what is the maximum resistance the analog inputs can read from?
i have seen examples of 10k potentiometers all over, is that what it needs to work?
The datasheet recommends a source resistance of 10K or less for the analog inputs, however it gives the input resistance as 100Mohms typical. This means that you can get away with much more than 10K if the input you are measuring does not change very fast. 1750K should be no problem. However when using a source resistance greater than 10K, you are liable to get unstable readings which depend on what other analog inputs you have read recently, because of the time that the internal sample capacitor take to charge through the source resistance. There are a number of ways to avoid this, any one of the following will do:
1. Call analogRead twice on the pin with the high source resistance, and use the second reading. This is the simplest but slowest method. If the source resistance is more than about 200K then I recommend putting a delay between the two calls as well.
2. Modify the analogRead code in wiring_analog.c. Look for the lines:
// without a delay, we seem to read from the wrong channel
and insert a delayMicroseconds(...) call at that point. I have found that a delay of 10us works well for source resistance of up 100K, and a little over 100us is sufficient for a source resistance of 1M. I normally use a modified version of this file that allows me to insert a configurable delay that this point.
3. Before calling analogRead, switch the multiplexer to the required channel, then do the delay as in the previous solution, then call analogRead.
4. Connect a capacitor of about 10nF between the analog input pin and ground.