In this paper, we propose to study voltage controlled oscillators (VCOs) based on the equivalence with pulse frequency modulators (PFMs). This approach is applied to the analysis of VCO-based analog-to-digital converters (VCO-ADCs) and deviates significantly from the conventional interpretation, where VCO-ADCs have been described as the first-order ΔΣ modulators. A first advantage of our approach is that it unveils systematic error components not described by the equivalence with a conventional ΔΣ modulator. A second advantage is that, by a proper selection of the pulses generated by the PFM, we can theoretically construct an open loop VCO-ADC with an arbitrary noise shaping order. Unfortunately, with the exception of the first-order noise shaping case, the required pulse waveforms cannot easily be implemented on the circuit level. However, we describe circuit techniques to achieve a good approximation of the required pulse waveforms, which can easily be implemented by practical circuits. Finally, our approach enables a straightforward description of multistage ΔΣ modulator architectures, which is an alternative and practically feasible way to realize a VCO-ADC with extended noise shaping.

This paper presents a compact-area, low-power, highly digital analog-to-digital converter (ADC) for audio applications. The proposed converter is implemented using only oscillators and digital circuitry, without operational amplifiers nor other highly linear circuits. The ADC consists of two twin secondorder ΣA modulators, which can work both individually or in a pseudodifferential configuration. The proposed system has been implemented in a 0.13-μm standard CMOS technology. The single-ended configuration occupies an active area of 0.02 mm ² , is powered at 1.8 V with a current consumption of 155 μA, and achieves an A-weighted dynamic range (DR) of 98 dB-A. The pseudodifferential configuration achieves 103 dB-A of A-weighted DR at the expense of doubling the area and power consumption.

This paper analyzes the influence of phase noise and distortion on the performance of oscillator-based sensor data acquisition systems. Circuit noise inherent to the oscillator circuit manifests as phase noise and limits the SNR. Moreover, oscillator nonlinearity generates distortion for large input signals. Phase noise analysis of oscillators is well known in the literature, but the relationship between phase noise and the SNR of an oscillator-based sensor is not straightforward. This paper proposes a model to estimate the influence of phase noise in the performance of an oscillator-based system by reflecting the phase noise to the oscillator input. The proposed model is based on periodic steady-state analysis tools to predict the SNR of the oscillator. The accuracy of this model has been validated by both simulation and experiment in a 130 nm CMOS prototype. We also propose a method to estimate the SNDR and the dynamic range of an oscillator-based readout circuit that improves by more than one order of magnitude the simulation time compared to standard time domain simulations. This speed up enables the optimization and verification of this kind of systems with iterative algorithms.

After introducing the basic concepts, we will describe the different VCO-based ADC architectures for instrumentation and biomedical applications published in the recent years. Both open loop and closed loop, high order architectures will be analyzed and compared to other standard instrumentation ADCs. The main solutions given to problems associated with VCO-based ADCs in instrumentation, like VCO's phase noise, linearity and sampling metastability will be reviewed. The comparison with other architectures reveals that the FoM of VCO-based ADCs lags that of conventional ΔΣ modulators. Nevertheless the total power consumption, area and performance make them fairly superior when we consider the total efficiency of the instrumentation system, including signal conditioning pre-amplifiers and references.

A theoretical approach to the problem of replicating continuous-time systems with frequency modulated oscillators is proposed. It has been shown that continuous time filters and especially, the loop filter of a sigma-delta modulator, can be implemented with VCOs and digital circuitry. The analysis of these architectures is usually based on the integral relationship between frequency and phase in an oscillator. However, the spectrum of a modulated oscillator contains components at the harmonics of the oscillation frequency that cannot be easily described using the phase-frequency relationship. We propose a model in which a VCO is considered a signal encoder instead of an integrator. This model is applied then to the design of a band pass-sigma delta modulator.

In this brief we propose a technique to compensate the distortion of an open-loop VCO-based ADC. The VCO is linearized by placing an analog feedback loop based on a frequency-to-current converter which uses an asynchronous switched capacitor circuit. The feedback loop relies on the VCO gain to linearize the ADC without requiring additional amplifiers. This permits the use of a ring oscillator with small input range to efficiently encode a large signal. The proposed technique has been experimentally demonstrated in an audio ADC. The proof-of-concept chip was fabricated in 130nm CMOS and achieves 76.64 dBA of peak SNDR with a Dynamic Range of 90dB and a power consumption of 482 $\mu \text{W}$ in 20kHz.