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Measuring small current, complicated
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Gavin Melville:
A circuit I’m designing runs on a smallish current, around 300nA.  Every 30 seconds the circuit draws 200mA for a few msec, and it needs to draw that without dropping any significant voltage, ie 1 ohm as a shunt would be about the limit the circuit can tolerate, .1 ohm would be better.  I don’t need much bandwidth, so uCurrent would be OK, but way short on gain.

Even with good bench meters and careful shielding, trying to measure .3 uV DC on a 4V DC rail is very hard work. 

Am I missing any obvious tricks?
BravoV:
Not an expert, assuming you have a decent bench PSU that has remote sense lines, how bout this trick ?
exe:
What's the end goal of measurement? As I see you already know idle consumption, and consumption when it is active. What else you need to know?

BTW, may be a source measurement unit (SMU) could help, esp. if you don't need much bandwidth.  It's essentially a power supply with built-in DMM. The trick is there is no drop on output as psu compensates for drop on the shunt. But they are expensive. Also, a few msec might not be enough to switch ranges.

I think it should be possible to compensate shunt drop with an opamp. My brain doesn't work right now, but one could try something like this: http://www.vk2zay.net/article/251 (of course resistor values to be adjusted for your project). So, basically, opamp in inverting configuration keeps it's input at zero and compensates for any drop on shunt. I'm not sure about performance.

There are also app notes and videos from vendors how to do such measurements. It's sort of active topic in IoT and battery-powered devices.
Gavin Melville:

--- Quote ---What's the end goal of measurement? As I see you already know idle consumption, and consumption when it is active. What else you need to know?
--- End quote ---

I'm beating the idle current down nA by nA.


--- Quote ---a source measurement unit (SMU) could help
--- End quote ---

I've got a Keithley 2450. Surprisingly -- it's not much help.  It switches ranges not quite fast enough and browns out.

The only thing I can think of at the moment is to use a high value shunt (say 200 ohms), and short the shunt when the high current pulse comes along.
mzzj:

--- Quote from: Gavin Melville on November 08, 2018, 08:32:36 am ---A circuit I’m designing runs on a smallish current, around 300nA.  Every 30 seconds the circuit draws 200mA for a few msec, and it needs to draw that without dropping any significant voltage, ie 1 ohm as a shunt would be about the limit the circuit can tolerate, .1 ohm would be better.  I don’t need much bandwidth, so uCurrent would be OK, but way short on gain.

Even with good bench meters and careful shielding, trying to measure .3 uV DC on a 4V DC rail is very hard work. 

Am I missing any obvious tricks?

--- End quote ---
Are you actually interested in average current consumption or do you NEED to know if the 200mA pulse is 198mA?
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