So this is the Micsig CP2100 series current probe, it’s my first piece of equipment from Micsig but I’ve been hearing good things about them so I kinda have high expectations for this product.
What I have here is the CP2100A variant which is rated for 800KHz bandwidth, there is also a B variant which is rated for up to 2MHz in the latest revision, that one is a bit more expensive and I would only recommend getting it if you really need that bandwidth, otherwise there shouldn’t be any other difference between the two models. I don’t know if I mentioned this already but obviously it can measure AC and DC, it has two ranges, 10A and 100A. There is a zero function on the module for automatic zero adjustment and you can also do manual offset adjustments with these arrow keys.
Micsig specs this as 3% ±50mA accuracy for the 10A range and 4% ±50mA for the 100A range but from what I’ve been reading on the forums, this is actually better than the spec, you can pretty much measure down to 50mA without having to worry about that ±50mA but we’ll put that to a test later. Included below you see a set of images from the teardown.
If you are looking for the correct way to select, place and route your current measuring shunt resistor on a PCB then stick around as I will be sharing a bunch of tips & tricks that I learned while designing and building my own boards over the past years.
A shunt resistor can be used in multiple ways but for the purpose of this video we are interested in using it for measuring current, this means it needs to be inserted in series with the device under test. It can either be inserted in a high side configuration as shown in this example but it can also be inserted in a low side configuration. These two typologies have their advantages and disadvantages.
Here is a side by side comparison, when used in a low side configuration there is a ground offset which depending on your application may or may not pose problems. When used in a high side configuration, you must use a differential input amplifier for sensing the voltage drop while in a low side configuration you can get away with single ended.
Another thing that might be important, in a high side configuration you have the ability to detect a short on the load, while on the low side configuration you can’t do that. Also in a low side sensing configuration, you are not able to sense and account for additional leakage currents through stray secondary paths from the load to ground. This list might not be complete, there might be other differences to consider, these are just the ones that I had to consider in my designs.
But let’s say you’ve figured out the topology you want to use and you are now faced with choosing your current shunt resistor. Can you just pick your typical metal film 1ohm resistor from your favorite distributor? You can, but you won’t get the best results. Probably the most important factor you want to consider when choosing your shunt resistor is the temperature coefficient or the temp co as engineers like to refer to it. This will tell you how much your resistor value is going to change with variations in temperature.
Every system you build will probably see a variation in temperature at board level and so if you calibrate your system to calculate the current for a given resistor value and that resistor value changes with temperature, you are going to introduce significant errors in your measurement.
