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mcovington
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2019/08/12 08:12:38 (permalink)
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Measuring currents smaller than 1 uA

Hello, this is a bit off topic for this forum, but I couldn't come up with a better place to ask it...
 
What are some good ways to measure currents smaller than 1 uA?  The purpose would be to make sure that a micro has gone into a very-low-power sleep mode, and to distinguish various sleep-mode settings.  The test setup needs to pass higher currents (tens of mA) without appreciable voltage drop, to power the PIC before it goes to sleep.
 
My ordinary multimeter has a resolution of 0.1 uA on its lowest range, which probably means its accuracy is 0.2 uA.  And when measuring the current to a sleeping PIC, it often reads 0.0. 
 
I can think of various tactics such as a resistor with a switch across it -- when the PIC has gone to sleep, open the switch and measure the voltage across the resistor.  Or maybe a resistor in parallel with a diode so that with appreciably higher current, the voltage drop still won't exceed 0.6 V or 0.2 V (Schottky); then measure the voltage across the resistor.  But I haven't tried it.
 
Do I need to either spring for a high-end benchtop multimeter or build myself an instrument?  Other ideas?
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    mbrowning
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    Re: Measuring currents smaller than 1 uA 2019/08/12 08:40:33 (permalink)
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    mcovington
    I can think of various tactics such as a resistor with a switch across it -- when the PIC has gone to sleep, open the switch and measure the voltage across the resistor.  Or maybe a resistor in parallel with a diode

    I've done both of these things when in a similar situation (current sense resistor for low power mode must be too big for normal operation). The first one when a diode drop was not acceptable in the higher power mode (I used a 2pin header and jumper instead of a switch), second when it was.
     
    Why do you think these methods might not be good enough?
     
     
     

    Go Navy! Beat Army!
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    mcovington
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    Re: Measuring currents smaller than 1 uA 2019/08/12 09:36:36 (permalink)
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    mbrowning
    Why do you think these methods might not be good enough?

     
    Just wondering if there was something else I'd overlooked.  As fast as things change these days, for all I know maybe the Chinese are selling a nanoammeter for $5 and I'm the only one who hasn't heard about it. Smile
     
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    PStechPaul
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    Re: Measuring currents smaller than 1 uA 2019/08/12 09:42:02 (permalink)
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    A switched MOSFET across the sense resistor might be better. Otherwise, an instrumentation amplifier with gain of 1000 can read 10 uV on 10 ohms (1 uA) as 10 mV, with resolution to 0.01 uA (10 nA), and will drop only 100 mV with 10 mA current draw.
     
    Nanoammeter from China is $250 with shipping:
    https://www.aliexpress.co...2_7,searchweb201603_53
    post edited by PStechPaul - 2019/08/12 09:45:43

     
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    Number_two
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    Re: Measuring currents smaller than 1 uA 2019/09/04 13:46:46 (permalink)
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    I would trust that Chinese nanoammeter about as far as a could throw it...and I'm sure I could make it in the trash.
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