But the Arduino analog input pins really wants to see a source impedance of 10k ohms or less for best results. He may have to take multiple analog reading and average the results if working with higher resistances.Lefty
Quote from: retrolefty on Dec 14, 2012, 02:35 amBut the Arduino analog input pins really wants to see a source impedance of 10k ohms or less for best results. He may have to take multiple analog reading and average the results if working with higher resistances.LeftyNot necessary if you only want to measure slowly varying signals and don't need the full 10-bit accuracy. The input resistance of the ADC is given as 100Mohms typical in the datasheet. There is an issue in ensuring that the sample capacitor can charge between setting the multiplexer and starting the conversion, but that is easily solved by either inserting a small delay in the analogRead code or by connecting a capacitor to the analog pin, as I suggested in my post.I used a potential divider made from two 4.7Mohm resistors to monitor the voltage of a 9V battery voltage in a recent project. This was for a system that had no power switch and put the mcu in power down mode when turned off, so it was important that the potential divider didn't drain the battery. Using such a high source resistance introduces an error of around 2.5%, but for a battery monitor this was acceptable.
Are you sure about that?
The 200K is high enough to limit the current into the pin protection diode to a safe value even if the voltage exceeds 15V.
QuoteThe 200K is high enough to limit the current into the pin protection diode to a safe value even if the voltage exceeds 15V.Are these "pin protection diodes" internal zenners connecting the pin to ground?
An external diode to clamp the input from going above the AVR VCC voltage might be a good idea - I wouldn't want to put that kind of stress on the internal clamping diodes (if there are any).
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