Noise on input signal

[fanman]

Hi guys,

new to this so looking for advice. I have a PIC16F88 that has been configured to receive analog inputs on pin 12 using ADCIN command and then strobe a LED as an output on pin 8 using HIGH/LOW commands. I noticed that when the LED strobes, it induces the strobe signal on the analog input. I have played around with the circuit to ensure good ground planes exist etc but still same problem. Has anyone experienced this before and how can I eliminate it?

thanks

mark.

[aratti]

If you want real help, you have to post the circuit.

Cheers

Al.

[fanman]

Sure Al, please see attached image. Thanks Mark.

[aratti]

Did you set all the unused pins as outputs (using the tris register) and set them to low? If not please do it! Never leave floating inputs. If they cannot be set as outputs, then you must ground them.

Cheers

Al

[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