Laboratory Notes

Breaking the Analog Circuit Bottleneck

Hae-Seung Lee
Microsystems Technology Laboratories

Digital circuits have enjoyed an exponential increase in performance during the past several decades from CMOS technology scaling. However, since most real world signals are analog, analog circuits are an essential part of most electronic systems. They are used to amplify, process, and filter analog signals, and to convert analog signals to digital signals and vice versa. Unfortunately, analog circuits have not benefited in similar fashion from technology scaling. On the contrary, the technology scaling has had a predominantly negative impact on analog circuits.

Fig. 1. Microphotograph of prototype analog-to-digital converter in CBSC technology

Fig. 1. Microphotograph of prototype analog-to-digital converter in CBSC technology

Traditionally, many analog circuits in mixed-signal environments have relied upon operational amplifier based sampled-data signal processing. Two adverse side effects of technology scaling that have a significant negative impact on analog circuit design are the reduced signal swing and the decrease in intrinsic device gain. Gain is important in feedback-based, analog signal processing systems because it determines the accuracy of the output value. Cascoded amplifier stages have been a popular solution to increase amplifier gain, but they further reduce the signal swings of scaled technologies. In addition, the applicability of Op Amp based circuits is unclear in non-Si technologies such as compound semiconductors, carbon nanotube/nanowire devices, and molecular devices.

Prof. Lee’s research group recently demonstrated a new class of analog circuits called comparator based switched capacitor (CBSC) circuits (Fig. 1) that promises to eliminate the bottleneck caused by Op Amps. The CBSC technique is based on the observation that an accurate output voltage is necessary only at the sampling instant by the next sampling circuit. In conventional sampled data circuits, an operational amplifier drives its inverting input to virtual ground.

In a CBSC, a comparator detects the instant the input crosses virtual ground. The comparator output is used to determine the sampling instant. Since the comparator only detects the virtual ground crossing rather than driving as an Op Amp does, it is much more power efficient. Analyses indicate that CBSC’s have a potential for two orders of magnitude lower power at the same performance level. In addition, CBSC may enable high performance analog circuits in non-Si technologies because complementary devices are not needed in comparators.

The CBSC technique has a wide range of applications, including pipeline and delta-sigma A/D converters, D/A converters, discrete-time amplifiers, filters, as well as sensor and actuator interface circuits.

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