Someone Electro said:
2,5K?
The PICs ADC dosent take much curent from it.If you just slitly toch the positve rail and a floting ADC pin it reads almost full voltage.So it wod work for shure.
What acuracy do you want?(Most PICs have 10 bit ADC)
The spec for the maximum source impedance is 2.5K, which is why my analogue tutorial board uses opamp buffers, then a series resistor (actually 2.7K) feeding the PIC - the resistor is added simply to prevent anything 'nasty' happening - like you set the I/O pin as an output!.
The reason for the 2.5K value is due to the sample and hold internally in the PIC A2D, the capacitor in the sample and hold has to charge (or discharge) from the impedance of the source. The higher the source impedance the longer this takes, otherwise you won't get a true reading (and could be a HUGE amount off).
As long as you're only using one A2D input, and the source is only slow changing, it's not a problem. BUT, if you use more than one input you may have to wait a number of seconds for the value to stabilise.
For example, two inputs, with too high a source impedance, one with 1V input, and the other with 3V input. If you read them too fast the first will read something under 2V, and the second something over 2V. As you can imagine, neither of these are very acceptable!.
As long as you can afford the wait?, it's not a problem, something like this:
Select input 1
Wait 5 seconds
Read A2D and store
Select input 2
Wait 5 seconds
Read A2D and store
The 5 seconds was just a nominal value I used, you can find it out experimentally, connect two analogue sources (one high and one low), and display the values on an LCD or something.
Make a note of the two readings, then reduce the wait time, until the readings start to change - you then know that you are reading the values too quickly. Pick a wait time a fair bit above the critical value, to give a bit of tolerance.
Or, add an opamp buffer? - it really depends on what you are doing!.