Delta-sigma modulators using frequency-modulated intermediate values

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 multibit 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 has been implemented in a 1.2-μm standard digital CMOS process and the results confirm the theory. For the first-order modulator an input signal amplitude of 150 mV resulted in a signal-to-quantization noise ratio (SQNR) of ≈115 dB at 2 MHz sampling frequency and signal bandwidth of 500 Hz