+Two seperate power supplies are required for the DAQ, due to the presence of both digital and analogue electronics.
+
+\subsubsection*{Logic Power Supply}
+The AVR Butterfly runs off $3V < V_{cc} < 5V$ DC. Since $V_{cc}$ is also used as the reference voltage for the ADCs and DAC output, it is 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 DAQ 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.
+
+\subsubsection*{Op-amp Power Supply}
+The DAQ circuitry involves several operational amplifiers (LF356), which require dual $\pm 10-15V$ supplies. As there were no dual $\pm$ power supplies available, a single $30V$ power supply was used, with the circuit shown in figure \ref{fig_opamp_supply} used to produce $\pm 15V$ relative to ground.
+
+The buffer amplifier ensures that negligable current can flow from the power supply into the logic and ADC circuits, whilst the capacitor removes high frequency fluctuations of the power supply relative to ground.
+
+To simplify circuit diagrams, op-amps will be drawn with the power supply connections ommitted from this point onwards.