There's an old philips filter routine from an439
Basically a low pass filter and ideal for a 12 bit ADC's..


idata U16 filt1,filt2,filt3,filt4,filt5,filt6,filt7,filt8;


U16 filter0(U16 filt0){
U16 ftemp1,ftemp2,ftemp3,ftemp4,result1,result2;

ftemp1=((filt0/2)+(filt1/2));
ftemp2=((filt2/2)+(filt3/2));
ftemp3=((filt4/2)+(filt5/2));
ftemp4=((filt6/2)+(filt7/2));

result1=((ftemp1/2)+(ftemp2/2));
result2=((ftemp3/2)+(ftemp4/2));
filt7=filt6;
filt6=filt5;
filt5=filt4;
filt4=filt3;
filt3=filt2;
filt2=filt1;
filt1=filt0;

return((result1/2)+(result2/2));
}



called as ..
gas_sensor1.value = filter0(read_adc_byte(channel7));