AVR-Chat

<snip>
> You need to connect AVCC (always!), you need to supply an external 
reference, or use VCC for ratiometric measurements (the internal 
reference is pretty dodgy), 

This part is typical for internal or external ADC's.    the ADC value 
is referenced to the power supply or to another value.  if you have a 
good reference, then your ADC reading will be better.

>you need to supply a cap on the ref pin.  The ADC inputs as on most 
>micros, are relatively low impedance, so if you need high impedance, 
>you'll have to buffer.

Depending on the external value, you might need to use an op-amp to 
change the value to get it to read so the highest voltage is the same 
as the chips voltage.    This means that if you are running on 3 
volts and the thing you want to read is 0 to 10 volts, you can use a 
a voltage divider to change that to 0 to 3 volts.  But, if you want 
to measure 0 to 100mV,then you need to ampliphy the value and for 
that an op-amp would be called for.

Since you mentioned external components, this may be on of those that 
would be needed.   But it would be needed for any style ADC you chose.

Dave