# How to calculate ADC voltage divider maximum input voltage?

#### bobdxcool

Mar 9, 2012
98
I'm trying to calculate maximum input voltage that can be applied to a voltage divider based ADC circuit attached in the picture below along with converting the ADC raw value to actual voltage.

In the above attached image, P8 is a terminal screw block connector where the external analog voltage is applied. And as you see, the resistors form the voltage divider circuit. AN0-AN7 go to the ADC pins of a microcontroller. This is a 5 volt system and so I believe the reference voltage should be taken as 5 volts. I am trying to figure out what would be the maximum voltage that I can apply to the pins on P8 connector and also how to convert the raw ADC values to actual analog voltage that was applied. So, based on the circuit, 5 = (V x 10k)/30k -> V = 15 volts. [Here 5 volts was taken as the reference voltage since it is a 5 volt based system.] So the maximum voltage that can be applied is 15 volts. The resolution of the microcontroller is 12 bits. So, to calculate the actual analog voltage from raw ADC values, Actual Voltage = (Raw ADC value)*15/4095. However with this calculation I'm not getting the right voltage that was applied. Can anyone guide me regarding where I am going wrong in this whole calculation ?

#### Arouse1973

Dec 18, 2013
5,178
Hello
(15 V / 30 K)*20 K = 10 V not 5 V. Could that be your issue? Try 7.5 V for your max input voltage. Just checking it is 10K and 20K isn't it?
Thanks

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