I found a discrepancy in the ATtiny datasheet. While the section explaining the ADC shows a simplified schematic with a 1...100 kOhm series resistor, the characteristics table shows 100 MOhm.
Figure 17-8. Analog Input Circuitry
The ADC is optimized for analog signals with an output impedance of approximately 10 kΩ or less. [...]
With that max. 100 kOhm series resistance in the sample/hold path it makes sense to me, that they say that the source impedance should be about 10 kOhm. But with the 100 MOhm impedance from the characteristics table I'd say requiring 10 kOhm source impedance is unnecessarily high.
Table 21-8. ADC Characteristics, Single Ended Channels. T A = -40°C to +85°C
Table 21-9. ADC Characteristics, Differential Channels (Unipolar Mode). T A = -40°C to +85°C
Table 21-10. ADC Characteristics, Differential Channels (Bipolar Mode). T A = -40°C to +85°C
What is the reason for that discrepancy? Or can I assume the 100 MOhm is a wrong value and should actually be 100 kOhm typical?