I'm trying to understand a current measurement of a hobby project.
This project consists of several ICs and is battery operated.
At a certain mode (deep sleep) it should consume a very low current, which I estimated around 100μA or less.
To verify low current consumption I'm using the following setup for the measurement:
simulate this circuit – Schematic created using CircuitLab
Since my shunt resistor R1 is 0.5 Ohm, I would expect the current to be V / 0.5. (Actually I'm using 3 resistors of 1.5 Ohm in parallel, which I consider as a single 0.5 Ohm resistor).
The load R2 is the digital circuit with multiple components (ESP32, BQ21040DBVR, CP2104, M3406, XC61CC3302NR-G, resistors, capacitors, etc) at a deep sleep mode.
And this is the reading I'm getting:
This measurement is consistent. When removing power it goes flat so it originates from my circuit.
Questions
- Is the setup I'm using good enough for measuring sub-milliampere currents?
- Why Vmin is less than 0? The scope is calibrated and when removing power it goes to 0.
- Is it correct to use Vrms as the "average DC measured voltage"? It so, it means that the average current draw is 0.00056 / 0.5 = 1.12mA
- Regarding those 7.28Vpp spikes every 500μsec:
- I would like to find their source.
The circuit is a printed PCB with SMT parts so I prefer not to rework it, if possible. What would be a good way to debug it and find the part that creates these spikes? - Since these spikes are very short and at a (more or less) steady frequency, my guess is that they come from one of the ICs on the board, and not from analog discrete parts. But looking at their datasheets, I couldn't find any reason why any of them would consume ~14mA every 500μsec when the system is in a deep sleep mode.
Any idea?
- I would like to find their source.