\subsection*{Microprocessor}
The DAC/ADC box has been based upon Atmel's AVR Butterfly; an inexpensive and simple demonstration board for the ATMega169 16 Bit microprocessor. The features of the AVR Butterfly include easily accessible ports for Analogue to Digital Convertor (ADC) inputs and digital input/output, an onboard Universal Asynchronous Reciever/Transmitter (USART) for RS-232 serial communications, and a 6 character Liquid Crystal Display (LCD). The AVR Butterfly can be programmed using a conventional computer over the USART using a RS-232 COM port. For modern computers (which do not usually posess COM ports), a RS-232 to USB converter may be used.
-Figure \ref{avr_butterfly.pdf} is a labelled photograph of the AVR Butterfly, showing the use of each port in this project.
+Figure \ref{avr_butterfly.pdf} is a labelled photograph of the AVR Butterfly showing the use of the available ports for this project.
+
%Figure of Butterfly
\begin{center}
- \includegraphics[scale=0.50]{figures/atavrbfly.jpg}
+ \includegraphics[scale=0.70]{figures/avr_butterfly.pdf}
+ \captionof{figure}{AVR Butterfly} \label{avr_butterfly.pdf}
\end{center}
Unless otherwise stated, all voltage differences are specified relative to the power supply ground of the AVR Butterfly.
For making voltage measurements above $V_{cc}$, a voltage divider allows reduction of the voltage at the ADC. By constructing the voltage divider using a variable resistor, the range of measurable inputs could be manually adjusted.
-The diodes shown in Figure Figure \ref{fig_ADC_normal} ensure that the ADC is protected from accidental exposure to voltages outside the acceptable range. In normal operation both diodes are off. If $V_{\text{adc}}$ were to become greater than the reference point $V_{cc}$, current would flow between the ADC input and the reference point, acting to reduce $V_{\text{adc}}$ until it reached $V_{cc}$. Similarly, if $V_{\text{adc}}$ fell below ground, current would flow from ground to the ADC input, acting to increase $V_{\text{adc}}$ until it reached ground.
+The diodes shown in Figure \ref{adc_normal.pdf} ensure that the ADC is protected from accidental exposure to voltages outside the acceptable range. In normal operation both diodes are off. If $V_{\text{adc}}$ were to become greater than the reference point $V_{cc}$, current would flow between the ADC input and the reference point, acting to reduce $V_{\text{adc}}$ until it reached $V_{cc}$. Similarly, if $V_{\text{adc}}$ fell below ground, current would flow from ground to the ADC input, acting to increase $V_{\text{adc}}$ until it reached ground.
The voltage at the ADC input can be related to the input of the voltage divider using Kirchoff's Voltage Law and Ohm's Law:
\begin{align*}
$V_{\text{in}}$ can be therefore be determined from the registered ADC counts by:
\begin{align*}
- V_{\text{in}} &= \frac{\text{ADC counts}}{2^{10}} \times \frac{R_1 + R_2}{R_1} V_{cc}
+ V_{\text{in}} &= \left(\frac{\text{ADC counts}}{2^{10}}\right) \times \frac{R_1 + R_2}{R_1} V_{cc}
\end{align*}
\subsubsection*{Differential ADC Input}
During the testing of the TCS experimental apparatus, it became desirable to measure the emission current of the electron gun. The electrometer used for this current measurement was capable of producing an analogue output in the range of $0-1V$. However, the negative terminal of this output was not at ground potential, but rather at the same terminal as the negative input terminal. Directly connecting the electrometer output to one of the ADC inputs discussed above would create a short circuit between the initial energy power supply, and ground (refer to Figures \ref{} and \ref{}). Therefore, it was decided to add a differential stage before the input of one of the ADCs.
-Figure \ref{} shows the modification made to the input for ADC5 on the AVR Butterfly. The original voltage divider and input protection discussed above are still present. The modifications include the addition of an instrumentation amplifier, and low pass filters.
+Figure \ref{adc5.pdf} shows the modification made to the input for ADC5 on the AVR Butterfly. The original voltage divider and input protection discussed above are still present. The modifications include the addition of an instrumentation amplifier, and low pass filters.
+
+\begin{center}
+ \includegraphics[scale=0.70]{figures/adc5}
+ \captionof{figure}{Differential Input stage for ADC5}
+ \label{adc5.pdf}
+\end{center}
+
The instrumentation amplifier consists of two stages of operational amplifiers (op-amps); input buffers, and a difference amplifier.
The difference amplifier can be shown using the ideal op-amp model to produce an output voltage proportional to the difference between its inputs:
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.
+\begin{center}
+ \includegraphics[scale=0.70]{figures/logic_ps}
+ \captionof{figure}{Logic Power Supply}
+ \label{logic_ps.pdf}
+\end{center}
+
\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.
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