What is the purpose of having ISOLATED inputs on a scope? All the grounds should be at the same potential...

Question

In fact, why are there several different ground inputs (clips on each probe). If all these should be at the same potential, isn't connecting multiple ground clips the same as introducing ground loops to the system?

Thanks.

Answer 1

they are not grounds. they are just the COM (reference) for the probe. you need them to choose the reference point you want to measure. if they were ground you could not use a duel input scope to measure the source voltage and the voltage across a circuit component without shorting something out.

the COMs on the probes are not connected to each other. they set the ground potential in the circuit that measures the wave form. like you said they are isolated so multiple connections do not form ground loops.

good question. on a side note the circuit that you are measuring needs to be isolate from the source that powers the oscope. i have made that mistake before. that will cause a very "lively" ground loop.

Answer 2

Most electronic instrument connect its chassis to the power source Ground for safe reason. Some electronic device does not like to connect a real Ground to its chassis.Either it causes a short circuit (blow a fuse) or it affects the actual measurement. To prevent this happen when you test with a scope has a real Ground connect to the chassis,it has two ways to do it.

[1] Insert a no ground plug in front of scope power plug,so that no more

     real ground connect to the chassis of scope.

[2] Using a one to one transformer to supply power to the scope,and

     no real ground connect to the scope.

This way,the ground (common) of input probe shall not link to the device under test through the real Ground of power line. We call it is isolated ground input.