Floating the 'scope

Why not invert one channel and switch to "add".

Then use both inputs differentially.

Most, if not all worthwhile instruments should do that.

Probe compensation needs to be accurate for best CMRR.

Or buy a differential probe?

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

Is this a two channel scope? If so, what does an ohmmeter measure between the shells of the two BNC inputs of the two channels?

Aren't the two BNC input grounds connected to the same internal ground reference?

A problem arises when you need to make measurements with a 2 or 4 channel scope at locations in the circuit which don't have a common reference.

For example, suppose you need to look at the gate-source voltages of the two top MOSFETs in an H-bridge configuration. Isolating the equipment doesn't allow you to connect the ground clips of the two scope probes to different points in the circuit that have a substantial voltage difference.

You need to use floating differential probes, or a scope with true isolated inputs.

In days gone by, cheap consumer devices like televisions and table radios often had a "hot" chassis; it was connected to one side of the line. And, in those days, the line cord wasn't polarized; there wasn't even a third ground prong on the outlet.

Using an isolation transformer when repairing such a device was a good idea.

Why would anyone take a scope apart and disconnect the line cord safety ground from the chassis inside the scope? Whenever I've seen anyone do this, they just use a ground buster, easily obtainable from the hardware store, to disconnect the safety ground at the outlet.

As I asked John, why would anyone go to the trouble to get inside a piece of equipment to disconnect the safety ground, when they can just use a ground buster at the wall socket?

What's the model number of that scope?

The 2 BNC's are isolated.

I have no idea how they are doing it how ever, I can say this, I don't like the BNC receptacles. It's a hard plastic shell instead of metal. I guess that's part of the isolation. It does work.. I've used it to measure signals around a resistor that is far from any common node with one probe while the other has it's common attached to the device common. It seems to work .. He DMM inputs are standard DMM probes.. and are also isolated.

I don't think its that hard to fab 20 mhz isolated analog amps.

This will not work for reading a voltage of 1 or 2 volts or less, which is floating as much as 400 volts above ground. For instance, reading the gate voltage of an SCR on a 480 VAC mains system. You must float the scope, and I've done it safely and effectively, with a hand-held scope. There may be some differential probes that can handle this; I don't know.

Paul

scopes,

Also available are these:

I think they may be made by the same folks that make the Yokogawa differential probes.

I've used the 100MHz Yokogawa probes, and they are very handy.

Most idiots just cut the ground pin off the power cord.

--
Greed is the root of all eBay.

HDS1022MN

and the 100 Mhz version

Sometimes when I have seen this, I complete the job by cutting off the plug. Makes it safe again, but not very useful.

Paul

The one can be done in five minutes for free, the other involves going out to the hardware store and costs money?

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

The OP was talking about 15 volts common mode, where the technique will work perfectly well.

56 dB CMR with a common mode voltage of 480 RMS is beyond the capability of even a 7A13. There are differential probes that will do it, but they have 4 figure price tags.

I've never needed to go to that extreme. Most of what I do is IGBTs with a DC bus of around 280V, and the (now occasional) SCR bridge at 200V RMS. A

7A13 will handle those, as will a fairly cheap differential probe (few hundred bucks).

I hope the handheld you used was battery powered. I wouldn't like to stick

670-something volts peak across the PSU isolation of a floating line-powered 'scope.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

7904, two 7A13s, job done.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

TPS2024. No plugins. 4 channels. Job done, and battery operation for ultimate portability.

No comparator / voltage slideback function.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
                                             (Stephen Leacock)

The CT2593-1 described here:

is specified for 86 dB CMRR at 50 Hz, 66 dB at 20 kHz, and 1400 volt common mode rated for only $331