| Miniaturization has primarily benefited digital, rather than analog, electronics both in speed and cost. This has led to an increasing need to convert analog signals to digital at high speed. The fastest analog-to-digital converters (ADC) today generally employ a "flash" strategy. In this ADC architecture the input analog voltage is sampled and then compared simultaneously with a range of reference voltages and the output code is derived from the output of the comparator corresponding to the highest reference voltage that is exceeded by the input voltage.;The resolution for the flash converter is limited by jitter in the sampling clock, and the speed is limited by the time taken for the comparator circuit to switch fully into one state or another, to overcome "ambiguity". Jitter can be significantly reduced (to as low as 20fs) through the use of a mode-locked laser to generate the sampling pulses. Ambiguity can be reduced by employing "spatial quantization" in which, at each sampling instant, the input analog voltage deflects an optical beam into an array of detectors. The output code is derived from the detectors on which the beam lands. Most previous examples of ADC's employing spatial quantization were based on cathode ray tube technology and were impractically bulky and power hungry for most of today's applications. Here we describe a miniature spatial quantizer based on the deflection of a light beam onto an array of photodiodes. The resolution demonstrated in a fabricated prototype is 3-bits and the speed consistent with sampling at 125GHz, and an energy of 7.2pJ/conversion. |
