I am trying to simulate a leaky integrator in LTspice using an AD8603 op‑amp. My long‑term goal is to model slow‑changing outputs from MOX gas sensors, but the gas sensor itself is not important for this problem — the issue happens even with a simple test signal.
To design the integrator, I followed the calculation steps from the application note (Design Steps, Page 2). I set Rin = 1 MΩ, calculated Cf using the recommended formula, and chose Rf = 10 × Rin, as suggested.
Input Signal
For testing, I am feeding the integrator with a triangular waveform with a frequency of:
f=8.33 mHz(one full cycle every 120 seconds)
Problem
In simulation, the integrator saturates (runs into the supply rails) instead of producing the expected slow rising and falling output. This happens immediately after the simulation starts.
My Question
Why is my integrator saturating even though the input frequency is extremely low (8.33 mHz) and the slope of the waveform is very small?
Could this be caused by my choice of Rin/Rf/C, the AD8603 biasing, or something about how LTspice handles very slow PWL waveforms?
