UM2243 Rev 2 23/41
UM2243 Static current measurement principle
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9 Static current measurement principle
The static measurement mode allows a stable current to be measured over a wide range
from 1 nA to 200 mA, with an accuracy of better than 2%.To use this mode properly, the
t
arget board current should not vary by more than 10% of its average value during
measurement acquisition.
It is also recommended to remove JP4 jumper (additional 2.2 µF capacitor needed for
d
ynamic measurement) especially for current consumption measurements below 50 nA.
Figure 16. Static current measurement principle
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The static current measurement principle is based on a shunt method: due to Ohm’s law, the
current flowing in the shunt resistor causes a voltage drop (Vi in static current measurement
principle) to occur. This voltage is amplified by a differential amplifier to deliver a voltage in
single-ended mode referenced to ground. The voltage is converted by one analog-to-digital
converter (ADC) of the embedded MCU, and is finally processed to deliver the current
measurement information.
To allow a wide dynamic range, the STM32L496 MCU’s firmware first performs an auto-
calib
ration, then switches-in the appropriate shunt resistor among five (ranging from few
hundred milliohms for high current measurement, to several kiloohms for the highest ohmic
shunt) in order to read the appropriate value on the ADC. This circuitry is designed to not
exceed 60 mV dropout on VOUT. In other words, if the output voltage (VDD) is set at 3.3 V,
VOUT d
oes not drop below 3.24 V during static measurements.
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