A 0.4–0.9 V Supply Voltage-Flexible Third-Order Passive ΔΣ Modulator With Switched-Capacitor Loop Filter Achieving 71.9 dB Peak SNDR at 4 MHz Bandwidth

This paper describes the design and implementation of a 4 MHz bandwidth $3{^{\text {rd}}}$ -order Discrete-Time (DT) Passive Delta-Sigma Modulator ( ${\mathrm {P\Delta\Sigma M}}$ ), which avoids the use of supply voltage-sensitive linear amplifiers. The Switched-Capacitor (SC) operation and amplifier-less nature of its loop filter render the proposed modulator highly supply voltage- and clock frequency-flexible. To explain our design choices, we investigate two key design challenges unique to ${\mathrm {P\Delta\Sigma M}}$ s. Firstly, the noise criticality of the quantizer ADC due to the lack of (voltage) gain in the loop filter preceding it. Secondly, the inaccessibility of the readily synthesizable and highly frequency-selective loop filter frequency responses typically employed in ${\mathrm {\Delta\Sigma M}}$ s, caused by the absence of linear amplifiers. We show that the noise-power trade-off of the quantizer ADC in 1 bit P $\Delta \Sigma $ Ms is the same as that of the first integrator amplifier in Active ${\mathrm {\Delta\Sigma M}}$ . Next, we cover the suitability of several passive filters for use in ${\mathrm {P\Delta\Sigma M}}$ loops. This leads to the selection of the voltage-sampled Charge-Rotating IIR (CR-IIR) filter, for which we show ${\mathrm {\Delta\Sigma M}}$ -oriented design equations, on which we base the design of our $3{^{\text {rd}}}$ -order DT ${\mathrm {P\Delta\Sigma M}}$ . Measurement results on the resulting prototype ${\mathrm {P\Delta\Sigma M}}$ , implemented in GlobalFoundries 22 nm FD-SOI CMOS, demonstrate that it achieves a 71.9 dB peak SNDR and 4 MHz bandwidth while consuming 4.4 mW from a 0.9 V supply and occupying 0.076 mm $^{\mathrm {\mathbf {2}}}$ of silicon area. The prototype ${\mathrm {P\Delta\Sigma M}}$ continues to operate at a $2.25\times $ reduced supply voltage of 0.4 V, albeit at a reduced bandwidth of 156.25 kHz, where it achieves a peak SNDR of 62 dB while consuming 37.8 $\mu $ W.