1 GHz, 16 mW, 2 Bit Analogue to Digital GaAs Converter

There will be in the near future, a strong demand for ultra high speed, medium resolution analogue to digital (A/D) converters to be used in various field of applications such as oscilloscopy, telecommunication systems and especially radar signal processing. 8 bit, 100 MHz converters are already available in silicon technology; but the need is rather in the gigabit range which seems, at least today, hard to achieve because of a much too high power consumption. GaAs has proved to be very attractive as far as speed is concerned especially for digital circuits, but also for analogue ones: high speed decision circuits (1), comparators (2) and even 3 bit A/D (3) have already been reported. However the limited sensitivity of the comparators and their high power consumption (450 mW for 2 bit A/D) made it difficult to envisage converters with higher resolution. Using a new comparator design and a N-OFF GaAs process, we have fabricated a 1 GHz, 2 bit, 16 mW** A/D converter. This represents a real breakthrough beause of the much lower power consumption than that of any other approaches. The converter is organised in a parallel architecture which is the best way to reach both a medium resolution and a high speed. Three comparators are utilized for implementing the two bits, BO and Bl which are available in a Gray code. A fourth one produces an overflow indication. Figure 1 shows the block diagram of the circuit and a microphotograph is given on figure 2.