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Could anyone please let me know which one is better for low current measurement? I'm going to measure power consumption of MCU and camera module, But it seems I can't use shunt resistor, because current will be less than 300mA and shunt resistor has large error for such low current measurement. However, I can't decide a proper tool to make it. If I could use the feedback ammeter, then I'd like to go for it to avoid bying the expensive current probe.

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  • \$\begingroup\$ is this for in-circuit measurement? or testing your device? \$\endgroup\$
    – hassan789
    Commented Aug 23, 2015 at 3:12
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    \$\begingroup\$ Please provide justification for "shunt resistor has large error for such low current measurement". You certainly ought to be able to get 1% accuracy. \$\endgroup\$ Commented Aug 23, 2015 at 4:34

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I assume the load takes large current pulses but with a low average current.

If you just need the average current you can use a normal ammeter (i.e. measure a voltage across a shunt) but add a large capacitor across the shunt (or across the load).

The capacitor will supply the current during the pulses and will let you use a shunt appropriate for the average current, not the peak. The capacitor may need to be many hundreds or thousands of microfarads depending upon the particular scenario.

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  • \$\begingroup\$ I need to measure detailed profile including rise and fall shape. \$\endgroup\$
    – SD11
    Commented Aug 23, 2015 at 3:49
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    \$\begingroup\$ I've done that by putting a small shunt (e.g 0.01ohm) then using a low offset opamp (10uV) configured as a differential amplifier to multiply it by 100 or so to display on an oscilloscope. The integral feature on many new DSO (I use an Agilent) then allows you to integrate the current to measure not just how much current is required at different phases of operation but also how much charge would be taken from the battery (this was for circuitry in a cell-phone). \$\endgroup\$ Commented Aug 23, 2015 at 4:01
  • \$\begingroup\$ Thank you for the answer. I think I should try it. But I wonder if the method guarentee good accuracy. \$\endgroup\$
    – SD11
    Commented Aug 23, 2015 at 4:18
  • \$\begingroup\$ Agreed it has its limitations but is probably at least as good as a current probe. Probably not as good as a feedback ammeter could be,although I've never used one. \$\endgroup\$ Commented Aug 23, 2015 at 4:24
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I would not agree that a sense resistor would have a large error.

A wide range of 1% or better current sense resistors is available from multiple vendors. Here is a typical series of parts.

My approach (for simplicity) would be to use one of these with a good instrumentation amplifier or perhaps a precision amplifier.

You have not stated what frequencies you are trying to capture which would influence my amplifier choice.

There are amplifiers specifically designed for current sensing, and even some with an integrated ADC.

If 1% to 2% accuracy is sufficient, then this is how I would probably measure current.

These types of devices are widely available from Maxim Integrated, Analog devices, Linear Technology and Texas Instruments to name but a few.

HTH

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