Noise on input signal

[fanman]

Hi Al,

Went back and grounded the unused pins. Still get the pulse on the analog signal. Anything else I missed?

cheers

Mark.

[aratti]

Is C11 (decoupling) connected directly to pin 14 ?

Add a large capacitor to the output of your regulator ( say 220 uF)

X5-3 point goes to a pic pin. Is it an input? If yes and you have nothing connected to the terminal, then you have to ground or pullup also X5-3

Cheers

Al

[Gusse]

X5-3 point goes to a pic pin. Is it an input? If yes and you have nothing connected to the terminal, then you have to ground or pullup also X5-3

Another vote for pull-up or pull-down (some high but reasonable value would be OK). Floating input will see your LED transient (voltage/current change) and interpret that as an input signal (=working as a sensitive receiver).

-Gusse-

[MikeWinston]

If X5-2 is power to your sensor, then maybe try a capacitor there.
But first I would try what aratti said and use bigger caps by the regulator.

Mike -

[fanman]

Thanks guys. I will try your suggestions. I will double check the TRIS registers to ensure they have the correct setting. By the way C11 is connected directly to Pin 14 and the input signal is from a low impedance source.

[Jerson]

You need to check your grounding paths. If this circuit is built on a breadboard, it may not be easy to resolve this. Ground loops are a concern in most ADC problems. Try to get your ground routed like this
Input -> 7805 -> Output transistor -> Microcontroller.

Any changes in this sequence can contribute to the problem.

Another possible problem is the rate at which you read the ADC. Do you filter the readings you get ??

[fanman]

Hi Jerson, yep have tried ground planes on a PCB. Slight improvement but same issue. Good point regarding sampling rate. Its 50us as defined in the code. The source has inbuilt filtering. I don't use any averaging of the sampled data.

Someone online mentioned that the PWM setting can affect the ADC?

thanx