Progress in Design of Improved High Dynamic Range Analog-to-Digital Converters

We describe several improvements that are being pursued to improve the dynamic range of lowpass phase modulation-demodulation (PMD) analog-to-digital converters (ADC). The existing ADC has been tested at sampling frequencies up to 29.44 GHz; a 89.15 dB signal to noise ratio (SNR) is achieved for a 10 MHz sinusoidal input, with the noise being measured in a reference 10 MHz bandwidth in the decimated band. The first improved approach involves a multi-rate ADC where the modulator sampling frequency is increased in multiples of the decimation filter clock. We have tested the multi-rate ADCs at sampling frequencies up to 46.08 GHz and 29.44 GHz for chips fabricated using the 4.5 and 1 kA/cm
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fabrication processes respectively. For a single channel ADC, with a 9.92 MHz sinusoidal input, sampled at 29.44 GHz, the SNR is 83.93 dB in a reference 10 MHz bandwidth. The spur-free dynamic range (SFDR) is 95 dB. In another improved architecture, called the quarter-rate ADC, the modified quantizer quadruples the input dynamic range by distributing the input in a cyclical fashion to four output channels, each operating at a quarter of the fluxon transport rate. This enables quadrupling the synchronizer channels, providing an opportunity for up to 12 dB performance enhancement. A parallel counter following the multi-channel synchronizer converts the differential code to a multi-bit binary code, which is further processed by the decimation filter. A prototype version of this ADC with a two channel synchronizer, fabricated using the 4.5 kA/cm
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process, has been tested up to a sampling frequency of 25.6 GHz. For a 10 MHz sinusoidal input, the SNR is 82.54 dB, with the noise measured in a reference 10 MHz bandwidth. We are also designing a subranging ADC with two PMD front-ends. Simulation results promise greater than 20 dB performance enhancement.