Do I need to use optical isolators to connect two probe grounds w.o. shorting? - Page 1
Forum section: electronics / beginners

author: NeverDie
author_rank: Regular Contributor
timestamp: September 16, 2015, 06:57:12 pm
content: I want to take two simultaneous measurements on a 3.3V circuit using two different channels. One channel would be measuring milliamp voltage across a 1 ohm sense resistor (to measure Rx current on an RFM69 radio), and the other channel would alternate between 0 volts and 3.3v (connected to an interrupt pin that goes high when the packet is in the FIFO and ready to read). I'd like to see both traces running at the same time. I can connect either one up individually and read it no problem, but I can't connect them both at the same time without creating a short through the ground clips of each probe. That's because the two points I want to connect the ground clips to are not equipotential. The radio circuit itself is battery powered (floating), so this isn't a case where there's a short through earth-ground. And, yes, I did see the Dave Jones video on how not to blow-up your oscilloscope, but it just said doing this would be a problem and didn't appear to offer a solution appropriate to this simple scenario.I thought there might be simple optical isolators to handle a situation like this (each isolator is connected to the points I want to probe), and then the oscope probes would connect to the other side of the isolators, and their grounds could then be the same no problem. Do I have to make my own special purpose circuit just to do this measurement? Or is there something I can buy off the shelf? I did try looking, but didn't see anything inexpensive for doing low voltage measurements like this.I'm using a Rigol 1054Z oscilliscope.

author: free_electron
author_rank: Super Contributor
timestamp: September 16, 2015, 07:01:54 pm
content: optical isolators dont work that way ...simply do not connect the ground lead of the second probe.the scope will be measuring referred to whatever its groudn is connected to ( in this case the voltage on one side of the sense resistor.so the scope will give you a signal based off that.

author: Hideki
author_rank: Frequent Contributor
timestamp: September 16, 2015, 08:12:40 pm
content: If you really want to have different ground references for each probe, then you need to buy a differential probe. They come in all sorts of price ranges, but none of them are particulary cheap.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 16, 2015, 08:23:46 pm
content: Or get a second scope.

author: gilbenl
author_rank: Regular Contributor
timestamp: September 16, 2015, 09:13:58 pm
content: I may not understand what's going on here, but it sounds like you're measuring two points on the same device that is floating (battery powered) using a scope that is ground referenced. Since the DUT isn't ground referenced, whats the issue?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 16, 2015, 09:49:00 pm
content: The scope in question is a 1054Z, a _four channel_ scope.So perhaps the right and easy way to do this would be to make the first measurement (current, i.e. voltage drop across a current-sense resistor) with one probe, and then use _two probes_ , two channels to make a differential measurement of the second signal, with all ground clips connected to the same point (at the current sense resistor). Use the scope's math capability to do the A-B (or A+B) math for the differential measurement.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 16, 2015, 09:50:07 pm
content: Quote from: gilbenl on September 16, 2015, 09:13:58 pmThe issue is that the scope's probe Ground clips are all connected together at the scope. So they can't be connected at different points in the DUT without shorting those points together.

author: rs20
author_rank: Super Contributor
timestamp: September 16, 2015, 10:33:54 pm
content: It seems to me that the common mode voltage on the (presumably?) High side current sense resistor will be quite constant... So if this device under test is definitely battery powered or powered by a floating lab supply, then:Connect both ground leads to one side of your current sense resistor. Connect one probe to the other side of the resistor. Connect the other to your interrupt pin. Your interrupt pin on your scope will go from -3.3V to 0V, but that's fine?

author: newbrain
author_rank: Super Contributor
timestamp: September 18, 2015, 02:23:24 pm
content: Quote from: John_ITIC on September 16, 2015, 08:23:46 pmUnless I'm mistaken and misunderstood the problem (and Dave's video) this is not going to work.Both scopes will have the ground alligator clip connected to earth, so that would just be a "longer" short, through the main sockets, instead of the scope's case.

author: Wim_L
author_rank: Regular Contributor
timestamp: September 18, 2015, 02:45:17 pm
content: free_electron has it right. Use the first probe to measure the voltage across the sense resistor. Ground clip on one side, probe tip on the other. Use the second probe to measure the second channel, and either don't connect the ground clip or connect it to the same place as the first probe's ground clip.This makes sense, because for the first channel you want to measure the values as accurately as possible. For the second, you just want to know when it goes high or low, and you can see that even when there's a substantial DC offset on that signal.

author: NeverDie
author_rank: Regular Contributor
timestamp: September 18, 2015, 04:24:44 pm
content: Quote from: Wim_L on September 18, 2015, 02:45:17 pmThis is a good summation. (By the way, thanks free_electron!) I got lucky this time because the second signal was digital. I think it only rose to about 2 volts, but that's all I needed to see that it was triggered.What about next time, though, if I'm not so lucky in a similar scenario where I want to compare two analog signals and where accuracy matters? Then the same technique wouldn't work.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: September 18, 2015, 08:01:57 pm
content: Quote from: newbrain on September 18, 2015, 02:23:24 pmActually, if you have enough scope channels, could you not ground reference all scope channels, connect Ch1 before the current sense resistor, Ch2 after the current sense resistor and use the scope's math function to calculate the actual voltage drop (Ch1 - Ch2)? Then you are free to use the remaining two scope probes for other ground-referenced signals. A differential probe would do the same thing but only using a single channel.This also answers the recent question why to have a four-channel scope!

author: Wim_L
author_rank: Regular Contributor
timestamp: September 22, 2015, 08:56:18 am
content: Quote from: NeverDie on September 18, 2015, 04:24:44 pmIdeally, differential probes or scopes with independent isolated inputs. Unfortunately, neither of those are particularly cheap.Using two scope channels with math mode to make an improvised differential probe works, but depending on your needs accuracy and common mode rejection may be disappointing.In cases where only the AC component of a signal matters, coupling the signal to the oscilloscope using a suitable transformer may work - ideally one capable of high frequencies and with shielded windings. You'd have to check the transformer doesn't introduce unacceptable signal distortions for the frequencies and signal amplitudes you're interested in, and that it doesn't cause excessive loading (particularly problematic if there's a DC component too - it won't be measured on the output side, but it may drive the transformer into saturation!).Coupling using capacitors may also work. Using capacitors designed for use in the signal path (i.e. not electrolytics or those ceramics with high voltage coefficient), and keeping in mind you need capacitive coupling on both contacts (you can't do it with just the AC coupling mode of the oscilloscope, as that only AC couples the central pin of the BNC connector, not the ground). When first attaching the ground clip with added capacitor, there will be a brief current spike through that ground lead until the capacitor is charged. Some circuits may be sensitive to this.These two last methods may be cheaper, but you have to customise them for every specific measurement. They don't offer the general purpose ease of use you get with differential probes.

author: NeverDie
author_rank: Regular Contributor
timestamp: September 25, 2015, 02:20:36 am
content: Quote from: John_ITIC on September 18, 2015, 08:01:57 pmTo what degree does accuracy diminish if doing that?

author: rs20
author_rank: Super Contributor
timestamp: September 25, 2015, 12:22:58 pm
content: Quote from: NeverDie on September 25, 2015, 02:20:36 am? Not at all? Unless you do something silly, like not zoom the individual signals vertically so that they fill the ADC range. The math function that does the subtraction after is just a digital subtraction, and is perfectly accurate as such.

author: Wim_L
author_rank: Regular Contributor
timestamp: September 25, 2015, 09:38:26 pm
content: It depends on the common mode range. If, say, you have a 1 V differential signal, and 1v common mode, you lose about half of your range, but that's still pretty good.If you have a 10 mV differential signal and 1 V common mode, it's going to be terrible on scopes using 8 bits for amplitude. And it will be terrible at high frequencies even if you've got a high resolution measurement, as the common mode rejection ratio of most measurement devices drops significantly as frequency increases.

author: rjeberhardt
author_rank: Regular Contributor
timestamp: September 26, 2015, 08:14:34 am
content: Using two channels and maths to get a differential measurement can give significant errors. OK the digital subtraction is accurate to within one bit but the analogue side of the two channels, including the probes, can have slightly different gains.A quick check is to take a reading then swap the probes and channels over and take the reading again. If the readings are the same all well and good. If they are different you can take the average of the two.Russell.