: new299 on April 18, 2015, 10:50:39 amThey are. That's why every channel is isolated against the chassisand every other channel, so that you cannot touch the GND of a channel while it potentially carries lethal voltages. Notice that the probes sold for battery powered scopes have completely shrouded BNC connectors, again, to prevent the operator from touching lethal voltages.

author: MarkL
author_rank: Unknown
timestamp: April 21, 2015, 01:23:41 am
content: Quote from: dom0 on April 20, 2015, 09:54:19 amThe Fluke 91 (and 92) do not have the channels isolated from each other. The user manual says this very explicitly. They rely on "no exposed metal" to prevent carriage of dangerous voltages to the operator.Scopes like the Tek TPS2000 series do have the BNC shells completely isolated from each other.Maybe you meant "insulated", not "isolated" ?

author: new299
author_rank: Regular Contributor
timestamp: April 21, 2015, 08:11:19 am
content: Quote from: JDubU on April 20, 2015, 05:18:14 amThanks! As far as I can tell from that schematic the BNC ground is not isolated and just goes to system ground. Am I reading this correctly?

author: new299
author_rank: Regular Contributor
timestamp: April 21, 2015, 08:30:14 am
content: Quote from: MarkL on April 21, 2015, 01:23:41 amRight, what threw me though is Dave mentions that there's a capacitor that's "AC coupling ground though to signal common and internal shielding".I'm slightly confused, as the schematic (image of coupling section posted) doesn't seem to match exactly what Dave is showing. Unless that's actually the signal and there's also a sneaky resistor. In which case that cap is for AC coupling and appears to that off an LF component of the signal...

author: MarkL
author_rank: Unknown
timestamp: April 21, 2015, 11:22:05 am
content: Quote from: new299 on April 21, 2015, 08:30:14 amYou're right that the schematic doesn't match what we can see in the teardown.As I said in my first post, Dave's link is the WRONG service manual. It is for the 93 and not the 91. Same for JDubU's link.The 91 service manual does not seem to be on line. That's why I was asking if someone could post the schematic to the 91.

author: new299
author_rank: Regular Contributor
timestamp: April 21, 2015, 01:31:07 pm
content: Quote from: MarkL on April 21, 2015, 11:22:05 amAh I assumed JDubU had found the correct manual. Should have checked it! From the photoset it looks like there could be something else coming off that cap but it's hard to tell. Hopefully one of the Dave's will see this and perhaps post some more pics. I'll also try and search out a service manual or see if I can pick one up on ebay at some point.