internal resistance ADC

[Jad]

Good morning. I am working with the Discovery kit with STM32H7B3LI MCU and want to get its internal resistance. I want to use 10 buttons, each with one resistance, and then get the correct value for my MCU. I already know how to get the values between 12000 and 64000. but I do not get what Voltage they describe. I saw on the datasheet of the Chip that it has a linear relation with the voltage and receives a voltage between 1.62 and 3.6V. Still, I have no accurate Information on the internal resistance to calculate my voltage divider. I can do it with the information I have, but I feel like I am missing something. When I put a 120-ohm resistance or shortcut directly to the ADC with 3.3V, it also gives me the maximum value.

[TDK]

The ADC doesn't have a resistor to ground. You can't use it as one half of a resistor divider. It has some effective series resistances and capacitances.

 

See here for the effective circuit:

 

You can do what you're doing, but you will need to use external resistors and have the voltage of the net change based upon what button is being pressed.

[Jad]

ok so i will add a pull down resistance then. Does the value then will go until 3,6V? because i still do not understand why i get the max value when I send 3,3V

[TDK]

The convertible input range for the ADC is (0, VREF+) which is typically (0, VDD). It sounds like your board has VREF+ = 3.3V.