How to increase the accuracy ?

[ahmedragia21]

Hi, I'm using a 10k POT to change the position of the servo in degrees , im using an ATmega8 , codes are in C , im using a 5V source for the pot , and 5V for AVCC .
I get for 50mV about a 100 count change in the adc , i wanna get a 0.5Degree resoultion , what to increase ?? increase the resoultion of the ADC ? or the POT value ?

 

[smanches]

Post your schematic. What needs to be changed is dependent on how your voltage divider is setup.

 

[millwood]

.5 degree out of 360 degree (assuming your pot is single turn) = 1/720. so you need a 10bit or higher adc to do it.

 

[ahmedragia21]

smanche, its a simple voltage divide , 5V---POT ----> GND

millwood, its a servo motor , it rotates from -90 to 90 ;
so only changing the ADC resoultion not the Pot's value ?

 

[Nigel Goodwin]

The pots value has nothing to do with it, as long as it's low enough in value to correct charge the sample and hold.

Does a servo even give 0.5 degree resolution?, how wide is the dead band on one?, and what is the backlash like?.

 

[ahmedragia21]

Your right nigel , but why a 50mV , puts a 100 increase count in the digital value ?
would you tell me the calculations of how the required accuracy affects the resolution of the ADC

 

[Nigel Goodwin]

Your right nigel , but why a 50mV , puts a 100 increase count in the digital value ?
would you tell me the calculations of how the required accuracy affects the resolution of the ADC

It's dead simple - divide 1 by the resolution of the A2D and multiply by 100.

For 10 bits this gives roughly 0.1% resolution, and for only 8 bits it gives 0.4% resolution.

For 180 degrees travel, and 0.5 degree resolution you need 0.3% resolution, so 8 bit isn't good enough - but 10 bit should be fine.

 

[smanches]

The pots value has nothing to do with it, as long as it's low enough in value to correct charge the sample and hold.

Misunderstood the question. Thought he was saying he was only getting a 50mV rise from the POT.