This might be old news in a half a decade or two but by today's means, I am referring to electronic prototypes and designs which would draw in a μA (uA) and even nA range of current.
Some recent MCUs, such as SAMD21 that I am using atm are armed with internal clocks such as ,always on, Ultra Low Power Internal 32kHz RC Oscillators which would draw only 125nA, and the whole microcontroller is capable of consuming only 6.2μA on STANDBY mode with a live RTC.
In these type of quiescent current and power consumption levels the smallest limitations in the internal machinery of bench measurement devices such as multimeters and oscilloscopes could add a fair bit of error to the overall measurement or even measure a flat out wrong value in situations like a different relay kicking in when changing the resolution from 6 to 8 decimal places accuracy on your multimeter.
What is the most precise method of measuring the overall quiescent current/power consumption for such applications?
Update:
As I mentioned in one of the replies, measuring low currents is hard but very possible, however, making conclusions on the integrated amount of current consumption to come up with numbers for the realistic over all power consumption is more what I had in mind.
I have bumped into some solutions such as wide range current to frequency converter, however the wide range in this application note is only limited to the max of 200uA and in my case, my max current can rise to milliamps when my radio is transmitting and could drop to as low as 3uA when the whole system goes to sleep.
Best Answer
One solution is to use an instrumentation amplifier to measure the voltage drop across a shunt resistor. These are designed to offer an extremely high input impedance to both inputs of the amplifier (in excess of 1 giga-ohm), while allowing you to amplify this signal by relatively large factors (1000x is not uncommon). Note that the fact that there is a really high input impedance isn't too terribly important for this particular application, however the high amplification factor is.
The basic schematic looks like this (I'm using
IAis a self-contained package for an instrumentation amplifier; often, these have an external gain resistor so you can choose whatever gain you want):simulate this circuit – Schematic created using CircuitLab
The large amplification factor allows you to use a relatively small sense resistor, mitigating a large portion of the effect of the burden voltage on your DUT.
If you're just looking to buy an off-the-shelf solution which does effectively this, you could look into something like the uCurrent. There are probably also specific IC's designed for this current range.
Since the outputs of these type of current sensors is just a relatively isolated analog voltage, you can use any standard oscilloscope or voltage meter to measure the current.
These very simple devices are good enough for things in the nano and micro ampere ranges and are relatively easy to use.
For even smaller currents (pico or fempto ampere ranges), there are specially designed chips such as the LMP7721, along with a few pages of application notes on low current design. It's unlikely you'll want something like this for measuring power current draw. These are typically used by the scientific community for measuring sensor outputs (photodiodes/other very low current sensors).