Analog voltage monitoring

AN4112

4/17

Doc ID 023195 Rev 1

1

Analog voltage monitoring

The STM32F05xx devices embed a 12-bit analog-to-digital converter (ADC) which is very

fast with a sampling rate of 1 Msample/s. However, with a 1.5 mA typical consumption, it can

jeopardize battery life time if left powered-on continuously. It is therefore recommended to

use analog comparators in application cases where the analog input voltage (sensor output)

needs to be measured as soon as a pre-defined threshold is exceeded.

In the STM32F05xx devices, analog comparators are useful for monitoring the analog input

voltage and powering-on the ADC when required. While monitoring the analog voltage, the

device can enter Stop mode at the same time that both comparators are still powered on.

Consequently, better consumption is achieved and power is saved.

Note:

used.

In an analog voltage monitoring application, where the sensor output voltage is lower than

the threshold, the MCU remains in Stop mode, thereby saving power. As soon as the sensor

output exceeds the threshold, the MCU is woken up, the ADC is powered on and the analog

input voltage is measured. When the sensor output is under the threshold, the MCU re-

enters Stop mode.

The average power consumption is dramatically reduced when compared with an

application that continuously measures the analog voltage whatever the input value.

Figure 1 shows how to connect a sensor output (temperature sensor, pressure sensor,

pyroelectric infrared detector, photodiode sensor) to an STM32F05xx device in an analog

voltage monitoring application using comparator 2 (COMP2). COMP2 monitors the analog

voltage in Stop mode while the ADC measures it in Run mode.

Figure 1.

Sensor output connection to COMP2

1.

ADC_CH1: AC channel 1

COMP2_INP: comparator 2 non-inverting input

2.

Only if required.