Design of an Input Multiplexer for Video Applications

Abstract: In modern home entertainment video systems the digital interconnection between the different components is becoming increasingly common. However, analog signal sources are still in widespread use and must be supported by new devices. In order to keep costs down, the digital and the analog receiver chains are implemented on a single die to form a system-on-chip (SoC). For such integrated circuits, it is beneficial to reduce the number of power supply domains to a minimum and preferably use the core voltage to power the analog circuits. An eight-to-one input multiplexer, targeted for video digitizer applications, is presented. Together with the multiplexer, a simple current-mode DC restoration circuit is provided. The goal has been to design the circuits for a standard, single-well, 65 nm CMOS process, entirely using low-voltage core transistors and a single 1.1 V supply domain, while allowing the input signal voltages to extend beyond the supply rails. To fulfill the requirements, a bootstrap technique has been proposed for the implementation of the multiplexer switches. Bootstrapping a CMOS switch allows high linearity, as well as wide bandwidth and dynamic range, to be achieved with a very low supply voltage. The simulated performance is: 3 dB bandwidth of 536 MHz with a 1.5 pF load at the output of the multiplexer and a SFDR of 65 dBc at 20 MHz and 1 Vp-p input signal. It has been verified that no transistor is stressed by high voltages, therefore, the circuit reliability is guaranteed. The DC restoration circuit utilizes the main video ADC, for measuring the DC level, and is capable of setting it with an accuracy of 60 μV within the range of 100 mV to 500 mV.