Modelling and Simulation of a SAR ADC with Internally Generated Conversion Signal

IOSR Journal of VLSI and Signal Processing (IOSR-JVSP)
Volume 5, Issue 1, Ver. I (Jan - Feb. 2015), PP 36-41
e-ISSN: 2319 – 4200, p-ISSN No. : 2319 – 4197
www.iosrjournals.org
DOI: 10.9790/4200-05113641 www.iosrjournals.org 36 | Page
Modelling and Simulation of a SAR ADC with Internally
Generated Conversion Signal
T. Vimal Prakash Singh
Department of Electrical Engineering,NERIST,Nirjuli,Arunachal Pradesh,India
Abstract: This paper presents the modeling and simulation of a 833.33 kS/s, 51.279µW successive
approximation register(SAR) Analog to Digital Converter(ADC) using 0.18μm CMOS technology that uses
internally generated signal for approximation for low power applications. The ADC is powered by single supply
voltage of 1V. In our scheme, comparator output time and bit settling time of the Digital to Analog
Converter(DAC) are utilized to generate a signal level such that the next step of the conversion can take place.
This model is significant for Globally Asynchronous Locally Synchronous(GALS) system integration.
Keywords: Globally Asynchronous Locally Synchronous (GALS), Asynchronous ADC.
I. Introduction
Analog to Digital Converters (ADC) are used to interface the analog domain with digital domain in
order to take the advantage of the high-speed digital Signal Processing (DSP) algorithms in variety of
applications. In Systems on Chip(SOC), large number of ADCs are used for interfacing purpose. And in such
systems, power consumption is one of the prime concern in order to increase the battery life [1],[2],[3],[4]. And
perhaps for reduced power density in the chip. ADCs are the prime component in these system. SAR ADC is
widely used for medium resolution because of its low power consumption and high speed [5],[6],[7].
However capacitive Digital to Analog Converter (DAC) used in a SAR ADC increases exponentially
as the number of bit increases. And it affects the speed of conversion. The choice of the unit capacitance also
depends on the thermal noise and capacitor mismatch.
In a conventional synchronous SAR ADC, for a 8 bit SAR ADC 8 clock cycles are required to
complete the conversion. Clock cycle time is determined by considering charge distribution time of the DAC
and comparator response time. And clock cycle period is uniformly kept looking at the worst case time of all the
blocks of a SAR ADC. Due to requirement of such a high speed clock, power consumption is high and
conversion time is also fixed.
This paper proposes a technique to implement a SAR ADC that does not use a global clock for the
conversion steps. A self timing like strategy using DAC settling time and comparison settling time of the
comparator is used to implement the asynchronous ADC. In our model, settling times of DAC and Comparator
are determined and a signal level is generated for the conversion steps.
II. Architecture Design
The architecture of the proposed ADC is shown in Figure 1. It consists of a comparator, and Sample and Hold
(S/H), binary weighted switched capacitor DAC , SAR logic and conversion signal generator.
Figure 1: ADC Block Diagram
(A) Sampled and Hold Circuit: In order to reduce the non-linearity introduced due to the VGS of the sample
switch NMOS MS, we use a bootstrapped sample and hold circuit as shown in the Figure 2. Initially the
capacitor CG is charged to Vdd and in the track phase a constant voltage of VDD is applied across VGS of the

Modelling and Simulation of a SAR ADC with Internally Generated Conversion Signal
sample NMOS MS. Charge in CG is also replenished in every phase of sample/track and hold with appropriate
signals. Sample/track and Hold transient simulation is shown in Figure 3.
(B) Design of Comparator: Comparator is a main module in a SAR ADC design. Comparator circuit which is
used in our implementation is shown in Figure 4. It should be able to detect the difference of two LSBs and it is
also to be noted here that in our architecture sampled input is compared with the DAC output voltage.
Therefore the sampled input noise is also considered in our design and implementation. In order to verify the
working of the comparator, we applied an input reference voltage of 500mV at one of the comparator input and
a square wave input voltage from 499.97mV to 500.03mV was swept at the other input and transient analysis
was performed. Transient response was plotted in the Figure 5. In the plot, Vconvsig is the signal to enable
comparator and Voutnr and Voutmr are the comparator outputs.
Figure 2: Sample/Track and Hold Circuit
(C) Design of DAC: In our designed DAC, we use a simple binary weighted capacitor array shown in the
Figure 4. instead of split capacitor array. The split capacitor array DAC is prone to capacitor mismatch and
parasitic capacitance. The unit capacitor should be as small as possible to reduce the power consumption
although it is decided by the thermal noise and capacitor mismatch[7]. The conversion starts with closing the
MSB switch initially. When the MSB switch is closed and connected to Vref and remaining capacitors are
connected to Gnd, the circuit acts like a voltage divider. The voltage at the DAC output is
6
0
* ( 1)
(0)
( 2 )
ref
dac
i
V C n
V C
C n i


 
 
. The DAC output
Figure 3: Sample/Hold and Hold Transient Simulation
voltage is compared with the sampled voltage. If the DAC output is less than the sampled input voltage, MSB is
set to 1 otherwise the MSB is set to 0. This process of conversion is carried out till LSB bit is determined.

Figure 4: Comparator Circuit
Figure 5: Comparator Transient Simulation
Figure 6: DAC Circuit
Fig. 7 shows simulation results of few comparisons. From bottom to the top, analog input, DAC output voltage
which is one of the comparator input, internally generated conversion step signal, comparator output and start of
conversion are plotted.
(D) Design of SAR Controller: In a synchronous 8-bit SAR ADC, 8 clock cycles are required for the whole
conversion cycle. Clock cycle period is designed considering charge redistribution time, settling time of the
DAC and comparator response time. As such clock cycle period is kept uniform considering the worst time of
all the three blocks of a SAR ADC.

Figure 7: Transient Simulation of DAC
In our designed self timed like SAR ADC, control signal for Successive Approximation is generated
internally without global synchronization by considering the variation in the comparator settling time and
comparator response time. In a binary weighted capacitor array DAC, settling time for MSB capacitor is longer
than that of LSB capacitor. Therefore in order to take advantage of these variation in the settling time of each bit
, we propose a completion detection scheme and it is shown in Figure 8. Similarly settling time of the
comparator also depends on the comparator input voltage. Comparator settling time completion detection
scheme is shown in Figure 9. The process of successive approximation is started with by determining the DAC
completion time and comparator completion time. After determining the completion time of both the comparator
and DAC, a signal is generated to start the binary approximation. Control signal generation scheme for our
proposed SAR ADC implementation is shown in Figure 10 and control signal generation is plotted in Figure
11. In the plot, clkgb is the control signal and Vaa is DAC completion signal and Vbb is the comparator
completion signal.
Figure. 8: DAC completion detection scheme
Figure 9. Comparator completion detection scheme
Figure 10: SAR Controller signal generation scheme

Figure 11: SAR Controller signal generation
III. Simulation Result
Transient simulation of the designed SAR ASC was done with LTSPICE using 180nm CMOS process
library and is plotted in the Fig. 12. We apply an input ramp signal having a slope of 0.00125 V/μs and
simulation was run for a period of 400μs to cover all the codes.
Figure 12: Transient Simulation of ADC
Table 1 summarizes the performance comparison of the proposed work with state of the art ADC available in
the literature. Power consumption of the proposed ADC with VDD was simulated and the result is plotted in the
Figure 13. The proposed self timed like SAS ADC was able to give correct code at VDD of 0.8V also.
IV. Conclusion
A successive approximation SAR ADC using self timed like strategy without requiring a global clock
is modeled and simulated using 180nm standard CMOS Process. Conversion signal was generated internally
after completion of the comparator activity and DAC activity. Our self timed like SAR ADC consumes
51.279µW at 833.33 kS/s and VDD of 1 V. Our self - timed like SAR ADC is suitable for low power
applications and asynchronous SOC implémentations
Figure13. Power with Vdd
Table 1: Performance comparison
This work [3] [7] [4] [1]
Technology(μm) 0.18 0.18 0.13 0.065 0.13
Resolution 8 4 - 10 8
Global Clock no yes yes no no
Supply voltage 1V 1V 1V 1V 1V
Sampling rate 833(kS/s) 25(kS/s) 1 25(kS/s) 10.24(MS/s)

Power(μW) 51.34 0.160 0.053 0.281 26.3
References
[1]. Harpe, P.J.A.; Zhou, C.; Yu Bi; van der Meijs, N.P.; Xiaoyan Wang; Philips, K.; Dolmans, G.; de Groot, H., "A 26 W 8 bit 10
MS/s Asynchronous SAR ADC for Low Energy Radios," Solid-State Circuits, IEEE Journal of , vol.46, no.7, pp.1585,1595, July
2011
[2]. S. Mukherjee, D. Saha, P. Mostafa, S. Chatterjee, C.K. Sarkar, A 4-bit Asynchronous Binary Search ADC for Low Power , High
Speed Applications, International Synposium on elecronic System Design, 2012
[3]. Anh Tuan Do; Chun Kit Lam; Yung Sern Tan; Kiat Seng Yeo; Hao Cheong, Jia; Xiaodan Zou; Lei Yao; Kuang Wei Cheng;
Minkyu Je, "A 160 nW 25 kS/s 9-bit SAR ADC for neural signal recording applications," New Circuits and Systems Conference
(NEWCAS), 2012 IEEE 10th International , vol., no., pp.525,528, 17-20 June 2012
[4]. Chao Yuan; Lam, Y.Y.H., "A 281-nW 43.3 fJ/conversion-step 8-ENOB 25-kS/s asynchronous SAR ADC in 65nm CMOS for
biomedical applications," Circuits and Systems (ISCAS), 2013 IEEE International Symposium on , vol., no., pp.622,625, 19-23
May 2013
[5]. C. Yuan and Y. Lam, "Low-energy and area-efficient tri-level switching scheme for SAR ADC," IET Electronics Lett.,vol. 48, no.
9, Apr. 2012
[6]. Y. Zhu, C. H. Chan, U. F. Chio, S. W. Sin, S. P. Martins, and F. Maloberti, "A 10-bit 100MS/s reference-free SAR ADC in 90nm
CMOS," IEEE J. Solid-State Circuits, vol. 45, no. 6, pp. 1111-1121, Jun. 2010
[7]. D. Zhang, A. Bhide, and A. Alvandpour, "A 53-nW 9.1-ENOB 1-kS/s SAR ADC in 0.13-μm CMOS for Medical Implant Devices,"
IEEE J. Solid-State Circuits, vol. 47, no. 7, pp. 1-9, Jul. 2012

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