What is the internal impedance of the ADC in a ESP32?

[ullixesp]

I need to measure slowly changing voltages (so internal resistance is sufficient) from a voltage source having a high internal resistance, like some 10MegOhm. The source's voltage is a few hundred volt, which will be divided down to under 3V (the limit of the ADC, if I am not mistaken) by using a 1 GigaOhm resistor, followed by an appropriately sized resistor.

I don't find the input impedance of the ADC in the data sheets. Is it even known?

[ullixesp]

Nobody? Not even a little hint? Or direction where to search any further?

[ESP_Sprite]

Given that the ADCs are on standard GPIO pins, I imagine you can take the specs of those as a reference: 2pF input capacitance and 50nA leakage current.

[ullixesp]

Not sure what to do with these numbers. As my voltages change very slowly, I don't need the impedance and can ignore the 2pF.

When I naively(!) take the 50nA and the 3.3V, I get by Ohm's law 3.3/50E-9 = 66 MegOhm.

Hmmm. This would be very high. Realistic?

[ESP_Sprite]

You assume that the input leakage is resistive and linear when you calculate a resistance like that but generally, yeah, that makes sense.