Isolated USB oscilloscope - how can it help

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Isolated measurement

Sometimes we would like to measure a signal with oscilloscope on a device, we are not familiar to. The reason might be that we don’t have a datasheet or a scheme available, or simply a common ground potential is not physically accessible. In such cases isolated USB oscilloscope can help. What’s more, I think we can use the USB isolator with any USB device if there is a risk of damaging the computer’s USB port.

When we take measurements we must pay attention how and where to connect probe’s ground (alligator clip). Almost all oscilloscopes have probe’s ground connected to a main AC power socket earth potential. Wrong probe connection can cause short circuit and device damage.

My experience with isolated USB oscilloscope

Not long ago I wanted to measure current through a coil on a device for so called magnetic levitation. I inserted a small serial resistor in the circuit so that I could measure a voltage on it. That way I could indirectly measure the current through coil. As I already mentioned, there was a problem when the ground wasn’t common for the device on which I wanted to take measurements and our measurement equipment. I had a few options how to cope with this issue.

Common GND point can’t be reached

First of all, I could use two-channel oscilloscope with maths functions. Measuring signal at each end of the resistor without applying probe’s ground and than subtracting these two signals (channels) would give me a result.

Another option would be isolating the device I would like to take measurements on, or isolating the oscilloscope. Only one probe would be used but in both cases I would need to find somewhere a transformer. Isolated power transformers are usually heavy; they use a lot of space and they are never there when you need them.

Solution

Fortunately, there is one more simple solution. Use of a USB oscilloscope and a USB isolator makes that kind measurement easy and efficient. There is no need for additional workspace or moving heavy equipment.

Common GND point can’t be reached

I managed to solve my issue pretty easily by using an oscilloscope probe at one end of the resistor and its alligator clip at another. I didn’t need to worry anything would “blow up”. The USB isolator makes the USB oscilloscope a floating device or if you prefer an isolated device, in the same way as you would do it with the transformer. The oscilloscope has 1MOhm input impedance therefore I practically didn’t change any conditions in the circuit. That way we can measure signals anywhere in the circuit.

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