TMR Based Current Prove
Using an Tunneling Magnetoresistance sensor to create a DC-10mhz(maybe) current probe for hobby use
Why?
Current probes are a very useful piece of kit. Usually I would use a Current transformer when necessary but these dont work for DC. The TMR sensors can work with DC into the high frequency range, and a DIY option would make them accessible to the Hobbyist, as professional options cost thousands of dollars. I first became curiuos about TMR sensors after reading this page on the TDK website:
https://product.tdk.com/en/techlibrary/productoverview/tmr-angle-sensors.html
there are some clear benefits with these chips over Hall effect or AMR devices, and since I was looking for a new project I thought making a useful device out of these relatively new devices would be a perfect fit!
Topology
I used TMR based sensors from Multi-Dimension. They are a company that specializes in this kind of sensor. The sensor outputs a differential signal depending on current fields. They come in a few different flavors but the one I picked, the TMR2583, is a Wheatstone bridge sensitive in the Z-direction. The datasheet is a little sparse so I am curious to see what kind of performance I can get out of these chips. I added plenty of compensation and the analog section has a 10mhz bandwidth so I should be able to take full advantage of them.
Schematic
The circuit uses a simple instrumentation amplifier circuit using the AD8421. There is some compensation between U2 (the TMR sensor) and this IA. The rest is some power supply things, something of note is the isolated dc to dc converter I used to get my 12v rails. Although it is very convenient in the future I will probably replace it with a buck boost converter since they are hard to find and expensive. The circuit I use to control the relay is a D type flip flop that acts as a latching circuit for a push button. The relay controls the gain of the IA for hopefully 10x and 1x modes in practice. If this circuit works for rev2 I would like to add a microcontroller for auto zero and some other nice to have things.
Circuit and PCB Design
PCB
Using Kicad I created a quick 4 layer board. I dont have too much to say about it, I choose four layers so I could keep the layout clean and small, I tried to follow best layout practices and took full advantage of the suite of tools Kicad has, although this project did show me some of the programs limitations. The boards will be ordered on JLCPCB and populated by hand.
