4. ADC In ATmega328p

4.1 ATmega328p ADC Specifications

The ATmega328p (used in the Arduino Uno) has a built-in ADC with the following specifications:

4.2 Successive Approximation Method

The ATmega328p uses the Successive Approximation Register (SAR) method. In this method, the ADC works by performing a binary search for the Vin value.

At each step:

• The SAR sets a trial bit
• The DAC generates a voltage (Vdac)
• A comparator compares Vin with Vdac
• The bit is either kept or changed based on the comparison result

This process is repeated N times (N = ADC resolution, which is 10-bit for the ATmega328p).

An example of SAR implementation can be seen in this image:

1. Started from the middle value (1000) This is the representation of ½ of Vref (MSB = 1).
2. First comparison (Comparator)
   • If Vdac > Vin → move down (red arrow)
   • If Vdac < Vin → move up (green arrow)
3. Determining the next bit Each step determines one additional bit. Example:
   • 1000 → 1100 (if Vin is larger)
   • 1000 → 0100 (if Vin is smaller)
4. Repeat process (Binary Search) The value range is continuously narrowed until all bits (MSB → LSB) are determined.
5. Final result The rightmost nodes show the final binary code. Example results: 1011, 0110, etc.