Op-amps

Condition a sensor for the ADC

Your CA-2 capstone built a light sensor whose output swung from about $0.3$ V to $2.2$ V, and your CA-6 capstone filtered its flicker. Both had a flaw this unit can now fix: the divider's output impedance changes with light level, and anything you connect to it loads it differently at different brightnesses. This capstone conditions that signal properly — buffer it, scale it, and hand the ADC a clean full-range voltage.

1. From CA-4's method, state the Thevenin resistance the LDR divider presents in bright light and in darkness (you computed roughly $3.3$ kΩ and $9.1$ kΩ). Explain in two sentences why a buffer's infinite input impedance makes that variation irrelevant, and what would go wrong without it.
2. Design the buffer: a voltage follower. State its gain, its input impedance, and its output impedance under the ideal assumptions, and name which of Lesson 1's rules gives each.
3. The buffered signal spans $0.3$–$2.2$ V, but the Pico's ADC reads $0$–$3.3$ V, so most of the range is unused. Design a non-inverting stage that maps the sensor's span onto a wider one: compute the gain needed for $2.2$ V to reach $3.2$ V, then choose stock resistors for it and state the resulting full-scale value.
4. Check the real limits: with a single $3.3$ V supply, can the output actually reach your computed full scale? Name what would have to be true of the op-amp (rail-to-rail output, and by how much of a margin), and what the reading would look like if it were not.
5. Bandwidth check: the filtered sensor signal contains nothing above a few hertz (CA-6's capstone put the corner around $5$ Hz). Using the gain-bandwidth relation, state the minimum GBW your stage needs and note by what factor a typical $1$ MHz part exceeds it — an illustration that op-amp bandwidth is rarely the binding constraint in sensor work.
6. In PCB Studio, build the board you can build: Pico 2 W, the LDR divider on an ADC pin, and a header_1x3 to bring the conditioning circuit off-board (signal, power, ground). Generate the schematic and identify which header pin carries each. As in ME-3, the curated parts library has no op-amp — write one or two sentences on what a verified op-amp part would need to carry in its pin map before the generator could place one safely.