Delta-sigma modulators using frequency-modulated intermediate values

Abstract

This paper describes a new first-and second-order delta-sigma modulator concept where the first integrator is extracted and implemented by a frequency modulator with the modulating signal as the input. The result is a simple delta-sigma modulator with no need for digital-to-analog converters, allowing straightforward multi-bit quantization. Without the frequency modulator, the circuit becomes a frequency-to-digital converter with delta-sigma noise shaping. An experimental first-and second-order modulator have been implemented in a 1.2-¼m standard digital CMOS process and the results confirm the theory. For the first-order modulator an input signal amplitude of 150mV resulted in a SQNR of ³115dB at 2MHz sampling frequency and signal bandwidth 500Hz.

A 0.2V, 7.5 μW, 20 kHz ΣΔ modulator with 69 dB SNR in 90 nm CMOS

“…It can be shown that multiple taps improve the signal-toquantization-noise (SQNR) of the digital version of θ(t) in proportion to the number of taps used [3]. For a first-order single-bit modulator using m taps, the SQNR is given by [3] SQN…”

“…In the FDSM, the input signal v in (t) is first integrated by the RVCO, which encodes the signal in the phase θ(t) of the oscillation waveform, since θ(t) is given by [3] θ(t) = 2π…”

“…The topology use in this work is based on the work from [3]. This topology has no feedback, which means that the input signal does not need to be amplitude-limited as in traditional Σ∆ converters; if the RVCO is controlled through the bulk terminal, it is even possible to let the input (control) voltage exceed the supply rails.…”

Linearity of bulk-controlled inverter ring VCO in weak and strong inversion

“…A frequency ΔΣ converter (FDSM) is a ΔΣ converter without feedback [1]. The feedback in a ΔΣ converter can be eliminated by building a system consisting of an integrator and a differentiator.…”

Dual-clock MASH delta-sigma modulator employing a frequency modulated intermediate signal

“…However, its use is restricted to relatively low frequency applications due to the limitation of the sampling frequency. The DSM using frequency modulated intermediate signal (FMDSM) [2,3,4] has significant advantages that it has no feedback digital-analog converter (DAC) restricting the operation frequency, and hence, is suitable for high frequency operation. However, the FMDSM has been not widely used so far, since it is difficult to apply the FMDSM concept to higher order DSMs.…”