Products > Test Equipment
Floating Scopes
alm:
That is a perfectly safe solution (assuming suitable probes) that I am pretty sure was discussed in this topic. The main limitation is that the common mode rejection ratio is quite limited, i.e. you are likely to see a signal (for example mains frequency) superimposed on your trace. That is where a 'real' differential probe performs much better.
Safar:
Yes, CMRR is a problem in this method. Especially that is need to accurately setup signal level on both channels to avoid ADC overload. And useful signal can be very low after math op
David Hess:
--- Quote from: alm on July 03, 2017, 03:34:46 pm ---That is a perfectly safe solution (assuming suitable probes) that I am pretty sure was discussed in this topic. The main limitation is that the common mode rejection ratio is quite limited, i.e. you are likely to see a signal (for example mains frequency) superimposed on your trace. That is where a 'real' differential probe performs much better.
--- End quote ---
The big problem is usually the limited common mode input voltage range for a given sensitivity. The common mode rejection ratio is less of a problem if you have an analog oscilloscope because:
1. The vertical variable controls can be used to match the DC gain on each input for maximum common mode rejection ratio. (1) Compensation can be used to trim the AC common mode rejection ratio.
2. Subtraction in a DSO *adds* the quantization noise of each channel and reduces the number of significant bits. Some very early DSOs did the add and invert in the analog domain before the digitizer and do not suffer from this problem as much.
(1) It is actually a little eerie to perform this adjustment. You can *see* the noise "null out" when the variable gain control is adjusted correctly.
tronde:
--- Quote from: IanMacdonald on July 03, 2017, 12:02:01 pm ---Slightly oftopic as it's not a test instrument, but everything said here also applies to earthed soldering irons. (I was going to say in spades, but actually it applies to all shapes of tip :groan:)
If you use an iron with a 240v element then earthing is essential for safety, since an insulation failure could put mains on the tip. No question about that, and it's one of the reasons that direct-mains irons are a poor choice for any serious work.
Most decent bench irons run from 24v though, and the question then arises, is earthing desirable or not?
Static discharge is necessary if working on MOSFETs, but that does not actually require a hard, zero-ohm earth.
The downside of earthing irons which don't actually require an earth for safety, is the risk of damage to equipment due to residual charge on electrolytics being dumped to earth through any sensitive component you are soldering on or near.
That, and if soldering on high current gear with an earthed iron you are in the same position, safetywise, as using a multimeter which does not have proper HBC fuses. If a terminal is unexpectedly live, that is much the same as forgetting to take your unprotected DMM off the 10A range before measuring voltage.
--- End quote ---
I don't consider this offtopic, because it can affect the safety on the workbench you also use for measurements.
As I wrote in reply #52 you need to be aware of the entire system. You can not just rely on a differential probe to be safe, so the grounding of the soldering iron is certainly worth looking at.
As you say, a mains supplied soldering iron will need a "hard earth" for safety, while a low voltage iron will need a "soft earth" for slow / controlled discharge of static electricity.
This is OK as long as you know what kind of earth you have. I have a Weller WD2 transformer. On that transformer you can change the resistance from the tip to earth by means of a 3.5mm plug / socket with a built in switch. No plug gives 0 ohm (hard earth). Plug only gives no connection to earth, and a resistor installed in the plug gives you an earth resistance with that value. At first this seems to be rather smart. BUT - Weller goofed. The plug / socket is just a normal el-cheapo thing, and the plug can easily work loose. Then you will have a hard earth while you believe it is a soft earth. The plug they use is rather small, and I doubt you can find space for a resitor that will safely withstand 250V. I modified my transformer so I have a proper resistor installed inside the enclosure.
The grounding of the antistatic mat is also something to consider. With 230V mains it will normally be connected to earth by a 1Mohm resitor. If we place something with an earthed chassis on the mat it can bypass the safety resisor. A normal antistatic mat will have a rather high surface resistance, but the resistance to earth will decrease and a leakage current will increase.
JDW:
A lot of good info in this thread, but one thing overlooked now in 2023 are newer scopes like the Rigol DHO800/900 series come floated due to having a USB-C type power plug and a power brick from Liteon that is only 2-prong at the wall socket. Sure, they include a separate grounding wire (with banana connectors at both ends) "for safety," but the fact the scope comes this way is a recipe for unsafe practices, especially here in Japan where 3-prong wall sockets are utterly impossible to find in most homes. Indeed, here in Japan, the only time you'll see a ground is near a wall socket in a room where there is a toilet (Japanese love their electronic bidet devices), or a refrigerator in the kitchen. They are almost always screw terminals for connecting bare wires that hang off the appliances or bidets. Any other place, which is the most likely place you'd be using a scope, the wall sockets are 2-prong only. But even if they were 3-prong, the design of the power adapter is such that the user isn't FORCED to use a grounded wall socket plug. That Rigol grounding wire is "optional" in that it isn't built into the main power cord. That is a big issue.
Tektronix offers battery powered scopes which are basically floating devices, but even Tektronix cautions you about them, properly saying not to test voltages above 30Vrms or 42Vpeak. They also warn against the use of Isolation Transformers as being "Dangerous."
https://www.tek.com/en/documents/technical-brief/floating-oscilloscope-measurements-and-operator-protection
But even if you are using less than 30Vrms (which is basically all I test, personally), grounding the scope provides a way to avoid Common Mode Noise on your measured waveforms.
Lastly, the Rigol DHO800 documentation that came with my scope doesn't even use terms like "floating," and references to Earth Ground are few, which mean that people searching the documentation for important safety info might miss the topic altogether or think they are safe to use their new 12-bit scope in a floated condition.
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