avr comparator problem

[yas_iiee]

peace be to all,

i m working on interfacing a flame sensor with avr as a part of FYP. the sensor gives output around 1-2µA (current source). i m just passing this current thru a resistor and comparing it using 'avr analog comparator'. the comparator works fine when i connect a POT at AIN0 and smple VDR at AIN1. when i disconnect the vdr and connect AIN1 with sensor resistor ACO goes high no matter what the difference b/w the voltages be.

what might be the problem?

 

[bobcat1]

Hi

Probably your AIN1 low impedance overload your sensor - add a buffer amplifier

between the resistor and AIN1 input

All the best

Bobi


The microcontroller specialist

 

[millwood]

check the datasheet and it is likely the leakage current isn't that far off from 1ua.

plus, if the sensor is a current type, you should use a resistor to form a loop for it and measure the voltage across the resistor, not put the resistor in serial with the input pin.

 

[yas_iiee]

i m using a resistor of fairly high value 500K in the loop not in series with AIN1.
I couldn't find the input impedance of this pin in datasheet but if it is lower than the reisistor i m using it must be reducing the the drop(being in parallel), hence making ACO low according to my code but it remains high.