The datasheet for the ATtiny861a states that "If the ADC is used AVCC should be connected to VCC through a low-pass filter". The datasheet doesn't specify what values I should use or what style of low-pass filter I should use. The ATMega datasheets on the otherhand state that the low-pass filter should be a 10uH inductor connecting AVcc to Vcc with a 100nF capacitor connected to ground. Is this the same style of low-pass filter I should be using for my ATtiny or is there something better?
I think the ATmega advice is pretty good. With L1 = 10uH and C1 = 100nF, the 8MHz processor frequency is down over 60dB. I added a series resistor R2 = 10Ω to reduce the resonant peak of the LC pair. It may be not necessary, though.