Oscilloscope shows a voltage present when probe is disconnected

Thread Starter

aguaman99

Joined Dec 23, 2012
20
I have two scopes one analog and one digital. A Rigol DS4024 and the other a Tek 2465B. They both seem to be operating and matching readings on my other exuipment when connected to a circuit. When probes are disconnected they both show the exact same thing and it has me thinking i just don't understand something..... both scopes are set the same, 50mV per division and 5ms per division and are coupled AC. both probes are lying on the bench and not connected to or touching anything. Im displaying a 60Hz signal with an amplitude of 150mV on one scope and 200mV on the other. I'm thinking they are picking this up from the mains somehow but im not sure why that would happen or if this could be messing up signals when im connected to a circuit. Im guessing there is a very logical answer to this but it is not clear to me... Is this normal? If so why is this?
 

crutschow

Joined Mar 14, 2008
29,792
When the probes are not connected, they are a high impedance that picks up the 60Hz electric field radiation from your house wiring.
When it is connected to some circuit node, which is a relatively low impedance, it sees only the circuit signal, not the 60Hz pickup.

You can test this by connecting various resistor values from the probe tip to the probe ground connection and see how much you pick up under those conditions.
 

Thread Starter

aguaman99

Joined Dec 23, 2012
20
Thank you for the quick reply that’s what I figured. Basically acting like an antenna. I didn’t notice when the ground clip was connected both signals went completely flat. I’ll try the resistor trick
 

Audioguru again

Joined Oct 21, 2019
4,685
The cable for the scope probe is supposed to be a shielded cable with the shield wire connected to the scope's 0V.
At the probe end, the ground clip should be connected to the cable's shield.
 

Thread Starter

aguaman99

Joined Dec 23, 2012
20
Thank you for the quick reply that’s what I figured. Basically acting like an antenna. I didn’t notice when the ground clip was connected both signals went completely flat. I’ll try the resistor trick
it is, not sure what your trying to point out i guess....
 

Thread Starter

aguaman99

Joined Dec 23, 2012
20
The cable for the scope probe is supposed to be a shielded cable with the shield wire connected to the scope's 0V.
At the probe end, the ground clip should be connected to the cable's shield.
I knew a technician who worked for the Tennessee Valley Authority, sometimes out in the middle of nowhere. He said it gave him an earie feeling to turn on his battery powered scope and not see 60 Hz noise,
I look forward to the no noise situation.... I'm in the middle of a relocation to northern Idaho but that's another story. Its funny this situation had me stumped, later after posting i was doing something else and then went duh.... sometimes the obvious can be illusive.
 

graybeard

Joined Apr 10, 2012
31
I had a cheap Chinese switching bench power supply for a while. You should have seen the noise on my not-connected scopes when that bad boy was turned on. That bench supply is now in the E-waste bin...
 
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