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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?

It would also be a bonus if the method would include an oscilloscope (CRO) which is also capable of capturing extremely short current spikes for when your device wakes up to do a measurement or send data over the radio.

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Well, if the current is DC a good bench top multimeter can measure it... – Vladimir Cravero 5 hours ago
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@VladimirCravero, but can it measure it without actually affecting the measurement. Heisenburg rocks. – Trevor 5 hours ago
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But you may be able to get a fairly good guestimate by powering the device from a capacitor instead of a battery and comparing the discharge plot with and without the device connected. – Trevor 5 hours ago
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A 125nA current can easily be measured with a 1M current sense R to create 125mV and thus the source voltage can be raised by the same amount. What's the problem? – Tony Stewart. EE since '75 5 hours ago
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The EEVBlog uCurrent GOLD is cheap, has a bandwidth of >300 kHz, and a resolution of 1 pA with a 5.5 digit meter when measuring nA. You could connect it to a multimeter for accurate DC measurements and an oscilloscope for transient measurements. – uint128_t 4 hours ago
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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 IA is a self-contained package for an instrumentation amplifier; often, these have an external gain resistor so you can choose whatever gain you want):

schematic

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).

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Heh, I commented above, and then noticed your uCurrent link. The uCurrent is definitely the easiest OTS solution to this, and +1 for explaining the DIY approach. – uint128_t 4 hours ago
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The problem will arise when the device will wake up, and the current will jump many orders of magnitude. To accommodate possibly enormous range, the shunt must be either dynamically variable, or have logarithmic resistance. One solution (using schottky diode as variable shunt) was discussed here few moths back, but I can't find the link. – Ali Chen 3 hours ago
    
Ok, here is the link: electronics.stackexchange.com/q/255646/117785 – Ali Chen 2 hours ago
    
That was a great discussion link you provided @AliChen, thanks. The uCurrent is a great product, but for my home dev I can do without accuracy, providing I can do reasonable comparisons down to about 1 uA. I work a lot with AVR's and simply put a 1N914 and parallel 100k resistor in the ground lead. I use my CRO to watch the current across the resistor. I manually set the power supply to 5 V initially and manually adjust it down to 3.3 V when it's asleep. I have no problem comparing deep sleep options this way. No great accuracy, but it works for comparing both processors and sleep options. – Jack Creasey 1 hour ago
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An off-the-shelf solution is a uCurrent from CMicrotek, worth the price. I easily measured 1uA currents. With a scope I can see when different functions of my application are running. You can connect it to a scope or a benchtop voltmeter.

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EEVBlog uCurrent GOLD is much cheaper. – Chupacabras 6 mins ago

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