+The two op-amps at the inputs to the differential amplifier act as unity gain buffers. Although the output of the unity gain buffer is equal to the input on its positive terminal, the buffer prevents current from flowing from the positive terminal to ground. With the buffer amplifiers absent, a current of: would flow between each of the input terminals and ground.
+
+Instrumentation amplifiers are usually constructed in the schematic shown in Figure \ref{}. In this version, the gain of the amplifier can be changed by altering a single resistor. However, more resistors are required. The version actually constructed was designed based upon the small number of resistors available, within a short time frame. Although the design could have later been changed, this would have been of no real benefit, since there was no requirement to adjust the gain of the amplifier.
+
+In principle, two ADC channels could be used to record the positive and negative outputs of the electrometer seperately, with differencing done in software. However this would require modification to the output cable of the electrometer, which may prove inconvenient for future uses.It was decided that the modification of the cable and added complexity of the software required would be more time consuming than differencing the two inputs using the hardware methods described above.
+
+The low pass filters were added to the inputs of ADC5 after it was found that an unacceptable level of AC noise was being output by the electrometer. The level of noise was too high to be filtered in software, for reasons that will be discussed in Appendix D.
+
+\subsection*{Power Supplies}
+Due to the presence of both analogue and digital electronics in the DAC/ADC box, three seperate supply voltages were required:
+\begin{enumerate}
+ \item Digital logic in the range $3 \to 4.5$V
+ \item Positive op-amp supply in the range $10 \to 15$V
+ \item Negative op-amp supply in the range $-10 \to -15$V
+\end{enumerate}
+
+Circuitry was designed which allowed two seperate single pole power supplies to be used for Digital logic and the op-amps. A dual 0-30V DC power supply has been used for both digital and analogue circuitry.
+
+\subsubsection*{Logic Power Supply}
+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.
+
+\subsubsection*{Op-amp Power Supply}
+The DAC/ADC box 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.
+