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What is the internal impedance of the ADC in a ESP32?
Posted: Sat Jan 14, 2023 10:08 am
by 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?
Re: What is the internal impedance of the ADC in a ESP32?
Posted: Tue Jan 17, 2023 7:50 am
by ullixesp
Nobody? Not even a little hint? Or direction where to search any further?
Re: What is the internal impedance of the ADC in a ESP32?
Posted: Wed Jan 18, 2023 3:22 am
by 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.
Re: What is the internal impedance of the ADC in a ESP32?
Posted: Thu Jan 19, 2023 9:39 am
by 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?
Re: What is the internal impedance of the ADC in a ESP32?
Posted: Thu Jan 19, 2023 10:27 am
by 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.