ichionid 
Hello guys,
I Have a problem with the accuracy of my ADC. Basically, i think it has to do with the capacitors. I connected one capacitor 100 nF between AREF and GND, but still the results are poor.Any ideas?Is it necessary to place another capacitor between VCC and GND?
:(
Koshchi Is it necessary to place another capacitor between VCC and GND?
ichionid Thanx, I'll follow your advice and see what happens.
John
Also what's the variation in the accuracy.
ichionid Vin = 4,17 volts while the ADC gives as output 2.93, so around 1 Volt variation, pretty big :S.
I can't figure out how to connect the capacitor between VCC and GND. I can't find VCC on STK500(same with 501)
Kleinstein The STK500 board should allready have the capacitors from VCC to GND and AVCC to AGND. They are allways needed, so they are alleady on board.
There are a few other things to go wrong with the ADC:
- is the adc clock right (50-250 kHz)
more than 1 MHz can cause trouble
- is the signal impedance less than about 20 KoHms ?
- enough delay afer changing the ref. settings ?
There may also be a fault in your software.
So give us exact schematic and software.
ichionid Thanx for the responseses guyz,
We tryied all the ADC clock prescalers but there was no change.
The only thing that worked right was changing our Voltage reference to the Internal 2.56 that worked with great accuracy and no capacitor.
We can't get a decent result with all other Voltage references. I can't understand why that happens.
I 'll post some code in a few minutes. Really thanx for the responses guyz, you are a great help
ichionid 
adc_init:
ldi r16,0x00//(1<<REFS1) | (1<<REFS0) //| (1<<ADLAR)
out ADMUX,r16
ldi r16,(1<<ADEN) | (1<<ADPS1) | (1<<ADPS0)
out ADCSR,r16
readTemp:
sbi ADCSRA,ADSC;start conversion
wait1:
sbis ADCSRA,ADIF;wait for conversion to happen
rjmp wait1
in ResL,ADCL
in ResH,ADCH
sbi ADCSRA,ADIF
ret
AndersAnd 
Read these application notes and follow the hardware design and layout guidelines:
AVR040: EMC Design Considerations
AVR042: AVR Hardware Design Considerations
2.2 Analog supply
The AVR devices that have built-in ADC have a separate analog supply voltage pin, AVcc. This separate voltage supply is provided to make the analog circuits less prone to the digital noise originating from the switching of the digital circuits.
To be able to obtain good accuracy with the ADC the analog supply voltage must be decoupled separately, in the same manner as the digital supply voltage. If a separate analog ground (AGND) is present, the analog ground should be separated from the digital ground, - so that the analog and digital ground are only connected at one point, - the origin of the GND i.e. at the power supply GND.
JohanEkdahl 
Read these application notes and follow the hardware desig and layout guidelines:
ichionid All the connections are done with the AVR(AtMega128) being on the STK501. I'll try the code in another STK and see what happens.Thanx for interesting pdf's!
ichionid finally we figured it out..The problem was that the GND of the analog input deffired around 0.7 Volts from the GND of the ADC. We thought that since everything was routed to a central voltage supplier we wouldn't have to connect the GND's together. Once again thanx for the response guys! Spamming over:P