A question about differential probes.

Hi John,

You're right, this is the main use-case for the type of probes I think you're referring to (the ones that look like a box with two flying leads with large probes/clips.

Since the usual 'scopes and probes have their metal shell connected to the mains earth connection, it's hard to connect to circuits where you may want to have that shell connected to a different voltage level (e.g. to see signals that are riding on a high voltage level, although to some extent you could try to do it with the 'scope set to AC input and shell to earth, but it isn't always ideal). Worse still, if you've got two or more signals, and you want each one to be referenced differently, it isn't possible with usual 'scopes and probes since all the shells are connected together and to earth. The portable 'scope solves the first problem, but not the second since the shells are all still connected together, although not to earth. It's also nice to know there is some isolation close to the circuit when probing, so having differential probes that isolate is useful, rather than run the risk of sending high voltages into the 'scope directly.

So, those box-type differential probes are serving a few different purposes. Also, those type tend to be useful for high voltage or mains work - they are handy for seeing very high voltage signals (i.e. no need to use the 'scope attenuator to see a thousand-volt signal) as well as the ability to have input referenced from very high voltages too.

They are really handy, and the main design effort is to make sure they are safe, and that they have a flat response. It would be nice to see a project for a differential probe, at some point I purchased a load of high-voltage capacitors and resistors, to attempt such experiments, but not had time to work on it : (

There are other types of differential probes too, such as those intended for high frequency (e.g. RF) work, where the aim isn't to isolate very high voltages, but more that they behave like a differential equivalent of a FET probe (i.e. ability to have a high-ish impedance for high frequency signals that would ordinarily be loaded by too high a capacitance with normal 'scope probes). That type look very different of course, with no large test leads/clips.

shabaz , some recent handheld scopes have the inputs isolated from each other, also solving the second problem.

jw0752 , for lower voltages, you can use two scope inputs and the math "-" function of the scope to do differential measurements without a diff probe.

Disadvantages:

• you use two inputs. Most scopes only have one math signal. So that limits you to one differential measurement.
• noisier
• using math limits the bandwidth for some scopes.

Thanks Jan,

This makes more sense to me now. Thank you for the additional information. I will check my inexpensive Rigol this evening to see if I have the Diff function in the Math index.

John

When using the Scopes Math function for this (And there is nothing wrong with that function), be aware that your grounds for each scope probe are still connected together and still to ground, there has been many a scope killed because this was miss-understood.

Actually that applies even when your not using the differential function but folks tend to make the mistake more in this mode... just an FYI

Use this on a low power board, John

Maybe a AAA battery with 3 series resistors over it.

Try to measure the voltage over the middle resistor.

Your Rigol has the function.

Use this on a low power board, John

Maybe a AAA battery with 3 series resistors over it.

Try to measure the voltage over the middle resistor.

Your Rigol has the function.

Hi Jan,

This experiment and a few others are on my schedule tonight.

John