High Performance GaAs/AIGaAs Heterojunction Bipolar Transistor 4-Bit and 2-Bit A/D Converters and 8-Bit D/A Converter

GaAs/AIGaAs heterojunction bipolar transistors (HBT) have been used to develop high performance MSI-LSI circuits for analog-to-digital conversion (ADC) and digital-to-analog conversion (DAC) functions. A 4-bit parallel (flash) ADC with 850 transistors has an input bandwidth of >1.0 GHz at a clock rate of >1.0 gigasamples/second. The 4-bit ADC has on chip 15 current-mode logic (CML) master-slave comparators with preamplifiers, encoding logic for a binary output, and a reconstruction DAC output. To date this is the largest reported HBT CML circuit. The development of the 4-bit ADC was preceded by the design and evaluation of a 2-bit flash ADC (145 transistors) implementing similar on-chip functions. It has an input bandwidth of 1.25 GHz at a clock rate of 2.5 gigasamples/second. An 8-bit parallel DAC with 125 transistors is monotonie (without trimming) and has a DC RMS level error of 0.6 mV, less than a quarter of a least significant bit (LSB). The DAC settles to 8-bit accuracy in 1.7 ns. High speed functional circuit yield on the best wafer is >70% for the 2-bit ADC and >85% for the 8-bit DAC. The performance of these building block circuits demonstrates the potential for realizing 8-14 bit, 10 Ms/s to 3 Gs/s HBT A/D converter systems.