How can I measure low power energy usage of bluetooth device?
You can power your device with a micropower LDO, and put the current sense resistor before the LDO. Thus you can use a much larger current sense resistor. You can also bypass the resistor with a diode to have a better look at very low currents, but still allow the device to draw more current when needed. You can also bypass the resistor with a cap for averaging.

You can also use a capacitor as power source for the LDO. Using a LDO means you can let the voltage on the cap drop a lot more (like 5.5V down to 2V), than if you were powering the device directly (like 3V to 2V).

But you need a rather accurate electrolytic cap. Well, you can always calibrate it with a known resistor as a load...

You can also use an IV converter:







simulate this circuit &ndash; Schematic created using CircuitLab

The opamp replaces the LDO and will make sure your device is powered by a constant 3V, while giving a current reading on the 1k resistor. The cap keeps the opamp stable. The diode should be enough diodes in series to allow the opamp to supply the device directly, bypassing the resistor, if needed. It is optional.

If your device has onboard caps, you'll need to add a resistor in series because the opamp won't like the capacitive load.