Is it possible a biggest voltage in the ADC input than the voltage supply ?

[Juan Moya]

Hello,

I want to supply the nRF52832 with a power supply of  2.2Vdc. The uC has to measure an analog voltage between 0 to 2.5V using the internal reference.

Is it possible a biggest voltage in the analog input than the voltage supply of the uC?

Best regards,

Juan

[ketiljo]

Nope, that's not possible. In any case, the voltage on the AIN pin must be lower than the reference voltage to the ADC.

[AmbystomaLabs]

I think ketilijo worded their response a little wrong. I believe they meant to refer to nRF_Vdd and not Vref.

You can measure voltages at or less than nRF_Vdd and indeed this can be and normally is greater than Vref.  The prescaler on the SAADC is what allows you to do that. The purpose of using the internal reference is to make sure the SAADC measurements don't vary with nRF_Vdd. This is especially a problem with a battery operated device.

But, you cannot apply a voltage greater than nRF_Vdd + 0.3V to any pin.  You might then think, "hey I'm right there at 2.2+0.3 = 2.5." While true that would not be a good idea.  Once you get above nRF_Vdd your source at Ain will start providing power to the nRF via the gpio/esd structure.  So, the reality is any voltage above nRF_Vdd you try to measure will be wrong since the load isn't constant.

You should put a voltage divider on your analog signal to make sure it is below nRF_Vdd (ie, 2.2V). It can easily be in the 500kohm range or greater to minimize power consumption.  Just pay attention to the time constant of the SAADC input and your Analog thevenin equivalent source resistance to make sure the SAADC input cap fully charges during the sample period.

[zuhaib]

(You can measure voltages at or less than nRF_Vdd and indeed this can be and normally is greater than Vref.  The prescaler on the SAADC is what allows you to do that. The purpose of using the internal reference is to make sure the SAADC measurements don't vary with nRF_Vdd. This is especially a problem with a battery operated device.)

I want to measue a NTC resistance how will i connect the pins using single ended and differential mode?

and is it true IF  I use internal  reference voltage it will not change with the change in vdd_nrf(i.e battery voltage ).

how will i use internal reference voltage?