A wideband, time-interleaved CMOS A/D converter using photoconductive sampling switches

High-speed analog-to-digital (A/D) conversion of signals with bandwidths of several tens of gigahertz would enable a number of digital signal processing applications in fiber-optic communications, wideband radar and high-speed instrumentation. However, current electronic A/D converter technology is at best capable of 4 to 8 effective bits of resolution for signal bandwidths as high as a few GHz. A number of optical sampling techniques have been proposed to provide much higher input bandwidths while taking advantage of low-jitter mode-locked laser sources to improve resolution. However, most of these techniques have been difficult to integrate with conventional electronic A/D converters.; This work introduces a parallel A/D conversion architecture wherein a large number of time-interleaved channels each combine photonic sampling with a 4-bit, 800-MSample/s CMOS A/D converter. Each channel samples the electrical input signal using ultrafast photoconductive switches that are fabricated in low-temperature-grown (LT-grown) GaAs and flip-chip bonded onto a CMOS integrated circuit chip. When optically-triggered with a short-pulse laser, the photoconductive switches provide sampling apertures of a few picoseconds with about 6 bits of linearity, but they have limited drive capability.; The design of the 4-bit CMOS A/D converters used to digitize the sampled signals is constrained by the need for a very low input capacitance and a small area, the latter necessary to allow the integration of many converters in a single chip to maximize the aggregate sampling rate. The A/D converters each consist of a linearized open-loop buffer amplifier followed by a 4-bit flash quantizer. Dynamic averaging of offsets in the flash quantizer relaxes transistor matching requirements, thus reducing the circuit area. The use of optically-triggered sampling and time-interleaving relaxes the jitter, timing skew and conversion rate requirements of the CMOS A/D converters.; Measurement results are presented for an experimental 2-channel, 4-bit photoconductive-sampling A/D converter fabricated in a 0.25-mum CMOS technology with flip-chip bonded LT-grown GaAs switches. The prototype provides about 3.5 effective bits of resolution for input frequencies up to 40 GHz, with an estimated sampling jitter of less than 80 fs. Each channel dissipates 70 mW of power at a 640-MHz conversion rate and occupies an active area of 150 mum by 450 mum.