NRF1DK PCA 10028 and ADC Voltage Value

Question

Hello 
 I am trying to move an arduino project where I read an ADC value from a FSR sensor. In arduino the FSR is powered by 5V and has 10Kohm pulldown resistor. The board is currently connected using the USB cable and will remain that way 
 When I've looked at the ADC examples, SDK documentation and forums I have seen that 
 
 Max Input on ADC Pin should not be higher than 2.4V 
 You can configure different prescale and reference values 
 
 My questions are : 
 
 
 How can I check if I have damaged the ADC Pins, as I have already tried the FSR board in some pins? 
 
 
 I don't quite understand the prescaling and reference constants. My guess on the Reference is used to calculate the actual Analog value = RefValue * ValueRead / Resolution. But I don't have a clue on the prescaling purpose and whether is referenced to VDD of the Ref value selected 
 
 
 If I change the Vcc=5V and use VDD instead, what configurations of reference and prescaling shall I use for a quick test 
 
 
 Regards

alvaromat · Accepted Answer

I will close my own ticket. 
 Considerations: 
 
 
 Section 31 in infocenter.nordicsemi.com/.../nRF51_RM_v3.0.pdf (It took more than usual googling time to find the last version) 
 
 
 I used NRF_ADC_CONFIG_INPUT_2 and the physical pin was the first one in the PCA10028 board (PIN 1) 
 
 
 A) Check damaged PINs? Directly connect the PIN to GND and the value will be 0 or 1 in your ADC program 
 B) Prescale happens internally before entering the ADC conversor so if you have 3V signal and prescale of 1/3 then you get 1V at the ADC conversor (in Section 31 is explained very good) 
 C) You CAN'T Have a voltage higher than 2.4V after prescaling so if I use 3V as input source I need to prescale either with 1/3 or 2/3 ... 3V * 1/3 = 1V or 3V *2/3 = 2V 
 Another important thing is that the pin itself has a limitation on the voltage (VDD + 0.3V). So the max voltage supported is 3.6V and you will need a voltage divider if your voltage is higher.

NRF1DK PCA 10028 and ADC Voltage Value

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