Download Theoretical Design

Project
Diagnostic for Viral Diseases
Project Record:
Detector Circuitry Design
Version 1
11/12/26
Alex Roth
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Version History
Version
Date
Description/Notes
1
11/12/16
Abstract, Goal, Theoretical Design, Prototyping and
Construction, Conclusions, References
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Table of Contents
Abstract ........................................................................................................................................... 4
Goal ................................................................................................................................................. 4
Theoretical Design .......................................................................................................................... 4
Voltage Divider ........................................................................................................................... 5
TTL Output ................................................................................................................................. 5
Design Considerations................................................................................................................. 5
Prototyping and Construction ......................................................................................................... 6
Eagle CAD .................................................................................................................................. 6
Breadboard Prototype .................................................................................................................. 7
Circuit Board Design and Production ......................................................................................... 7
Future Considerations ..................................................................................................................... 7
References ....................................................................................................................................... 8
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Abstract
Circuitry is required to allow for a single voltage source to provide power to components that
need input voltage of several different levels. There is also a need for a circuit to change the
pulse outputs from the detector into Transistor Transistor Logic (TTL) for data analysis. To
meet these needs, a circuit was designed which utilized a series of voltage dividers to provide
appropriate voltage levels to components. It also features an op amp set as a comparator that is
able to compare two input signals and only output a hi signal when the input pulse exceeds a
reference voltage. Theoretical designs for the circuit have been completed and modeled on
computer CAD software. Breadboard prototype testing is currently in progress, and will lead to
final component selection and circuit board design and construction.
Goal
In order to create a complete system, circuitry is needed to provide power to different
components of the detector and to transform the pulse output data from the Silicon
Photomultiplier into a TTL pulse for the Fully Programmable Gate Array (FPGA). This portion
of the project is working to design a circuit that will take a voltage input from a single source and
incrementally step it down to provide the appropriate voltage levels for each detector component
needed. Additionally, the circuit should include means to send a signal to the FPGA each time
there is a peak output from the detector.
Theoretical Design
A sketch of the proposed circuit is provided as Figure 1. Each part is explained in more detail
below.
Figure 1: Hand Sketch of Circuit Design
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Voltage Divider
The first requirement of the circuit is that is to take a single voltage supply and steps it down
incrementally to the supply voltage needed for each component of the detector. This is
accomplished by a series of voltage dividers. The circuit is designed for an input voltage of 27V,
the voltage needed to power the silicon photomultiplier (SiPM). Leads connect the voltage
source to the bias voltage of the SiPM and two resistors aligned as a voltage divider. The
resistor value ratio needed to attain the correct output voltage of 15V for the amplifiers was
calculated to be R1= 4/5R2. More leads were connected in parallel before a second voltage
divider is implemented. This time, in order to get the needed 5V output for the comparator op
amp, the resistance ratio needed was R3=2R4. Finally, after connecting leads from the output of
the second voltage divider to the comparator, a third voltage divider, with resistance ratio
R5=24R6 was implemented to get a final output of 200mV. This voltage was fed into the
negative terminal of the comparator. The function of the comparator is discussed in the next
section. It was also determined to use a potentiometer as a variable resistor for R6 to allow for
threshold adjustment in the future.
TTL Output
The circuit designed also needs to change the peak outputted by the SiPM int an output TTL
(Transistor Transistor Logic) for the Fully Programmable Gate Array (FPGA) to read (See Data
Acquisition Software Project Record). Instead of outputting data on pulse height and width, the
circuit only needs to output a “yes or no” signal as to if there was a pulse or not. The circuit is
designed to accomplish this by using an op amp comparator to distinguish between the pulse
signal in the positive terminal and a threshold voltage in the negative terminal. Any time a pulse
from the detector exceeds the threshold voltage, a signal is fired from the output of the
comparator. The threshold for the pulses we receive was determined in the Amplifier
Characterization Project Record to be 200mV. The comparator used will ideally have an output
delay of no more than 10ns in so that it is able to compare signals at the frequency needed for
this application.
Design Considerations
Because the amplifiers and SiPM are already on functioning circuit boards with SMA
connectors, Connection points will be added into the circuit design to incorporate them. Headers
will also be used to make connections between the circuit and the amplifier voltage inputs after
the appropriate voltage level has been achieved. An updated hand-drawn schematic
incorporating these connections is provided below.
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Figure 2: Circuit Design with Connections
Prototyping and Construction
Eagle CAD
The next step was to create a preliminary drawing of the circuit on Eagle CAD system. This
drawing is shown below as Figure 3. As the design progresses, specific parts will be
implemented into the design on Eagle. An actual circuit board design will then be created from
the schematic, which can be used to mill the board in house or order from a manufacturer.
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Figure 3: Eagle Circuit Schematic
Breadboard Prototype
Once the design was created on Eagle, a prototype of the board was created on a breadboard to
ensure that it worked as desired. The first attempt use resistance values R1=400, R2=500,
R3=1k, R4=500, R5=2.4k, and R6=100. This produced too large of current for the
detector, which was rated at a max current of 2mA in the datasheet. The power supply was set to
not exceed this current, so the voltage supplied was too little. It was calculated that the voltage
would need to be increased by 8x, so the each resistance value was increased by 8x to reduce
current flow and allow the appropriate voltage level. This solved the problem, but to this point,
the voltage output from the first divider is far below the expected value. Initial troubleshooting
attempts have not found the source of the problem so further exploration is required.
Circuit Board Design and Production
This will be undertaken once the breadboard prototype functions correctly. Cost-benefit analysis
will be performed to determine if the project will be better suited to purchase a completed board
from a supplier or produce it in house.
Future Considerations
Future work on the circuitry will focus on troubleshooting the prototype and completing the
current design with specific components. Design considerations will also be made to make the
circuitry compact. Work will also be done to make the power source interchangeable. This will
likely include attaching a barrel plug to the input end so that either external batteries or other
voltage sources can be connected.
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References
Amplifier Characterization Project Record
Link
Data Acquisition Software Project Record
Link
Silicon Photomultiplier Datasheet
Link
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