A 12-bit Two-Stage Pipelined SAR ADC design

Zhe Liu, Shida Zhang

Last updated on Feb 12, 2022 4 min read ADC

Image credit: predictabledesigns.com

Overview

In this project, we designed a 2-stage pipelined Successive Approximation Register (SAR) Analog to Digital Converter (ADC). SAR ADC architectures are popular for achieving high energy efficiency but they suffer from resolution and speed limitations. The pipelined SAR architecture divides a moderate resolution SAR ADC into some low resolution SAR ADCs. The target specifications are given in the following table:

Item	Specifications
Technology	45nm (NCSU FreePDK45)
Power supply	VDD = 1.2V, VSS = 0V
Resistor size	< 100kΩ
Capacitor size	2fF~ 10pF
ENOB@ Nyquist rate	>12bits
Sampling rate	>30MS/s
Walden figure-of-merit (FOM)	< 100-fJ/conversion-step

Get Started

High Level Block Diagram of the Pipelined SAR ADC

The diagram above shows the block diagram of the SAR ADC architecture. The ADC is a pipeline of a 6-bit and a 8-bit SAR ADCs. We decide to use differential sampling in order to cancel the common mode sampling offset. Furthermore, Vcm-based switching technique is used in both stages to lower the power consumption of the whole system.

The first stage has 6-bit effective resolution. The first stage sub-ADC and CDAC share the same sampling capacitor array. There are several typical switching techniques that are available. We decide to use Vcm based switching technique mainly because the low power consumption and faster settling speed. However, one biggest drawback of this method is that it requires extra switches to control each capacitor. There are three possibilities at all sampling capacitors. Before the certain bit compared, the corresponding capacitor’s bottom plate is connected to Vcm. After comparison, its bottom plate will be connected to either ground or Vref depends on the comparing result is 1 or 0. Notice that the positive pair sampling capacitor array and the negative pair sampling capacitor array are connected opposite to each other. These sets of switches are controlled by a SAR logic unit which I’ll go over in next section. Additional independent clock sets are used to make sure each capacitor is in the right phase.

The second stage is a 8-bit SAR ADC. The use of SAR architecture in the second stage enables a large resolution in this single stage. The implementation is very similar compare to the first stage but has two more capacitor sets.

SAR Logic

The classic SAR architecture is use due to its straightforward design technique. Figure above is the implementation on Cadence. It consists of a set of ring counter and a set of shift regisiter. There are 2x6+2 = 14 D flip flops in use. For each conversion, piror to the first clock rising edge, RESET will first clocked high to reset all flip flop’s output to 0. After that, the RESET signal remains low for the reset of the process. Once the first rising edge approaches, the MSB flip flop is set to 1. Then when second rising edge approaches, the counter shifts 1 to the next flip flop until it reaches the LSB. SAR logic determains the value of bit one by one based on the comparator’s output. For a 6-bit SAR logic, it requires 1+6 = 7 cycles to complete one single conversion. Table below is the algorithm of 6-bit SAR logic.

Clock	D1	D2	D3	D4	D5	D6	COMP. Out
RESET	0	0	0	0	0	0	–
1	1	0	0	0	0	0	D1
2	D1	1	0	0	0	0	D2
3	D1	D2	1	0	0	0	D3
4	D1	D2	D3	1	0	0	D4
5	D1	D2	D3	D4	1	0	D5
6	D1	D2	D3	D4	D5	1	D6
7	D1	D2	D3	D4	D5	D6	–

Below is the SAR Logic testing result. Notice that the input (Data) is set to 110000. Our SAR Logic output for each bit matches the input correctly.

D Flip Flop

Because SAR logic uses flip flop, we decide to implement a Positive-edge-triggered D flip-flop with Clear and Preset. Here, three input NAND gates are used. Below is the truth table of our DFF.