一种用于SAR ADC的高能效高面效DAC

胡云峰+易子川+何志红

摘 要 數模转换器（DAC）是逐次逼近型模数转换器（SAR ADC）能耗的重要来源之一. 为了降低DAC能耗，提出一种高能效高面效DAC结构，该结构包含四个子DAC. 在DAC转换过程中，通过采用附加步技术，使同边的两个子DAC结合产生所需要的DAC输出电压. 而且，子DAC结合可使所需的单位电容数量减少，能耗降低. 仿真结果表明，相比于传统的DAC结构，文中提出的DAC结构可降低99.89%的能耗，节省96.875%的单位电容数量.

关键词 高能效；高面效；逐次逼近型模数转换器；子DAC结合；附加步

In successive approximation register （SAR） analogue-to-digital converters （ADCs）， DAC consumes a significant part of the total power consumption [1]. Recently， several energy-efficient techniques have been developed to improve the power efficiency of DAC [1-10]. Compared to conventional technique， Split capacitor [1]， Set-and-down [2]， Vcm-based [3]， Tri-level [4]， Mohsen [5]， VMS [6]， Sanyal [7]， Asymmetric monotonic [8]， HCS [9]， Liang [10] reduced the energy consumption by 37.48%， 81.26%， 87.52%， 96.89%， 97.26%， 97.66%， 98.40%， 98.50%， 98.84% and 99.40%， respectively. In this paper， a more energy-efficient and area-efficient DAC is presented which can achieve the reduction of 99.89% in the energy consumption of the DAC.

1 The proposed DAC

1.1 The structure of the proposed DAC

The structure of the proposed DAC for SAR ADC is shown in Fig.1. The DAC consists of four sub-DACs， sub-DAC（p0）， sub-DAC（p1）， sub-DAC（n0） and sub-DAC（n1）， with the first two combined into one sub-DAC combination and the latter two into another one. Each sub-DAC has M（M=N/2， if N is even； M=（N+1）/2， if N is odd） sub-capacitor sections and the number in each box is the total capacitance of the sub-capacitor section. All the top plates of the sub-capacitor sections connecting serially through additional switches join to sampling port， and their bottom plates connect serially.

1.2 The application of extra-step in sub-DAC combination

The main idea of this paper stems from the method of using an extra-step to generate an extra voltage on the DAC， and combining sub-DACs （as shown in Fig.1.） to get a finer voltage. It can save more energy and reduce more areas for DAC than that by conventional techniques.

Fig.2 The application of extra-step in sub-DAC combination

As shown in Fig.2， in “Before extra-step”， the sub-DAC（p0） and sub-DAC（p1） have the same voltage as they use the same reference voltage. In “Extra-step”， the voltage of sub-DAC（p1） increases by Vref/4 through a shift in the reference voltage of the second capacitor from gnd to Vcm； the voltage of sub-DAC（p1） decreases by Vref/4 through a shift in the reference voltage of the first capacitor from Vcm to gnd. In “After extra-step”， the switches between the second capacitor and reference voltage in both of the sub-DAC（p0） and sub-DAC（p1） are opened， while the switch Sp between sub-DAC（p0） and sub-DAC（p1） is closed. In this way， a finer voltage of sub-DAC（p0） and sub-DAC（p1）， Vref/8 or-Vref/8， is achieved through the application of extra-step technique.

1.3 Operation of the proposed DAC

Before extra-step， The first M+1 bits are generated by using only the sub-DAC（p0） and the sub-DAC（n0）， while switches Sp and Sn are opened. During the extra-step， an extra voltage is produced through a shift in the reference voltage of the first or the second capacitor in sub-DAC（p1） or the sub-DAC（n1），according to the output in the （M+1）th comparison. After extra-step， the last N-M-1 bits are generated by combining sub-DAC（p1） with sub-DAC（p0） or combining sub-DAC（n1） with sub-DAC（n0）.

For simplicity， the proposed switching mechanism is described by using a 6-bit SAR ADC. Let the output be the digital word （b0， b1， b2， b3， b4， b5） =110 011. The steps of the conversion process are illustrated in Fig.3.

3 Conclusion

A novel energy-efficient and area-efficient DAC for SAR ADC， with an energy savings of 99.89% and an area reduction of 96.88% compared to the conventional technique， is presented. Thanks to the using of extra-step in sub-DACs combinations， the proposed DAC achieves lower energy and smaller area compared to the published DACs.

References：

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