An Energy-Efficient Area-Efficient DAC for SAR ADC

胡云峰，易子川，何志紅(. 電子科技大學(xué)中山學(xué)院，電子薄膜與集成器件國家重點實驗室中山分實驗室，中國 中山 528402；2. 深圳市國華光電科技有限公司，中國 深圳 580)

An Energy-Efficient Area-Efficient DAC for SAR ADC

Digital-to-analogue(DAC) is one of the important energy consumption sources of successive approximation register analogue-to-digital converter (SAR ADC). In order to reduce the energy consumption of DAC, an energy-efficient DAC structure which consists of four sub-DACs is proposed. By using extra-step technique during the DAC switching procedure, the two sub-DACs in the same side will combine to generate the required output of DAC. Moreover, Sub-DAC combination will reduce the required number of unit capacitors and decrease the energy consumption. Simulation results show that, in comparison with the traditional DAC structure, the proposed structure can decrease the energy consumption by 99.89% and reduce the number of unit capacitors by 96.88%.

energy-efficient; area-efficient; SAR ADC; sub-DAC combination; extra-step

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 hasM(M=N/2, ifNis even;M=(N+1)/2, ifNis 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.

Fig.1 The structure of the proposed DAC for SAR ADC

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 byVref/4 through a shift in the reference voltage of the second capacitor from gnd toVcm; the voltage of sub-DAC(p1) decreases byVref/4 through a shift in the reference voltage of the first capacitor fromVcmto 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 switchSpbetween 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 firstM+1 bits are generated by using only the sub-DAC(p0) and the sub-DAC(n0), while switchesSpandSnare 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)thcomparison. After extra-step, the lastN-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.

Fig.3 Operation of the proposed SAR ADC with the digital word being 110 011

Initially, in the sampling phase, all switches are closed, and the reference voltage of all capacitors is set toVcm. The input voltage is sampled onto the top plates of capacitors.

In the 1stcomparison, all switches are opened, the output voltage of the capacitor array are found to be

(1)

The comparator compares the sampling signals(VipandVin) and getb0.

In the 2ndcomparison, sinceb0is 1, the reference voltage of capacitors in the sub-DAC(n0andn1) is changed fromVcmtoVref, and the output voltages are found to be

(2)

The comparator comparesVp0(2) withVn0(2), and getb1.

(3)

The comparator comparesVp0(3) withVn0(3), and getb2.

(4)

The comparator comparesVp0(4) withVn0(4), and getb3.

During the extra step, sinceb3is 0, the reference voltage of the second sub-capacitor section in the sub-DAC(p1) is changed from gnd toVcm, and the output voltages are found to be

(5)

(6)

The comparator comparesVp0(5) withVn0(5), and getb4.

(7)

The comparator comparesVp0(6) withVn0(6), and getb5.

The energy of DAC consumed[1]at each step is as follows.

(8)

WhereE0,E1,E2,E3,E4andE5represent the energy of DAC required to determineb0,b1,b2,b3,b4andb5, respectively.Eextrarepresent the energy consumption in extra-step.

2 Simulation results

Fig.4 DAC energy against output code

Tab.1 Comparison of DAC techniques for 10-bit SAR ADC

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.

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(編輯 CXM)

2017-03-09

國家自然科學(xué)基金資助項目(60976026)；廣東省科技計劃資助項目(00760211330304099，2014B090914004，2016B090918083)；廣東省引進創(chuàng)新科研團隊計劃資助項目(2013C102)；深圳市科技計劃資助項目(GQYCZZ20150721150406)；中山市科技計劃資助項目(2016B2146)

HU Yun-feng1, YI Zi-chuan1,2, HE Zhi-hong1

(1.State Key Laboratory of Electronic Thin Films and Integrated Devices, Zhongshan Branch, Zhongshan Institute, University of Electronic Science and Technology of China, Zhongshan 528402, China; 2. Shenzhen Guohua Optoelectronic Technology Co., Ltd., Shenzhen 518110, China)

TN453

A

1000-2537(2017)03-0058-06

數(shù)模轉(zhuǎn)換器(DAC)是逐次逼近型模數(shù)轉(zhuǎn)換器(SAR ADC)能耗的重要來源之一. 為了降低DAC能耗，提出一種高能效高面效DAC結(jié)構(gòu)，該結(jié)構(gòu)包含四個子DAC. 在DAC轉(zhuǎn)換過程中，通過采用附加步技術(shù)，使同邊的兩個子DAC結(jié)合產(chǎn)生所需要的DAC輸出電壓. 而且，子DAC結(jié)合可使所需的單位電容數(shù)量減少，能耗降低. 仿真結(jié)果表明，相比于傳統(tǒng)的DAC結(jié)構(gòu)，文中提出的DAC結(jié)構(gòu)可降低99.89%的能耗，節(jié)省96.875%的單位電容數(shù)量.

高能效；高面效；逐次逼近型模數(shù)轉(zhuǎn)換器；子DAC結(jié)合；附加步

10.7612/j.issn.1000-2537.2017.03.011

一種用于SAR ADC的高能效高面效DAC

胡云峰1*，易子川1,2，何志紅1
(1. 電子科技大學(xué)中山學(xué)院，電子薄膜與集成器件國家重點實驗室中山分實驗室，中國 中山 528402；2. 深圳市國華光電科技有限公司，中國 深圳 518110)

*通訊作者,E-mail：shanhuyf@163.com