Designing Opamps for Low Voltage, High Speed, High Accuracy Analog to Digital Converters

This paper presents two opamp design examples for modern analog-to-digital converters. The first opamp, designed for a low-voltage low-power high-speed pipeline ADC, is a two-stage with folded-cascode as the first stage and feature common-mode stabilized active load and closed-loop pole placement techniques. The second opamp, designed for a high-accuracy high-speed sigma-delta ADC, is a two-stage opamp employing a modified cascode compensation to improve the bandwidth without increasing the power consumption. Both opamps are designed in a 0.5-µm CMOS technology and achieve DC gain over 90dB and unity-gain bandwidth over 200MHz.

Many modern electronic devices are mixed-signal systems where analog signals are quantized into digital data for processing in the digital domain. Hence, the performance of the system inevitably relies on the performance of analog-to-digital converters. The demands for high-resolution and high-speed converters have continually increased in telecommunications, digital signal processing, and industrial applications. Meanwhile, the operating voltage of integrated circuits becomes lower every year following advances in CMOS technology , thus reducing the signal swing and increasing the power consumption. In contrast, portable devices require that the power consumption is minimized to maximize the battery life. All of these requirements imply that the opamps, the core of practically all analog-to-digital converters, need to have high speed, high gain, large output swing, and low noise, while can operate at low supply voltage and consume as little power as possible.

This paper discusses two opamps which have been designed for two analog-to-digital converters.The first is a 2.5-V 10-bit 40MS/s pipeline ADC converter featuring double sampling technique . The second is a 3.3V 16- bit 1-MS/s Nyquist-rate sigma-delta ADC . Both converters are designed in a 3.3-V, 0.5-µm CMOS technology. The outline of this paper is as follows. Section 2 describes the first opamp, while Section 3 describes the second opamp. Simulation results are summarized in Section 4. Section 5 is the conclusion.

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