I made a remote car battery voltage measurement with an ATtiny202 and BRKWS01 that transmits the measured values to the Sigfox network, which no longer works in my country in Europe.

Now I would like to do it with ATSAMR34 LoRaWAN Transceiver (Evoluation Kit) but first I need to solve the ADC measurement on SAMD21-XPRO Evaluation Kit.

Assignment:

1. Voltage is measured once per hour after the MCU wakes up from sleep mode
2. The maximum car battery voltage when charging with an alternator can be 15V
3. ATSAMD21/R34 can have on the ADC input pin 3.3V only
4. I want to reduce the voltage of 15 V to 3.3 V by resistive divider 12k and 3k3 (i.e. 1 mA), which will be switched by MOSFET for ADC measurement to avoid discharging the battery (measurement 1x/hr).
   
   For a smaller input voltage, I wanted to use a Texas Instruments TPS22916BYFPR power switch.
   
   SAMD21 ADC input has Rsample 3k5 and Csample 3.3 pF see pdf page 1017 and 781 for ADC in TDS. I am also considering to get some advice from "Getting the most out of the SAM D21's ADC".
   
   I assume to use the 48 MHz MCU clock and set the prescaler for ADC to 32 i.e. ADC clock will be 1.5 MHz.

What is your suggestion for a Resistor Voltage Divider (RVD), its power switch and ADC setup to measure the voltage once every hour?

 

I want to reduce the voltage of 15 V to 3.3 V by resistive divider 12k and 3k3 (i.e. 1 mA), which will be switched by MOSFET for ADC measurement to avoid discharging the battery (measurement 1x/hr).

The 12V system of cars have wide swings in voltage because of load dumps and cranking. Therefore you should design in some protection of your MCU.

For a smaller input voltage, I wanted to use a Texas Instruments TPS22916BYFPR power switch.

The TPS22916 has an operating voltage of only 5.5. Where exactly do you want to use it?

 

The 12V system of cars have wide swings in voltage because of load dumps and cranking. Therefore you should design in some protection of your MCU.

The TPS22916 has an operating voltage of only 5.5. Where exactly do you want to use it?

I know that when charging the battery with an alternator the voltage can go up to about 14.4V. When I ran it over the Sigfox network for over 3 years, I never had problems with voltages higher than about 14.8V.

I wrote I wanted to use it for smaller input voltage e.g. for three Energizer 675 Zinc Air (Zn/O2) 1.4V cells.

 

Hi Kevil,
You may find this PDF helpful.
E

 

I found an interesting article "Using a Voltage Divider on an ADC Input" which should help me to calculate the optimal values of resistor voltage divider.

 

I wrote I wanted to use it for smaller input voltage e.g. for three Energizer 675 Zinc Air (Zn/O2) 1.4V cells.

Sorry I missed that. Should be fine for the battery. But watch out the IC is very small (0,78x0,78mm)