It's just the analogue voltage jiggling a bit. If I feed in a constant voltage, eg. 1.0 volts fixed
, the ADC sees it jiggling up and down by +/- 0.1V.
On mine, I've got it interpreting 0V as 10:1 and 5V as 15:1... which means it's accurate to around 0.1 AFR - certainly good enough for me to tune to!
But accuracy would start to become a real issue if you wanted 9:1 <-> 18:1 range. That 0.1V jiggle would really start to hurt for a wider range.
Hope this helps!
PS. I'm not actually feeding in analogue to the rear-o2. Instead, I've patched into pin 8 of the OP2.0 itself. Seeing as our graphs look so alike, it's quite possible the jiggle is right back at the LC-1 analogue source, rather than an issue at the ADC (ECU, OP2.0). I'm sure we've both got our respective groundings and wiring good.