Voltage on AN0 of PIC16F676

[garg29]

lm324 + pic16f676

Hi,
I request you to please see the attachment. I'm trying to measure a voltage on AN0 of PIC16F676 but voltage before 10K resistor is 5.00volts (Which is proper) but after the resistor it becomes 4.84 volts, can't understand the problem.

I tried reducing 10 k resistor to 100 Ohms output improved but only to 4.90 volts.

can u please suggest me the proper way.

With best regards,
Garg

 

[blueroomelectronics]

You want a voltage divider not a clamp like you have.

Try (VIN) 6.8K -AN0- 2.2K - GND.

 

[garg29]

Thanks for replying "blueroomelectronics", but i think you are not 100% clear with my problem. I don't want to shift down the voltage range. The opamp is giving me 0-5 volt which is full range, but the 10k resistor is creating the proble by shifting down the range from 0-4.84 volts.

 

[yus]

have you tried removing first the zener diode?
perhaps the zener also introduces an impedance to the ground,creating a voltage divider network. (the PIC, I belive, also has an input impedance).
*btw, the 10kohm resistor is already big enough as a source impedance.

 

[Kripton2035]

well, change your program on pic to say that full scale is 4.84v instead of 5v
or
change the lm324 gain so that there are more than 5v at his output, and so 5v on an0 input ?

did you try to remove the 10k resistor ?
I dont think the zener is usefull as there are already clamping diodes in the pic

hope this helps.

 

[garg29]

Thanks everyone for replying. I found the problem, my zener was faulty.

 

[justusfrantz]

i had the same problem as yours. the zener somehow caused a drop in the voltage. by removing that i get full range of voltage. i dun quite understand why though. i even change the zeners but same problem arises.