Time-Interleaved Switched Emitter Followers to Extend Front-End Sampling Rates to up to 200 GS/s

Optical transceivers with more than 50 GBd are now being deployed, while the use of more than 100 GBd is currently under investigation. CMOS components, such as the analog-to-digital converter (ADC) in the receiver path, can be highly parallelized for higher sampling rates but are difficult to scale toward higher analog bandwidths. Thus, the hybrid integration of front-end circuits with the function of a bandwidth gearbox in other semiconductor technologies is an interesting research topic. In this article, we show an example of a time-interleaved analog demultiplexer (ADeMUX) with four track-and-hold (T/H) circuits based on switched emitter followers (SEFs) in a 90-nm silicon-germanium (SiGe)-Bipolar CMOS (BiCMOS) technology for parallel operation of four CMOS ADCs. The 50% duty cycle of the clock signal facilitates its generation and can save power consumption in the clock path or enable higher sampling rates compared with the 25% duty-cycle clock of other ADeMUX architectures. We show that using switched preamplifiers in front of the SEFs can double the bandwidth of each T/H lane. Experimental verification shows up to 57-GHz input bandwidth while requiring only 16-GHz input bandwidth for each of the ADCs that can be connected to the four 32-GS/s output channels. The circuit achieves 3-6-bit accuracy and is suitable for 100-GBd signaling and beyond with pulse amplitude modulation. To demonstrate the capability of higher sampling rates, measurement results at 4 x 50 GS/s = 200 GS/s are provided as well although significant advancement of the measurement environment is needed for a complete evaluation.