Understanding low current measurement

Question

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?

Answer 1

It's difficult to cover the dynamic range required to measure both peak current and sleep current at the same time.

When trying to verify very low sleep current requirements, I use the following circuit, with component values suited to the particular load.

^{simulate this circuit – Schematic created using CircuitLab}

R1 is a reasonable value to develop a measurable drop at the expected uA levels. C1 is large enough to source all of the high current pulse demanded periodically by the load with negligible drop during the pulse, say <100mV, verify this with your scope.

Of course a large electrolytic for C1 will have a large leakage current, and this current may vary with time, especially if your C1 has just come out of your junk box after sever years uncharged, but this can be measured, tracked, allowed for with a bit of care. Letting the capacitor sit at its maximum voltage for a while before using it will reform the insulating oxide, and should mitigate most of the high leakage and its variation before using.

Replace C1 and R1 with much smaller values to characterise your peak load current.