Ibias Current Measurement Procedure; Figure 11. Low I Range Measurement Timing Diagram - STMicroelectronics UM1079 User Manual

Hardware and layout
The Q13 output of the counter allows connecting the 1 KW resistor when the current I
becomes very low.
Figure 11
IDD_CNT_EN falling edge, the shunt resistor R22 is connected between VDD_MCU and
the power supply to reduce the measurement range to 60 μA for the full scale. Then after
another 150 ms required for current stabilization, R22 is shorted, the I
stored in C13, and the MCU is woken up. After wakeup, the MCU measures the I
corresponding to the Low-power mode stored in C13.
4.7.3 I current measurement procedure
BIAS
In low I
DD
negligible compared to I
measurement, it is mandatory to subtract the bias current from the low I
since this current is not sunk by the MCU. I
stored in the MCU data EEPROM. The discovery demonstration software uses this value to
display the correct I
The procedure for I
1. Power off the board (disconnect the USB cable).
2. Set JP1 OFF.
3. Push down B1 (USER button), power on the board from the USB.
4. Wait at least 1 second before releasing B1. The LCD displays the I
5. Power off the board (disconnect the USB cable).
6. Set JP1 ON. The I
subtracted from the I
20/38
shows how the counter and the FET transistor 1 of U20 ensure that, 150 ms after
Figure 11. Low I range measurement timing diagram
DD
range mode, the bias current of the operational amplifier input (U5 pin 4) is not
current (typical I
DD
.
DD
measurement implemented in the demonstration software is:
BIAS
value is stored in data EEPROM. The bias current is then
BIAS
measured in I
DD
DocID018789 Rev 4
is ~240 nA). To obtain a reliable I
BIAS
is measured during production test and
BIAS
range mode.
DD
UM1079
DD
measurement is
DD
current
DD
DD
current value
DD
measurement.
BIAS