Oversampled ADC based on pulse frequency modulator and TDC

Oversampled converters based on voltage controlled ring oscillators are an attractive solution because of their digital implementation and simplicity. However, the voltage-to-frequency conversion of ring oscillators displays a poor linearity. Replacing the ring oscillator by a pulse frequency modulator (PFM) that provides improved linearity at the expense of feedback and analogue amplification is proposed. Compared to the equivalent continuous time sigma-delta modulators, the PFM may be more tolerant to circuit impairments. In addition, the output data of the proposed architecture is a multibit sequence through the use of a time-to-digital converter TDC instead of a Flash quantiser or a multibit digital-to-analogue converter. A high dynamic range can be achieved without severe constraints on analogue mismatch or clock jitter.