Residue amplification plays a key role in determining the energy efficiency, area and performance of high-speed pipelined ADCs. At its core, a residue amplifier simply consists of four transistors that transfer a differential input voltage to a capacitive load with the desired gain. However, in order to ensure gain accuracy over PVT, the core amplifier has to be augmented with extra circuitry to achieve high settling accuracy, at the expense of area and power dissipation. In this dissertation, techniques to improve the power efficiency of residue amplifiers were investigated, by adopting a two-pronged approach - 1. Developing new amplifier topologies which can achieve high resolution without relying on high DC-gain and settling accuracy, with the help of linearization in the analog domain, and 2. a deterministic calibration architecture which allows the calibration of linear gain error and distortion in background while achieving a fast convergence. @en

This brief presents a linearization technique for open-loop amplifiers. It utilizes the exponential {V}{-}{I} transfer of MOS transistors in weak-inversion together with a weak form of resistive degeneration. By using a specific relationship between the input transconductance and the source degeneration resistance, the amplifier is shown to have a significant reduction ( \sim 50\times ) in its third-order distortion component. Based on this linearization principle, two amplifier topologies are proposed by implementing the degeneration in (a) pseudo-differential and (b) common-mode configuration. The pseudo-differential degeneration scheme allows the amplifier to achieve more than 80dB linearity for 150mV {\text {pp-diff}} input swing at the expense of 35% lower power-efficiency. The common-mode degeneration eliminates this power penalty but exhibits 10dB worse linearity. To ensure an optimal linearity over process variation, a foreground calibration scheme is used to detect the nonlinearity. The nonlinearity correction is done by adjusting the bias current of the amplifier. The proof-of-concept amplifiers are constructed on a stripboard to demonstrate the effectiveness of the proposed linearization technique. With an input signal of 50mV {\rm pp{-}diff} , the two topologies achieve-105dB and-95dB HD3, respectively, improving the linearity of the state-of-The-Art open-loop amplifiers by at least 30dB.

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A 12-bit pipelined analog-to-digital converter (ADC) using a new integration-based open-loop residue amplifier topology is presented. The amplifier distortion is cancelled with the help of an analog linearization technique based on a tunable input-driven active degeneration. Amplifier gain and nonlinearity errors are detected in background using split-ADC calibration technique. The mismatch between the two half-ADCs is minimized by sharing the residue amplifier between the two half-ADCs and adding the calibration offset over time. Based on this "split-over-time" calibration architecture, a two-lane prototype ADC was fabricated in 28-nm CMOS that achieves 64-dB signal-to-noise + distortion ratio and 77dB spurious-free dynamic range at Nyquist input after calibration. Operating at 280 MS/s, the ADC consumes 13 mW from 1-V supply, exhibiting a Schreier figure-of-merit of 164.3 dB. By dissipating only 0.4 mW in the residue amplifiers, the linearization technique helps the ADC achieve an improvement of at least 3 dB in Schreier FoM over existing state-of-the-art ADCs with comparable architectures.

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This paper presents a closed-loop class-AB residue amplifier for pipelined analog-to-digital converters (ADCs). It consists of a push-pull structure with a ``split-capacitor'' biasing circuit that enhances its power efficiency. The amplifier is inherently quite linear, and so incomplete settling can be used to save power while still maintaining sufficient linearity. This also allows the amplifier's gain to be corrected by adjusting its bias current. When combined with digital gain-error detection, in this case the split-ADC technique, the result is a power-efficient gain calibration scheme. In a prototype pipelined ADC, this scheme converges in only 12,000 clock cycles. With a near-Nyquist input, the ADC achieves 66-dB SNDR and 77.3-dB SFDR at 53 MS/s. Implemented in 40-nm CMOS, it dissipates 9 mW, of which 0.83 mW is consumed in the residue amplifiers. This represents a 1.8x improvement in power efficiency compared to state-of-the-art class-AB residue amplifiers.@en