need to set the Vref in the ADC of ATmega16 and use some buffers to the voltage to be measured. What should I use to produce a Vref of 5V,
You can buy a 5V voltage reference (search digikey for "voltage reference"), and hook it up to Vref (note the input voltage though).
You can also hook up Vcc to AVcc, and put a cap on Vref (if your Vcc is stable enough)
Or, you can use the internal bandgap reference, if the measured voltage is < 2.56 V (possibly through external attenuation).
You may not need an external buffer. Input leakage is < 50nA. If your source can't handle that, you'll probably want a proper differential ADC anyway.
a 100 Ohms from the %v supply onto a 100nF cap. As recommended by the manual
Rene
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