Delta-Sigma Modulators using Frequency- Modulated Intermediate Values

This paper describes a new firstand 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 straight-forward multi-bit quantization. Without the frequency modulator, the circuit becomes a frequency-to-digital converter with delta-sigma noise shaping. An experimental firstand second-order modulator have been implemented in a 1.21⁄4m 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 of3115dB at 2MHz sampling frequency and signal bandwidth 500Hz. I. IN T RO D U C T IO N The delta-sigma (1-6) A/D conversion technique [?] is currently receiving increased attention as an attractive alternative to traditional A/D conversion. Although the delta-sigma modulator (DSM) is well suited for VLSI implementation, multi-bit quantization is not straight-forward due to DAC linearity problems, and the sampling speed is limited by integrator and DAC slew-rate. By implementing the main integrator as a frequency modulator [?, ?], we achieve a simpler circuit without DACs. Straightforward multi-bit quantization, and a potentially higher sampling frequency is one amongst other features. The new DSM that will be referred to as a frequency DSM (FDSM) becomes a F/D converter with 1-6 noise shaping if the frequency modulator is removed. To illustrate why a frequency modulator can be used as an integrator we may look at the FM signal itself. An ideal FM signal may be expressed as fm.t/ D sin[ .t/]; (1)