While reading a research paper about enhanced coulomb counting, I found this peculiar sensing circuit (inside the red lines).
(Research paper: Xie, Jiale, Jiachen Ma, and Kun Bai. “Enhanced Coulomb Counting Method for State-of-Charge Estimation of Lithium-Ion Batteries Based on Peukert’s Law and Coulombic Efficiency.” Journal of Power Electronics 18, no. 3 (May 20, 2018): 910–22. doi:10.6113/JPE.2018.18.3.910)
From the context given in the research paper, one would guess that it is an integrator circuit even if it is called otherwise (Coulomb counting). After simulating the circuit using a universal 4 terminal op amp in LTspice, (all resistances set to 1 kΩ and capacitances to 0.1 µF) I found that for a sinusoidal input (1 Vpp, 1 kHz, 0°), we get an attenuated sine shifted by 180°.
My question is, why not use a simple integrator circuit like this one, or the one below it instead if DC gain was an issue? And what purpose does resistor R3 (on the circuit above) serve?
(Image source: Electronics Tutorials - The Integrator Amplifier)
Research paper: https://drive.google.com/file/d/1SP-tZ8bZTTTOs65L9g8-d4h0eKQqEkD1/view?usp=sharing