current limitations of adc analog input module of p16f877

[ronrayzap]

i just wana ask if there are any limitations in electric current being inputted in the adc module of the pic16f877?

i am presently working on a project using the adc module and i have configured it as a 4 7-segment digit voltmeter in the mean time to test it. when i input 5v from the pic supply itself using a potentiometer to vary the analog input, the adc reacts well. the problem is when i input voltage from a dc voltage supply through a voltage divider, my circuit doesnt read the input and just displays the digit "8" in the least significant spot.

im just wondering why this happens and if the current in the voltage source has an effect because the current of the dc supply is significantly higher.

has anyone else experienced this? thanx for all the help in advance.

 

[philba]

pay attention to the impedence of the input - this is described in the datasheet. IIRC, it must be below 10K.

also, make sure you have a common ground.

 

[vsgiri]

When you have an external supply, as Philba suggested, make a common ground. Then protect the input of ADC using a series resistance (1k) and a zener(equivalent to supply voltage) to gnd. Also put a .1uF capacitor to gnd to check stray noise.
If your voltage supply can supply high current that is good enough. You are actually choking it with voltage divider and series protection resistor. Is the ADC result register giving random values or is it stuck at some value?

 

[ronrayzap]

thanx for the replies, im gonna try those and update again..

vsgiri - i havent checked the adc result register itself but the output on the 7-segment digits only displays an 8 in the least significant digit.