ac decoupling from pic 18f ADC

[ahmed osama]

dear all
i have a pic 18f4550 , i am using its ADC + max232 and reading the data using rs232 in my pc
i am using 9volt battery then 7805 Reg as Vdd supply

my problem when the adc i/p is floating , i get an AC signal 50Hz about 2.5 Vp and when i touch the input i got a complete square signal 5Vp with 50Hz

i was going to use such system as a part of EGC project , when i touch the gnd by left hand and ADC i/p by my right hand i also got a complete square signal 5Vp with 50Hz ( i know egc needs extra circuits for filtering & amp.)

from where such AC signal came , is it AC signal coupled from my pc through Rs232 !!!

how could i solve that

thanks all

bye

 

[Tahmid]

A floating input pin can pick up stray noise from "anywhere". That's why you are always advised never to leave a pin "floating". If that pin won't be used for anything, make it an output pin. If it's going to be an input pin, make sure it's never floating. Maybe if the rest of the circuit allows, you can use a pull-down resistor to ensure that the input is "low" and not "floating".

Hope this helps.
Tahmid.

 

[ahmed osama]

A floating input pin can pick up stray noise from "anywhere". That's why you are always advised never to leave a pin "floating". If that pin won't be used for anything, make it an output pin. If it's going to be an input pin, make sure it's never floating. Maybe if the rest of the circuit allows, you can use a pull-down resistor to ensure that the input is "low" and not "floating".

Hope this helps.
Tahmid.

when i touch the gnd by left hand and ADC i/p by my right hand i also got a complete square signal 5Vp with 50Hz