AVR-Chat

At 04:05 AM 7/29/2004, Astria Nur Irfansyah wrote:

>Hi, my friend wants to use the ADC of an AT90s8535, but 
>she is not sure how. i never had any experience using 
>AVR's ADC. My friend told me that there is still some 
>external circuit to be made. Is this true? What is the 
>purpose of this external circuitry?

not really.
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), 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.

Most uP ADCs have these limitations to one degree or another.
Used within it's limits, the ADC in the AVR is pretty nice.


>And what are the limitations of AVR's ADC? Is is slow 
>compared to simple ADCs like 0804/0809?

Yes, it is successive approximation, at something like 200 kHz.
Definitely not what you'd call a flash converter.