Floating measurements and ground loops
Any voltage measurement is a differential measurement, a voltage difference between two points. The reference used for a measurement can be either:
- the mains’ supply ground (ground referenced measurement)
- anything else (floating measurement)
Common desktop oscilloscopes are ground referenced. There are multiple reasons for this, one of them being that they are powered from the mains (110/220V). This brings some limitations: the ground of the probe must be connected to the ground of the oscilloscope’s supply, if the DUT (Device Under Test) has any connection to the mains. This connection can be subtle, like a simple USB cable between the DUT and a computer. The following schematic shows the correct connection.
Always remember that earth, ground and live wires are not galvanically isolated. Misconnection can result in destroying the DUT and/or the measurement device and can also lead to fire in the most extreme cases. This is called ground loop. It is one of the most common pitfalls when measuring electronic signals. The schematic below describes a wrong measurement. Here the ground of the probe is connected to a voltage source on the DUT, resulting in a short circuit:
IkaScope can be used for ground referenced measurements as well as floating measurements because it is battery powered (no connection to the mains). Therefore there is no path for the current to make a ground loop.
But what happens if you recharge your IkaScope using a USB adapter? Well, IkaScope has an integrated isolation transformer, providing galvanic isolation, and can be used for floating measurements while charging. You must read the detailed datasheet to find the maximum voltage it can handle between the USB ground and the probe ground. Failure to respect the maximum voltages can cause an electric shock.