The Digital-Assisted Charge Amplifier: A Digital-Based Approach to Charge Amplification

A charge amplifier incorporates digital circuits as an alternative to the classic analog amplifier to achieve a high open-loop gain to maintain a consistent closed-loop gain over input capacitance variations. The digital-assisted charge amplifier employs a comparator, digital control logic, differential charge pump, current sources, common-mode feedback, and clock generator. A detailed analysis studies the amplifier stability and trade-off between ripple voltage and settling speed. A novel two-step reset scheme and tri-state charge pump minimize the output offset due to ripple residues. Fabricated in a 180-nm CMOS process, the digital-assisted amplifier achieves an open-loop gain of 101 dB, closed-loop gain of
$15.0~\mu \text{V}$
/e-, input-referred noise of 221
$\text{e}^{-}$
rms, and output swing of 3 V while consuming 2.19 mW. The amplifier also demonstrates the ability to amplify a dynamic input current using a custom opto-electronic test setup. The amplifier maintains a consistent closed-loop gain across parasitic input capacitance from 6 pF to 94 pF.