A deep-submicrometer analog-to-digital converter using focused-ion-beam implants

The use of focused-ion-beam (FIB) technology to realize multiple MOSFET threshold voltages in the comparators of an experimental 4-b flash analog-digital converter (ADC) is discussed. The result is a simpler design in that each comparator is a CMOS inverter and the resistor ladder is eliminated. The ADC was fabricated in CMOS/SOS with comparator channel lengths of 0.5 mu m and digital device channel lengths of 0.25 mu m. Measurements of signal-to-noise ratio and analog bandwidth indicate that the ADC can function as a Nyquist converter at sampling frequencies up to 400 MHz. Better than 3-b resolution was observed for a wide range of conditions, including sampling rates above 1 GHz. The power dissipation apart from the output buffers (driving 50- Omega loads) was less than 100 mW, even at sampling rates >or=1 GHz. This 370 MOSFET circuit is one of the most complex ICs yet built with 0.25- mu m CMOS. A promising application is in oversampled ADC architectures, where this circuit would function effectively as an internal high-speed quantizer.<
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