The ADC circuit is a 300-800 ohm resistor in series with a 2 pF capacitor, plus ESD protection diodes. There is no loading, and the maximum leakage on the input pin is 50 nA. This applies to series 0 EFM32 parts: Gecko, Giant Gecko, Tiny Gecko, Leopard Gecko, Wonder Gecko, Zero Gecko, and Happy Gecko. The equivalent diagrams in single-ended and differential mode are shown below: