Rigol DS1054z measurement questions

[drummerdimitri]

So I'd like to make some audio signal measurements out of my amplifier but since I don't want to risk shorting things out I have read somewhere a while back that I could use two probes without the grounding clip and the math function to see the signal on the scope.

How exactly is this done? What operation do I need to choose?

Also, can I make THD or DF measurements with this scope?

[AngusBeef]

I'm new at Oscilloscoping but I believe it's called a differential measurement


Subtract channel 2 from channel 1

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[Mechatrommer]

is your amplifier connected to mains (not isolated)? if no ie if its isolated, no need 2 probes trick, one probe, probe's gnd to circuit's gnd is the normal way. DS1000Z math function is hopeless unless for some very slow update rate applications or while noise figure is not important or when you have nothing else better. if you still insist, you can see for yourself by pressing math button, choose (A - B) choose which channel to become A and which to become B, and turn on the math display, its all in the menu.

[AngusBeef]

is your amplifier connected to mains (not isolated)? if no ie if its isolated, no need 2 probes trick, one probe, probe's gnd to circuit's gnd is the normal way. DS1000Z math function is hopeless unless for some very slow update rate applications or while noise figure is not important or when you have nothing else better. if you still insist, you can see for yourself by pressing math button, choose (A - B) choose which channel to become A and which to become B, and turn on the math display, its all in the menu.

Just to make sure I understand - if amplifier is isolated it doesn't matter - it's akin to using any ole battery powered multimeter because as long as one of the systems (measuring device or device under test) is isolated, it can't have that ground issue.

But if we are using an oscilloscope connected to mains which is by design referenced to ground, and if the amplifier is connected to the mains, the oscilloscope ground probe ground is connected to the mains power ground is connected to the neutral (in the US at the breaker box) which is connected to the neutral that the amplifier is connected to.... so then we can make a complete circuit from any "hot" wire in the device under test, through the probe through the oscilloscope to the ground back to the device under test neutral wire... or something like that

But I don't understand how you use the two probes without the ground clips...

[Mechatrommer]

if the amplifier is connected to the mains, the oscilloscope ground probe ground is connected to the mains power ground is connected to the neutral (in the US at the breaker box) which is connected to the neutral that the amplifier is connected to.... so then we can make a complete circuit from any "hot" wire in the device under test, through the probe through the oscilloscope to the ground back to the device under test neutral wire... or something like that

in any circumtances of where the probe's ground is connected, whether to neutral or any other node in offline circuit (worst will be the live) there is possibility that current will leak through earth connection (through probe's low impedance ground) and trip house's GFCI/RCCB, the probe input pin doesnt matter since its high 10Mohm impedance path so you can connect the input/positive/center pin to anywhere you want up to its rated limit. otoh if dso is not properly grounded, there is possibility the gnd exposed metal on the DSO will raise voltage to dangerous level without you knowing it until you touch it.

But I don't understand how you use the two probes without the ground clips...

your offline/floating circuit will have "common mode voltage" read by both probe the same, this common mode will cancel themselves when you substract both probe's values using (A-B) math, leaving you with value similar to whats read by a proper differential probe. for other DSO brand and older Rigol DS1000E series, math trace can give you descent result and refresh rate. but the near limited/full DS1000Z processor will do math function abysmally. downloading the traces to PC and do math processing in the PC will be much faster and acceptable, if that matters to you, ymmv.

[drummerdimitri]

is your amplifier connected to mains (not isolated)? if no ie if its isolated, no need 2 probes trick, one probe, probe's gnd to circuit's gnd is the normal way. DS1000Z math function is hopeless unless for some very slow update rate applications or while noise figure is not important or when you have nothing else better. if you still insist, you can see for yourself by pressing math button, choose (A - B) choose which channel to become A and which to become B, and turn on the math display, its all in the menu.

I can easily isolate it from the mains by using a sinewave inverter.

Good to know thanks!

[das_strobel]

Just to make sure. This

is your amplifier connected to mains (not isolated)? if no ie if its isolated, no need 2 probes trick ...

is not 100% true in all situations.

Both (in fact all) probes share a common GND. So doing 2 independent measurements, that don't share a common ground in the same circuit is not possible. You would need the "2 probes trick", i.e. 2 differential measurement with 2 sets of probes again.

[TK]

If your amplifier output is bridged, then you need to do differential measurement as ground is not present in the speaker output pins.

[Mechatrommer]

Just to make sure. This

is your amplifier connected to mains (not isolated)? if no ie if its isolated, no need 2 probes trick ...

is not 100% true in all situations.

his concern is not to use gnd pin. you can use 2 probes on floating circuit while connecting gnd pins to a common gnd on the circuit, and choose whether to use 2 probe single ended reading, or use the crappy math function in DS1000Z as differential reading. but you cant simply plug in the gnd pin of the probe to even the gnd (neutral or common) node of the offline/mains circuit.