Wideband Current Probe Design

I build embedded systems, both the hardware and firmware, and the one tool I reach for first when I am testing my system is my custom built current probe. If you have a scope and you don't have a current probe you're really missing out.

 

Yes, there are tons of solutions available on the web but I alway found they were either too $$$ or better suited to measuring large AC currents. I decided to design myself a probe suited to the systems I typically build. Here are a few of the specifications for my design:

 

  Bandwidth: 800kHz

  Sensitivity: 29.8 V/A

  Output Signal Range: +/- 400mA

  Accuracy: 1% of reading

 

As you can see it has lots of bandwidth, this is more than enough to measure supply currents for digital systems so you can discern activity. This is actually one of the primary ways I use mine, it's amazing how much information you can glean about your processors state from watching the current! The input will work up to +/- 10 V and yes, it is bipolar so you can monitor charge/discharge circuits.

 

The circuit itself is pretty straightforward. It is a difference amplifier with a DC offset zeroing pot

 

I decided to make it USB powered because you don't want to be in the middle of making measurements and realize the 'weird' current levels you have been seeing are because the batteries in your probe are nearly dead. And besides, USB ports seem to be lurking everywhere these days - I just use the one on the front of my oscilloscope.

 

There are a few photos of the assembled circuit below. Of course, like any piece of kit it needed a nice enclosure so I splurged and designed a custom aluminum CNC case anodized in black. Not really necessary but it looks much better and is rugged enough that can I take it with me out of the lab if need be.

 

Let me know if you decide to build one, you can tweet at me here. If you have any suggestions/questions you can reach me though the contact form here or just DM me on twitter.

 

Cheers,

 

Chad

 

Design Files:

 

SCHEMATIC

BOM

GERBER (coming soon!)

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