Block:SC LPF
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Switched-Capacitor Low-Pass Filter
A switched-capacitor low-pass filter follows the Gm-C integrator in the AFE to band-limit the neural/recording signal before digitisation by the SAR ADC.
Design equation
Corner frequency is set by the capacitor ratio and the switch clock: f_c = \frac{1}{2\pi} \cdot \frac{C_2}{C_1} \cdot f_{clk}
Budgeting procedure
- Choose f_c from the signal band of interest.
- Pick C_1 for kTC-noise floor; C_2 follows from the ratio.
- Verify switch on-resistance gives adequate settling within the clock phase.
- Run kTC noise budget (method) to confirm.
Known failure modes
- Clock feedthrough corrupting the passband.
- Incomplete settling (too-high switch R_{on}) causing gain error.
- kTC noise dominating the front-end floor if C_1 is too small.