Potentiometer interfacing with MCU

[gagakseto]

Dear All,

I am new here. This is my first thread.

I have a basic question regarding potentiometer interfacing with MCU. I found in some schematics that interfacing potentiometer to ADC input of MCU needs series resistors (of 100 Ohm) at the upper-end (to 3V) and lower-end (to GND) of potentiometer (5 KOhm). If we use 1 KOhm potentiometer then the note said we don't need the series resistor and the potentiometer could reach the maximum value. What is the meaning of this? why do we need series resistors for the potentiometer?

Thanks in advance for the enlightenment.

Regards

 

[sam185]

Hi,
If you are connecting a res. in series with pot there will be more drop across the series configuration hence voltage will not reach its maximum value.

 

[gagakseto]

Hi Sam,

Thanks for the reply. My curiosity is why the designer put 100 Ohm series resistor if using 5KOhm pot and omit it for 1K pot. Is it a mere max value limitation or other consideration?

 

[alexan_e]

There is no need for a series resistor in a A/D input, you only have to use voltages within limits.
I assume that the series resistor was there to protect the chip in case of a mistake in the pin configuration/direction.
If the pin is configured as an output and the pot is in either end (gnd or Vcc ) and oposite from the current state of the pin then you will have a short-circuit which will probably damage the output of the pin, using the series resistor will prevent that but depending on the source of the voltage there will be a (small) error in the measurement.

Alex

 

[gagakseto]

Hi Alex,
Thanks a lot. I get your points.