Noise on input signal

[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

[Jerson]

No, the 50uS is the sampling time. If you happen to read your ADC at varying intervals(due to different paths of code execution), that affects the reading stability. Try to put the ADC read inside an ISR so that it reads at a constant rate. Then perhaps the coupling will break and get you stable readings.

[fanman]

Hmmm...have tried the suggestions and still same problem. The interference is definitely induced by the strobe pulse. When the strobe is not triggered the signal is fine. Very strange.

[Jerson]

can you post your code? Might help identify any issues in code if present.