100KSPS 8-bit Fully-differential Successive Approximation Register (SAR) analog-to-digital converter (ADC) for Low-power Applications - Universalization of IC Design from CASS

1. Team Members (Department of Electronic and Telecommunication Engineering, University of Moratuwa)

• Sajitha Madugalle.
• Imasha Weerasingha.
• Lohan Atapattu.
• Erandee Jayathilaka.
• Hirusha Maduwantha.
• Udara Mendis (MASc reading).
• Kithmin Wickremasinghe (MASc).
• Dr. Nilan Udayanga (PhD).

2. Overview of the Project:

Introduction: We propose to design a 100 KSPS 8-bit successive approximation register (SAR) ADC for low-power applications such as battery-powered or RF-powered internet-of-things (IoT) devices and wearable/implantable medical devices. Our main goal is to achieve 8-bit resolution while minimizing power and die area consumption. Walden figure-of-merit (FoM) will be utilized to quantify the performance of the ADC. The choice of the SAR ADC for the UNIC-CASS program will enable us to master both analog and digital design techniques while providing practical experience with relevant tools. Additionally, designing the SAR ADC layout will teach us crucial layout methods, such as common centroid and noise reduction techniques.

Scope: Considering the limited timeline and the experience level of undergraduate students, we have proposed a challenging but feasible scope for the project. The SAR ADC will be a fully-differential design with external input and reference provided, as illustrated in Fig. 1. External low-noise buffers will be used to buffer the input signal and reference voltage. The 8-bit DATA[7:0] output will be captured from an external data capture unit when the DATAREADY signal is asserted. Both AVDD and DVDD supplies (typical 1.8 V) are provided externally, along with an external 1 MHz clock.

Architecture: The architecture of the SAR ADC, shown in Fig. 2, includes two capacitor arrays, a comparator, and digital SAR logic. It utilizes binary-weighted capacitor arrays with charge redistribution architecture. The comparator comprises a pre-amplifier followed by a regenerative latch. The pre-amplifiers offset is auto-zeroed before every sample. After each conversion, the DATAREADY signal is asserted to notify the external data acquisition circuit to capture the available data. If time permits, we will introduce asynchronous logic for the SAR logic block to increase the sampling rate.

3. Block diagram of the project

Figure 1 – SAR ADC Block Diagrams.

4. Specification table

Specification	Min	Typ	Max	Unit	Comments
Supply voltage (Design Input)	1.7	1.8	1.9	V	-
External clock frequency (Design Input)	-	1	-	MHz	-
Resolution	-	8	-	bits	-
Operational Temperature	10	36	45	C	-
Reference voltage (Design Input)	-	0.8	-	V	-
Temperature (Design Input)	20	-	50	C	-
ENOB	7.5	-	-	bits	-
INL	-0.5	-	0.5	LSBs	-
DNL	-0.5	-	0.5	LSBs	-
SFDR	55	-	-	dB	-
Sampling rate	100	-	-	ksps	-
Current consumption during conversion	-	-	20	uA	-
Power down current consumption	-	-	100	pA	-
Die area	-	-	0.25	mm²	500 um x 500 um
FoM	-	-	200	fJ/conversion-step	Walden figure-of-merit (FoM)

Table 1 – SAR ADC performance.

Pin Layout

#	Pin Name	I/O	Description
00	vccd	-	-
01	vddio	-	-
02	mprj_io[37]	-	-
03	mprj_io[36]	A_Data[0]	Output
04	mprj_io[35]	A_Data[1]	Output
05	mprj_io[34]	A_Data[2]	Output
06	mprj_io[33]	A_Data[3]	Output
07	mprj_io[32]	A_Data[4]	Output
08	vssd2	DVSS	-
09	vdda2	AVDD	-
10	mprj_io[31]	A_Data[5]	Output
11	mprj_io[30]	A_Data[6]	Output
12	mprj_io[29]	A_Data[7]	Output
13	mprj_io[28]	data_rdy	Output
14	mprj_io[27]	clk	Input
15	mprj_io[26]	rst_n	Input
16	mprj_io[25]	comp_out	Output
17	vssa2	AVSS	-
18	vddio	-	-
19	vccd2	DVDD	-
20	mprj_io[24]	ibias_1u	Analog Input
21	mprj_io[23]	Vrefp	Analog Input
22	mprj_io[22]	Vinp	Analog Input
23	mprj_io[21]	Vcm	Analog Input
24	mprj_io[20]	Vinm	Analog Input
25	mprj_io[19]	Vrefm	Analog Input

Table 2 – Caravan SKY130 pin mapping.

Refer to README for this sample project documentation.