Questions about shunt resistor choosing


Hi Colin,
I am trying to do power analysis on my target, which works on 3.3v. Without resistor, the average current while running is about 140mA.

I used a 10Ohm resistor as a shunt resistor with 5V power supply. I connected the shunt resistor between 5V
and target VCC pin, and measure the voltage change between VCC PIN and GND. (decoupling caps have been removed). However oscilloscope did not give a reasonable power consumption line.

Is there any thing wrong in my setup? Any guidance on shunt resistor choosing?

Thank you very much for any help!


Colin has much more experience here than me, but my first though is why use a 5V supply? I understand that with a 10 ohm shunt the target will get ~3.6V on average, but it will get a much higher voltage when its current draw is lower.

Why not stay with 3.3V and use a much smaller shunt resistor?


Thanks for the advice. I tried a smaller shunt resistor (1 Ohm). and then I could stay at 3.3v to drive my target.
However I still could not get a reasonable power trace from this setup, even I remove most of the decoupling cap of power source. Could you help me more on this?


Do you have a good trigger? It’s hard to say more based on what you’re reporting. What do you mean by “reasonable power trace”?


I haven’t got a trigger right now. What I am doing is starting the scope, connecting power to my target and watching power traces.
by “reasonable power trace”, I mean with the shunt resister, the power traces did not show a big difference against to measuring the power source only, It just looks like random noise, without any clues of power consumption.
I also connected my target to chipwhisperer, it showed almost the ripple wave of power source, no obvious consumption peak found.


Hi, of course there’s more than one way to do this but it can be harder to know what you’re looking at, or find what you’re looking for, without a trigger. What’s your target? Noise will always be present to some degree.
If you have a trigger and can capture the same identical operation many times, then you can average those traces to average out the noise and better see what you’re interested in.