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Optionally, AVCC or an internal 2.56V reference voltage may be connected to the AREF pin by writing to the REFSn bits in the ADMUX Register. The minimum value represents GND and the maximum value represents the voltage on the AREF pin minus 1 SB. The ADC converts an analog input voltage to a 10-bit digital value through successive approximation. The voltage reference may be externally decoupled at the AREF pin by a capacitor for better noise performance. Internal reference voltages of nominally 2.56V or AVCC are provided On-chip. See the paragraph “ADC Noise Canceler” on page 236 on how to connect this pin. AVCC must not differ more than ☐.3V from V CC. The ADC has a separate analog supply voltage pin, AVCC. A block diagram of the ADC is shown in Figure 108. The ADC contains a Sample and Hold circuit which ensures that the input voltage to the ADC is held at a constant level during conversion.

If 200x gain is used, 7-bit resolution can be expected. If 1x or 10x gain is used, 8-bit resolution can be expected. Seven differential analog input channels share a common negative terminal (ADC1), while any other ADC input can be selected as the positive input terminal. Two of the differential inputs (ADC1, ADC0 and ADC3, ADC2) are equipped with a programmable gain stage, providing amplification steps of 0 dB (1x), 20dB (10x), or 46dB (200x) on the differential input voltage before the A/D conversion. The device also supports 16 differential voltage input combinations. The single-ended voltage inputs refer to 0V (GND). The ADC is connected to an 8-channel Analog Multiplexer which allows 8 single-ended voltage inputs constructed from the pins of Port F. The Atmel ® AVR ®ATmega128 features a 10-bit successive approximation ADC. Optional Left Adjustment for ADC Result Readout.2 Differential Input Channels with Optional Gain of 10x and 200x.8 Multiplexed Single Ended Input Channels.Up to 76.9kSPS (Up to 15 kSPS at Maximum Resolution).