Electrical – Oscilloscope measurement: unsure how to connect the scope ground to the target


I have some control board equipped with a microcontroller.
This control board is supplied by 24V voltage source. I
would like to measure signals on some microcontroller's
output pin with a digital oscilloscope.

I have only two common probes with earthed crocodile jack connector. I am not sure whether the measurement with a probe connected to the output pin and crocodile jack connector connected to minus pole of the voltage source is correct.

Would be better to use both probes with their crocodile jacks connected together and then connect the first probe on the microcontroller's output pin, the second probe to the minus pole of the voltage source and use ChA-ChB function?

Best Answer

You still need to connect the junction of the joined probe earths to something on the target system. Why not use a multimeter and check that earth on the scope (AC plug) corresponds with 0V on the target system. If there is a significant fluctuation in that measurement (AC) it may be because the target system is floating i.e. does not form an earth connection at all.

If the latter is the case then try connecting its negative pole to earth potential through a resistor at first (maybe 100 kohm). Check to see the voltage across the resistor using just the probe end on the scope. Consider gradually lowering the resistor and see what happens to the scope voltage - does it reduce - if so then it's likely the target is floating from earth and can be directly connected to earth.