| Electronics > Projects, Designs, and Technical Stuff |
| Measuring small current, complicated |
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| David Hess:
--- Quote from: Gavin Melville on November 08, 2018, 08:32:36 am ---Am I missing any obvious tricks? --- End quote --- You are missing a not so obvious one. Use a cascode MOSFET for the positive supply; in this case the transistor is essentially the output transistor of a linear regulator but given your low current requirements, special attention will need to be paid to keep the feedback current low. Now the output current (and the regulator's feedback network current) appears on the drain side of the MOSFET and a high value resistor can be used to convert this current into a voltage referenced to the positive supply or a current mirror can be used to reference this current to ground for ease of measurement without affecting the output voltage. When high current is applied, a diode clamp prevents the drain voltage from falling below the dropout voltage of the regulator. If a suitable clamp diode with a low enough leakage cannot be found although I can think of several, then two diodes in series with a guard voltage applied between them can cancel the leakage. At this point junction capacitances at 300 nanoamps become a significant but tractable issue but probably not a problem over 30 seconds. Some attention will be required to get good settling times. In the past this might have been done with a JFET driving a bipolar transistor which might still be the best way to go. |
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