lpcware

IRRELEVANT ADC VALUES

Discussion created by lpcware Employee on Jun 15, 2016
Content originally posted in LPCWare by KalavathiPR on Thu Sep 24 02:19:13 MST 2015
Hi,
In presence of external pull up, I was getting proper ADC values corresponding to the voltage provided to the ADC pin of LPC1769.
When no input is provided, ADC value was 4095.

Now, I have removed external pull up because I don't want to use it.
I am getting proper ADC values corresponding to the voltage provided to the ADC pin.
But, when I remove the connection to the i/p pin or when no input is provided, ADC value is neither 4095 nor 0. It reads some other ADC values.

I have changed pinmode register for ADC to  pull-up resistor, repeater mode, neither pull-up nor pull-down and also pull-down resistor enabled. But did not work.

As per UM10360 LPC176x/5x User manual page 111's [2], due to internal pull up- Voltage on the ADC input pin measures around 2.3V. 
After power on reset, the voltage on the same pin is around 0V.
1) After power on reset, why there is voltage drop? Is this the reason for getting irrelevant ADC values?
2) What is the solution? Why are the ADC pins in floating state even in presence of internal pull ups?
3) Where should I modify the code to get get proper ADC values?
4) Will the ADC work without external pull ups?

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