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This paper presents a delta-sigma pulse-width digitizer (ΔΣ-PWD) architecture designed for a super-regenerative receiver (SR-RX) operated at the 400-MHz Medical Implantable Communications Service (MICS) band. The ΔΣ-PWD employs the operation principle of a ΔΣ modulator to convert the pulse-width (PW) value into digital domain and achieve fine resolution, which allows the SR-RX to support the 2-ASK/4-ASK modulations. It can be considered as the PW domain counterpart of a conventional ΔΣ modulator. The proposed architecture is capable of suppressing the PW quantization error over the direct-sampling-type PW detector by 23 dB. A linear model for the ΔΣ-PWD is devised in this paper for system analysis and design optimization. Fabricated in a 0.18-μm CMOS process, the whole receiver draws 700 μA from a 1.3-V supply, while the ΔΣ-PWD consumes only 100 μA. When operated on the 4-ASK modulation signal with 312-kbps data rate, the receiver achieves an energy efficiency of 2.9 nj/bit and -76-dBm sensitivity.