Hi, sorry if this is a dumb question but does anyone know if the analog pins on an Arduino Uno have a resistance and what this value would be. Also if you knew this value and the voltage flowing through the pin could you calculate the current flowing through the circuit.
Thanks for your help.
100meg ohms.
Voltage doesn't flow, current flows.
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helpsalot: Also if you knew this value and the voltage flowing through the pin could you calculate the current flowing through the circuit.
Not really. The Arduino input impedance is high enough to be considered an open circuit for most practical purposes. On the other hand, if you know the impedance of the thing you're measuring the voltage across, you can simply use Ohm's Law to work out the current in that part of the circuit.
saximus: Not really. The Arduino input impedance is high enough to be considered an open circuit for most practical purposes. On the other hand, if you know the impedance of the thing you're measuring the voltage across, you can simply use Ohm's Law to work out the current in that part of the circuit.
Thanks for your help this clears stuff up.
Note that the impedance of the analog pins is significant - there is capacitance associated with the pin, the analog multiplexer and the ADC itself.
The DC resistance can be considered infinite, but the settling time of the ADC depends on the source-impedance of the thing driving the pin. If that source impedance is greater than around 10k, you start to see slight errors when switching between different analog pins (ie analogRead called on several pins in succession). You can either cure this by doing two reads in a row on the same pin, buffering the signal source, or adding a capacitor to ground on the pin itself, 10nF or so.