# Non-pullup resistance value of INPUT pins

On the DigitalPins tutorial page is found the following text:

Input pins make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 megohm in front of the pin.

The value of 100 megohm seems extraordinarily high. Might this be a typo? Surely a resistor that makes "extremely small demands on the circuit" would be a very low value resistor.

Additionally, how might I actually measure this empirically? I am currently using an Arduino clone and would like to verify the value.

Thank you.

If I read Table 29 of the Atmega 328 datasheet correctly then an input pin will only draw a maximum of 1 microamp from a 5v external source which suggest a resistance of at least 5 megohms if my maths is correct.

Surely a resistor that makes "extremely small demands on the circuit" would be a very low value resistor.

No, a low resistance would allow a large current to flow from the external circuit.

...R

PS ... as your question has nothing to do with the working of this website I have suggested to the Moderator to move it to the Project Guidance section.

Robin2:
No, a low resistance would allow a large current to flow from the external circuit.

I see. For use in a circuit with a load, one would want low resistance to "make small demands on the circuit". However for use as a pullup resistor, in fact the goal would be to keep the current low and thus have high resistance. Thank you.

Robin2:
as your question has nothing to do with the working of this website I have suggested to the Moderator to move it to the Project Guidance section.

Thank you. As that page is hosted on the arduino.cc website (this website) I thought that this was the place for the question about it.

dotancohen:
The value of 100 megohm seems extraordinarily high. Might this be a typo?

No, not a typo, and the values could actually be way higher than 100Megohm.
These are fet inputs, with some leakage from protection diodes and I/O switching.
The given values are worst case, and temp dependent.
Not a fixed value, so hard/impossible to measure.
Leo..

Wawa:
No, not a typo, and the values could actually be way higher than 100Megohm.
These are fet inputs, with some leakage from protection diodes and I/O switching.
The given values are worst case, and temp dependent.
Not a fixed value, so hard/impossible to measure.
Leo..

Thank you.

It's also not a resistance to ground (pull-down) or a resistance to Vcc (pull-up). It's just an indication of how much leakage current you might get. In my very-limited experiments the analog inputs tend to float around the mid-voltage range. (The voltage/logic-state of a floating input is officially "undefined".)