I am designing a compact EC (electrical conductivity) sensor PCB for a bovine mastitis detection project.

The system measures the electrical conductivity of milk in mS/cm and sends the measurement to an ESP32.

Design the schematic for the EC measurement section first.

Requirements:
1. Use a two-electrode conductivity probe.
2. Use AC excitation for conductivity measurement rather than continuous DC excitation.
3. Target milk conductivity measurement approximately in the range of 0–10 mS/cm.
4. Supply voltage should be 3.3 V.
5. Provide a conditioned analog output suitable for an ESP32 ADC.
6. Include an appropriate signal-conditioning/amplifier stage.
7. Include filtering to reduce electrical noise.
8. Include protection for the ESP32 ADC input.
9. Include test points for excitation signal, conditioned signal and ground.
10. Include a connector for the external EC probe.
11. Include a connector for ESP32 power, ground and analog output.
12. Keep the analog EC measurement section physically separated from digital circuitry.
13. Include proper power decoupling capacitors.
14. Design the circuit as a low-cost 2-layer PCB suitable for a student prototype.
15. Do not assume arbitrary component values without explaining why they are selected.

Before creating the PCB layout, show me the complete schematic and explain:
- how the AC excitation works
- how the milk conductivity is measured
- how the analog signal is conditioned
- how the ESP32 receives the EC value
- how calibration will be performed
- what each resistor, capacitor, op-amp and protection component does.

Do not generate the PCB layout yet. Generate the schematic first.