Digital Background Calibration Techniques for Pipelined ADC Based on Comparator Dithering

This brief presents a digital background calibration technique for pipelined analog-to-digital converters (ADCs). It is a histogram-based technique and called the correction with histogram of decision points (CHDP). In this method, the capacitor mismatch and residue amplifier gain error and nonlinearity are corrected by estimating the output code of decision points in the residue characteristic. In order to achieve adequate points, the threshold level of sub-ADC is changed and to increase the estimation accuracy, an algorithm named the mapping histogram is presented. CHDP does not require any special analog circuit and its digital logic is simple. Behavioral simulation results of a 12-bit 100 MS/s pipelined ADC indicate that the proposed calibration scheme improves signal-to-noise and distortion ratio and spurious free dynamic range from 34.1 and 35 dB to 68.2 and 75.8 dB, respectively, while needing about $1.5\,\, {\times } \,\, 10^{6}$ samples for the calibration of five stages.

The paper deals with a novel 12-bit low power switched-capacitor (SC) pipelined analog-to-digital converter (ADC). The problems caused by using of SC technique are compensated or roughly attenuated by means of combination of well-known analog-domain techniques and new digital background calibration technique. Since portable applications demand for low power consumption, it is one of the most important issues considered in the design. A modified operational-amplifier (op-amp) sharing technique was used to decrease the power usage as well as capacitor scaling approach. To avoid the clock feedthrough from digital part through the switches the fully differential circuitry was utilized. The special op-amps and comparators were designed for this purpose and to obtain large bandwidth. The power consumption of the op-amps was taken into account too. The finite op-amp dc gain problem is solved in digital-domain using background calibration. The capacitor mismatch and op-amp offset are compensated in the same manner.

We propose a new digital background calibration method for capacitor mismatches in pipelined analog-to-digital converters (ADCs). It combines commutated feedback capacitor switching with a background digital correlation loop to extract capacitor mismatch information, which is subsequently used to correct errors caused by the mismatch. This is an all-digital technique requiring minimal extra digital circuits, and is applicable to both single-bit and multibit-per-stage architectures. Simulations with a 15-stage, 1.5-bit-per-stage pipelined ADC with capacitor mismatch of sigma = 0.25% in each stage show that the technique improves signal-to-noise-distortion ratio from 62 dB (10 bits) to 94 dB (15.4 bits).

A novel digital calibration technique to correct DAC errors from capacitor mismatch is presented in this paper. The jump height of transfer curve is measured for calibration and random interference introducing method helps to improve the measurement accuracy with seldom modification on analog circuits. An 11-bit 250Ms/s subrange ADC based on SAR architecture is designed to test this calibration technique. Simulation results show that significant improvements can be achieved with the proposed calibration technique.

Design of low-voltage high-resolution MOSFET-only pipeline analog to digital converters (ADCs) has been investigated in this work. The nonlinearity caused by replacing linear MIM capacitors with compensated depletion-mode MOS transistors in all 1.5-bit residue stages of the ADC has been properly modeled to be calibrated in digital domain. The proposed calibration technique makes it possible to digitally compensate the nonlinearity of a 1.8V 12-bit 65MS/s MOSFET-only ADC in 0.18 mum standard digital CMOS technology. It improves the values of signal-to-noise-plus-distortion-ratio (SNDR) and spurious-free dynamic range (SFDR) by approximately 27dB and 35dB respectively

The problem of slow convergence speed exists in the correlation-based digital background calibration techniques for the pipeline ADC. The proposed improved techniques reduce the signal-dependent interference amplitude by clipping MSBs off the ADC output. Meanwhile, the exponential averager is introduced into the LMS iteration. These means can not only calibrate capacitors mismatch in MDACs and the finite gain and bandwidth of the interstage opamp, but also converge more than 20 times faster than normal correlation-based calibration techniques. A behavioral model of 12bits pipeline ADC with 2% interstage gain error in the first stage is presented to verify the efficiency of the calibration techniques. With calibration, simulation results show that SFDR enhances from 69dB to 98dB and SNDR enhances from 53.8dB to 73.5dB.

Problem statement: Pipelined architecture is considered to be the most suitable for high-speed and high-resolution applications among varies Nyquist Analogue to Digital Converters (ADCs) in nowadays digital signal processing domain. But the pipeline power consumption is growing with the technology scaling. For a pipeline ADC with high speed and high resolution, the fore-end track and hold amplifier and residual amplifier occupies the most power consumption of the whole system, so new and novel methods is needed to lower the amplifier power consumption. Approach: Different from the traditional use of close-loop amplifier, open-loop amplifier is used as the first-stage residual amplifier, which greatly decreases the system power consumption and design difficulty. To correct the nonlinear error introduced by the open-loop amplifier, backend digital correction is applied. To validate the rationality and correctness of the method and confirm the design parameters, Verilog-A is used to build a behavioural model, Cadence simulation tool Spectre is used to get the result. Results: From the simulation result of the behavioural model, we get the Differential Nonlinearity (DNL) of the digitally backend correction ADC is -0.25∼0.25, Integral Nonlinearity (INL) is -0.5∼0.25, Spurious Free Dynamic Range (SFDR) is 77.8dB. The Total Harmonic Distortion (THD) of the system after correction is calculated to be 73.66dB, so the Effective Number of Bits (ENOB) of the is 11.78 bits. Conclusion: Digitally assisted backend correction is a novel approach to lower the power consumption of pipeline ADCs, which makes great significance in mixed signal system design. The use of open-loop amplifier instead of traditional closed-loop amplifier can effectively decrease the design difficulty and design process than before. Only the first stage residual amplifier is changed to open-loop in this article and this substitution can also be replicated in the track and hold circuit and the succeeding stages.

In the front-end of a conventional pipelined ADC, the embedded multiplying digital-to-analog converter (MDAC) has to meet very stringent noise, speed, and linearity requirements. As a result, the MDAC needs an operational amplifier (OP) with a very high open-loop gain and a large unity-gain bandwidth. Such an OP dominates the overall power dissipation of the pipelined ADC. Moreover, as the advanced technology keeps scaling, the reduced supply voltage limits the signal headroom and the reduced channel length decreases the intrinsic gain of transistors.Consequently, it is very difficult to design an OP that can achieve the stringent specifications with the advanced technology. In order to save power and alleviate the design difficulty in advanced technology,this thesis proposes a 13-bit 100-MS/s pipelined ADC with digital background calibration. The design replaces the conventional closed-loop residue amplifier with a simple open-loop architecture in the first pipelined stage which specifications is the most stringent in the pipelined ADC. The gain error and the third-order nonlinearity of the open-loop residue amplifier are calibrated using a digital background calibration technique. The calibration scheme continuously estimates and calibrates the errors introduced by the imprecise and nonlinear gain of the open-loop residue amplifier. By adopting the concept of digital-assist analog circuit design, this thesis implements a high performance pipelined ADC with the aid of CMOS scaling. The proposed pipelined ADC has been designed and fabricated in a TSMC 0.18-um CMOS process and the digital background calibration is implemented by a FPGA. The measurement results show that the ENOBs of the ADC output with calibration and without calibration at an analog power supply of 1.65V, a digital power supply of 1.8V, and a sampling rate of 12MS/s are 5.3 bits and 7.2 bits,respectively. There are two possible reasons that cause the performance degradation of the pipelined ADC. First, the pipelined ADC can’t work properly and the offset of the backend stage can’t be cancelled because of a timing error of the comparator design. Second, the operating speed of the flash memory on the FPGA board limits the sampling rate of the pipelined ADC to be less than 12MS/s.

The performance of a wideband receiver is highly dependent on the performance of the analogue‐to‐digital converter (ADC) used. Such applications require ADCs to operate at ultra‐high speeds with high accuracy for inputs ranging from 40 MHz ‐ 1 GHz. Conventional solutions either use filters and mixers to extract the frequency of interest and convert it using a low‐speed ADC or use a single ultra‐high‐speed flash converter. Both of these solutions consume ultra‐high power and require complex circuitry, which expands exponentially with the converter resolution. In this work, a 12 bit 3.072 GS/s time‐interleaved pipeline ADC is proposed. In total, 32 pipeline ADCs, each with a sampling rate of 96 MS/s, are interleaved in the time domain to achieve an overall sampling rate of 3.072 GS/s. In addition to the potential energy‐saving capabilities of a time‐interleaving structure, the circuit adopts amplifier sharing in sample‐and‐hold circuits, further improving the power efficiency. To account for interleaving mismatch, the circuit uses a pure background calibration technique, maintaining the system linearity and spectral efficiency. The proposed design achieves signal‐to‐noise‐and‐distortion ratio of 53.65 dB and spurious‐free dynamic range (SFDR) of 69.04 dB while consuming 820 mW of power at a 1.2 V supply, resulting in a figure‐of‐merit of 0.67 pJ/conv‐step.

In this paper, we present a histogram-based two-phase calibration technique for capacitor mismatch and comparator offset of 1-bit/stage pipelined Analog-to-Digital Converters (ADCs). In the first phase, it calibrates the missing decision levels by capacitor resizing. Unlike previous works which require large capacitor arrays, only few switches are added to the circuit. The second phase performs missing code elimination. It achieves better calibrated linearity and provides better mismatch tolerance than the traditional digital calibration technique. Simulation results show that the proposed technique effectively improves both the static and dynamic performance.

A new digital background calibration technique to compensate the capacitor mismatch in pipelined analog-to-digital converters (ADC) is presented. A digital signal from the ADC output is constructed so as to transform the capacitor mismatch to global gain error. A simple modification to the conventional MDAC allows the ADC to toggle between different configurations to create a reference signal used to calibrate blindly the ADC. This method does not produce any limitation of the ADC in terms of speed or input dynamic range.

This paper describes all-digital enhancements for digital fractional- ${N}$ phase-locked loops (PLLs) based on delta-sigma ( $\Delta \Sigma $ ) frequency-to-digital converters (FDCs). The enhancements include an improved dual-mode ring oscillator (DMRO)-based $\Delta \Sigma $ FDC architecture and a digital background calibration technique that compensates for the $\Delta \Sigma $ FDC’s forward path gain error. The improved $\Delta \Sigma $ FDC has significantly relaxed timing constraints and a $3\times $ smaller phase-frequency detector output pulse-width span relative to the prior art, which make it simpler to implement and amenable to higher-frequency reference signals. The calibration technique compensates for non-ideal DMRO frequencies in the digital domain. It eliminates the need to tune the DMRO instantaneous frequencies as a function of the PLL output frequency, thereby simplifying the DMRO implementation, and it also improves the phase noise performance of PLLs with high loop bandwidths.

This article presented a novel digital blind calibration technique of time‐skew mismatches for time‐interleaved analogue‐to‐digital converter (TI‐ADC). Based on the frequency‐shifted and derived operation, the spurious signals could be reconstructed and subtracted from the sampled signal adaptively. The main advantage of the proposed calibration technique is applicable to any channel TI‐ADC and could achieve higher performance in comparison with the state‐of‐the‐arts. Numerical simulations and experimental results have demonstrated that the proposed calibration technique could significantly improve the signal to noise and distortion ratio (SNDR) and spurious‐free dynamic range (SFDR) of the TI‐ADC system.

This paper proposes a high area-efficiency 14-bit column-parallel successive approximation register (SAR) analog-to-digital converter (ADC) for array sensors. A novel hybrid capacitor digital-to-analog converter (CDAC) based on the charge transfer is utilized to increase the area efficiency. It consists of a 9-bit split CDAC and a 5-bit serial CDAC. A foreground digital calibration is employed to compensate for the linearity error caused by the capacitor mismatch and bridge parasitic capacitor. The prototype was designed and fabricated in a 130-nm CMOS technology. Sampling at 200KS/s, the total power consumption is 57 μW. With the digital calibration, the proposed ADC achieves the Spurious Free Dynamic Range (SFDR) of 89.14 dB and the Differential Nonlinearity (DNL) of 0.87/-0.99 LSB. The single ADC occupies an active area of 15 × 1450 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and the area efficiency is only 6.77 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /code.

In this paper, an approach to estimate signal to noise ratio (SNR) and effective number of bits (ENOB) in nonideal multi-bit stages of pipelined analog to digital converters (ADCs) is presented. The most significant error sources in multistage ADCs are the capacitor mismatch and the finite and imprecise gain of amplifier. Output voltage of each stage in pipelined ADC is modeled by an ideal and a nonideal output, where nonideal output is the error due to circuit imperfections in each stage. Using an appropriate model, the SNR and ENOB due to circuit nonidealities and in terms of standard deviation of random errors are calculated. Simulation results show the accuracy of the analytical proposed approach in estimation of SNR and ENOB in multi-bit per stage pipelined converters.