The AVR Butterfly runs off $3V < V_{cc} < 4.5V$ DC. Since $V_{cc}$ was also used as the reference voltage for the ADCs and DAC output, it was desirable that $V_{cc}$ be kept constant, despite the absolute level of the power supply. A $3.3V$ voltage regulator has been used for this purpose. The capacitor further smooths the output by shorting high frequency fluctuations to ground.
When the DAC/ADC box was first constructed $V_{cc}$ was supplied by three $1.5V$ batteries. However, due to higher than expected power usage, and the unreliability of the voltage regulator as the input voltage fell below $4V$, inputs for an external power supply were later added.
The AVR Butterfly runs off $3V < V_{cc} < 4.5V$ DC. Since $V_{cc}$ was also used as the reference voltage for the ADCs and DAC output, it was desirable that $V_{cc}$ be kept constant, despite the absolute level of the power supply. A $3.3V$ voltage regulator has been used for this purpose. The capacitor further smooths the output by shorting high frequency fluctuations to ground.
When the DAC/ADC box was first constructed $V_{cc}$ was supplied by three $1.5V$ batteries. However, due to higher than expected power usage, and the unreliability of the voltage regulator as the input voltage fell below $4V$, inputs for an external power supply were later added.