Current Sensing

I've been working another project in parallel with my C++ programming stuff and it's been interesting to say the least. I think I may have mentioned a DVT on a small little board a few weeks back. Anyway, I've been given the task of doing tests on the power consumption of this board. Simple enough you may think. It was, but now it's slightly more complicated.

First off, I just attached a digital multimeter in series with the negative terminal to the DC power supply. I made all my measurements through this, and it worked grand. Unfortunately, we have an RF board attached and when this is powered, I need to measure for current spikes. The multimeter won't catch these as they'll be quick. So I found I need to attach a shunt and an oscilloscope to the works.

Now this is where it all became a little more complicated. I've probably got the worst setup ever, but it'll do for the moment. I started reading up on current sensing and found that if I had a current probe with a torsal ring, I could simply attach it onto the wire (unobtrusively), without worrying about change in voltage. Do I have one? No. So I have to figure out the other way to do this. So I need to setup a low-side current sensing circuit in order to take my measurements. What's low-side? This is where I attach a small series resistor with high power rating, breaking the negative terminal. I also have to connect the negative terminal to a grounding point for accurate results. Then I need two traces, one tracking the voltage across the 12V bench power supply and another across the resistor measuring the voltage drop across it.

Now I've used a 10ohm resistor, 5% tolerance and rated at 1/4W, this is not an ideal resistor to use at all. The value should be as low as possible, with a tolerance of 1% or even 0.1% for accurate results as well as a proper power rating. My circuit is operating at 12V, my current load is roughly 0.2 to 0.3A, which means, my power rating should be 4Watts. I'm way off the mark with a 1/4W resistor. As a result I'm going to lose accuracy due to temperature rise.

Now with a proper setup, I should be able to accurately measure any current spikes by recording voltage spikes and doing the necessary maths using good old Ohm's law (Voltage = Current x Resistance). Currently I'm not finding any specific spikes other than plain noise, so I guess my circuit is ok for now.