tinyCurrent, oscilloscope, USB, and ground loops

[jkomets]

I want to measure deep sleep currents of a microcontroller (and a module) and the best way to do it seems to be the tinyCurrent, which has a convenient BNC connector for output directly to an oscilloscope (unfortunately the µCurrent Gold is not in stock anywhere). The microcontroller sometimes has a USBasp programmer attached, which is mains-grounded through the computer, and the oscilloscope is mains-grounded too.

My reading and watching has left me with the impression that it's best to avoid probing a mains-grounded DUT with a mains-grounded oscilloscope because it can create a potentially dangerous ground loop. I could detach the USBasp and use my bench power supply every time I measure sleep currents, but I would prefer to solve it with a USB isolator so that I have less replugging to do and also avoid accidentally damaging something.

The documentation of the competing Current Ranger has a page on proper usage. Here USB isolators and ground loops are also mentioned:
But it’s not OK:
to measure a DUT that is mains/earth powered/connected, with an oscilloscope/DMM that is also mains/earth connected

Additionally, I watched a YouTube video comparing the µCurrent and Current Ranger, and the author also recommends using an isolator for his ESP32.

I see a huge range of prices from 6,87 € on Aliexpress to 62,10 € on Reichelt. Many of them are based on the ADUM3160. Also, the ones that I keep seeing recommended (in the video and the documentation above for example) look exactly like the cheap one on Aliexpress. Will the cheap one do or should I invest more?

Thanks for your input!

[thm_w]

The $5 USB isolator from aliexpress works fine for digital stuff: https://www.aliexpress.com/item/1005001369085297.html

There are a few threads on low current measurements you can read through. Oscilloscope, DMM, high precision PSU, or dedicated measurement device may all be options. You'd need to provide more actual details on the resolution and ranges used.

https://www.eevblog.com/forum/testgear/currenter-vs-ucurrent-gold/
https://www.eevblog.com/forum/chat/iot-consumption-loggingprofiling/

[jkomets]

I'm looking to measure an ATtiny84V and an nRF24L01, so I would like to have a range of about 1µA to 15mA. The attractive thing about the scope is the ability to measure current over time and plot it, and that I have everything I need except the tinyCurrent itself.

I'll order the isolator you linked.

[thm_w]

The nordic profiler mentioned in the second thread has graphing capabilities via PC, but costs a bit more than the tinycurrent ($100).
So depending on if you prefer to use the scope or the PC, that might help you decide.

https://circuitdigest.com/review/nordics-power-profiler-kit-2-a-must-have-tool-for-embedded-engineers
https://pallavaggarwal.in/dc-current-analyzer-for-embedded-iot-product-development/

[jkomets]

Thanks for the tip on the Nordic PPK2. I watched some Youtube videos on the PPK2 and the µCurrent/tinyCurrent and I decided to order the PPK2, because including VAT and shipping it was only 20,- € more expensive than the tinyCurrent and I liked the fact that the burden voltage is lower because there are more shunt values to choose from, and I also liked the auto-ranging. That means a smoother transition between deep sleep current and transmitting current values on the graph. The CurrentRanger which I also found in my research has auto-ranging as well and has the problem that it can create spikes in voltage right in the moment it switches ranges and the PPK2 seems to have avoided this. I'm excited to try it out!