Task I: Microcontroller Selection
To drive both a display and an analog circuit, an inexpensive microcontroller board needed to be selected. The group considered three main criteria for board selection: whether the board had a built in ADC/DAC, whether the board would could easily drive a display, and price.
PSoC
The team first considered a device under the PSoC line of boards by Cypress Semiconductor. Not only did the team have prior experience with PSoC devices, most of them have multiple built in ADC/DAC chips. Though PSoC boards do not have HDMI ports, our team also possessed prior experience in using SPI to drive a small display. Because of the $15 price, the team decided that the PSoC 5LP Prototyping Kit would be selected if they decided on a PSoC device.
Raspberry Pi
Though lacking ADC/DAC chips, the Raspberry Pi line of devices was considered for its HDMI port that would limit display driving challenges. If a Raspberry Pi was selected, the team would purchase a Raspberry Pi 3 for $40.
Arduino
Finally, the Arduino devices were researched. Possessing ADC/DAC functionality and lacking an HDMI similarly to the PSoC devices, Arduino boards have a suitable amount of libraries and documentation, though not as extensive as the PSoC libraries. An Arduino Uno for $22 would be purchased if the team decided that route should be taken.
Decision
For reasons of cost, prior experience, ADC/DAC chips, and Armstrong stock room availability, the team decided to purchase three PSoC 5LP Prototyping Kits.
Task II: Analog Design
PSoC VDAC / VADC
The voltage range for the integrated VDAC and VADC is [0, 4] V. To drive appropriate voltages and currents, operational amplifiers were needed.
Circuit Architecture
To drive the device under test, the first VDAC will step drain and collector voltages, while the second VDAC will step gate voltages and base currents. Originally, two separate circuits were planned to be built, one for MOSFET devices and the other for BJT devices. However, due to the negative voltage requirements of P-channel devices, the decision was made to design one circuit for N-type devices, NMOS and NPN, and another circuit for P-type devices, PMOS and PNP. Furthermore, to measure drain/collector currents for the curve traces, the VADC will read the voltage from a small resistance precision resistor.