Hi,
After much research, trial and error, I finished designing a low power Capacitive Moisture Sensor with an Op Amp to boost the signal from the probe. Before finalizing the PCB, I prototyped the oscillating circuitry (Op Amp on breakout board + Probe + Resistors) on a breadboard and got decent results. Once I fabricated the PCB, the Op Amp did not produce any oscillation and yet the output is over 1V. Naturally I thought I made a mistake so I redesigned the PCB and manufactured a second iteration which did not help. So I started troubleshooting the issue following these methods:
- Testing on BreadBoard ( Op Amp breakout + External Resistors): Success
- Hooking the PCB probe to the working breadboard model: Success
- Hooking the PCB to the Op Amp breakout board + PCB/SMD resistors: Failure
- Probing each lead on the PCB for voltage drops, continuity and joins: OK
- Populated the Op Amp and supporting resistors only: Failure
- Op Amp pin assignment between the PCB and Bread Board: Match
A 3V power supply has been used throughout the testing. I am running out of options, something tells me the circuitry on the PCB somehow creates the issue even thought I followed the same schema the PCB is based on to build a successful breadboard circuitry with same Op Amp and same resistor values.
I've attached the schema and PCB screenshots in the hope someone might find what's preventing the Op Amp from oscillating as in the breadboard.
Thanks you in advance.