1. Introduction to Potentiometric Digital to Analog Converter

This repository presents a 10-bit Potentiometric Digital to Analog Converter.

The target is to design a 10-bit potentiometric DAC with 3.3v analog voltage and 1 off-chip external reference using the osu180nm technology node. Here are the specifications from VSD Corporation Pvt. Ltd.

A DAC is a building block required to convert digital data into analog.

Table of Contents

• 1. Introduction to Potentiometric Digital to Analog Converter
• 2. Potentiometric DAC Architecture Design
• 3. Specification List
• 4. EDA Tools Used
• 5. Pre-layout and Simulations
  • A. Switch
  • B. 2-Bit DAC subcircuit
  • C. 3-Bit DAC subcircuit
  • D. 4-Bit DAC subcircuit
  • E. 5-Bit DAC subcircuit
  • F. 6-Bit DAC subcircuit
  • G. 7-Bit DAC subcircuit
  • H. 8-Bit DAC subcircuit
  • I. 9-Bit DAC subcircuit
  • J. 10-Bit-DAC
  • Vout v/s Digital Code Graph for 10-Bit DAC
  • INL(LSB) v/s Digital Code Graph for 10-Bit DAC
  • DNL(LSB) v/s Digital Code Graph for 10-Bit DAC
• 6. Layout and Simulations
  • A. Switch Layout
  • B. Resistor Layout
  • C. Capacitor Layout
    • Modification done in osu180nm to make use of a capacitor as a device -
    • Limitation of osu180 PDK -
  • D. 2-Bit DAC Subcircuit Layout
  • E. 3-Bit DAC Subcircuit Layout
  • F. 4-Bit DAC Subcircuit Layout
  • G. 5-Bit DAC Subcircuit Layout
  • H. 6-Bit DAC Subcircuit Layout
  • I. 7-Bit DAC Subcircuit Layout
  • J. 8-Bit DAC Subcircuit Layout
  • K. 9-Bit DAC Subcircuit Layout
  • L. 10-Bit-DAC Layout
  • Vout v/s Digital Code Graph for 10-Bit DAC
  • INL(LSB) v/s Digital Code Graph for 10-Bit DAC
  • DNL(LSB) v/s Digital Code Graph for 10-Bit DAC
• 7. Instructions to get started with the design
  • For Pre-Layout Simulation -
  • For Post-Layout Simulation -
• 8. Author
• 9. Acknowledgments
• 10. Contact Information -

2. Potentiometric DAC Architecture Design

The basic idea is to divide the voltage into N different voltage values in the range of Vref+ and Vref- for an N-Bit DAC. The design used here to achieve this is the simple resistor string DAC which consists of resistors in series. These resistors are then connected to various switches in such a fashion that it routes the exact voltage to the output.

The problem of the largeness of the circuit is reduced by building hierarchical subcircuits of switch, 2 Bit, 3 Bit, 4 Bit,....., 9 Bit DAC, and then two 9 Bit DAC is used to build the 10-Bit potentiometric DAC.

Have a look at the simplified architecture for potentiometric-DAC given below

Given below is the block diagram of the DAC -

3. Specification List

Parameter	Description	Min	Type	Max	Unit	Condition
RL	Load resistance	50			Mohm	T=-40 to 85C
CL	Load capacitance			1	pF	T=-40 to 85C
VDDA	Analog supply		3.3		V	T=-40 to 85C
VDD	Digital supply voltage		1.8		V	T=-40 to 85C
VREFH	Reference voltage high			3.3	V	T=-40 to 85C
VREFL	Reference voltage low	0			V	T=-40 to 85C
RES	Resolution		10		bit	T=27C
VFS	Full Scale Voltage	0		3.291627	V	T=27C

Parameter	Pre-layout	Post-Layout
DNL	-0.999893345 LSB to 2.03065020 LSB	-1.182952606 LSB to 2.380283181 LSB
INL	-1.953038429 LSB to 0.527216491 LSB	-3.698306813 LSB to 0.181125461 LSB
Gain Error	0	0
Offset Error	2.12E-07 V	2.14E-07 V

4. EDA Tools Used

The design has been built using open-source EDA tools. The library used is osu180nm.

I have used eSim to build schematic, ngSpice to run simulations and verify the circuit. Magic has been used to lay-out the circuit.

5. Pre-layout and Simulations

The complete circuit of 10-Bit potentiometric DAC is built hierarchically using the following subcircuits.

A. Switch

B. 2-Bit DAC subcircuit

C. 3-Bit DAC subcircuit

D. 4-Bit DAC subcircuit

E. 5-Bit DAC subcircuit

F. 6-Bit DAC subcircuit

G. 7-Bit DAC subcircuit

H. 8-Bit DAC subcircuit

I. 9-Bit DAC subcircuit

J. 10-Bit-DAC

The source details at Vref+ = 3.3V and Vref- = 0V are given here.

Vout v/s Digital Code Graph for 10-Bit DAC

INL(LSB) v/s Digital Code Graph for 10-Bit DAC

DNL(LSB) v/s Digital Code Graph for 10-Bit DAC

For pre-layout simulation please follow this.

6. Layout and Simulations

A. Switch Layout

B. Resistor Layout

value = 253 Ω

C. Capacitor Layout

value = 3.12nF

Modification done in osu180nm to make use of a capacitor as a device -

add

    device capacitor None glass poly,pc 9000 73000 

in the #devices section inside the osu180nm.tech node.

Limitation of osu180 PDK -

As the capacitor layer, or poly2 none are present in osu180nm, the capacitor can not be built in a straight forward way using the osu PDK.The above given modifications in the technology node were done to simulate the characteristics of the circuit. Here, defining the capacitor in a certain manner gives the required characteristics in the circuit.

D. 2-Bit DAC Subcircuit Layout

E. 3-Bit DAC Subcircuit Layout

F. 4-Bit DAC Subcircuit Layout

G. 5-Bit DAC Subcircuit Layout

H. 6-Bit DAC Subcircuit Layout

I. 7-Bit DAC Subcircuit Layout

J. 8-Bit DAC Subcircuit Layout

K. 9-Bit DAC Subcircuit Layout

L. 10-Bit-DAC Layout

size = 635.1 x 684.8 microns

The source details are here.

Vout v/s Digital Code Graph for 10-Bit DAC

INL(LSB) v/s Digital Code Graph for 10-Bit DAC

DNL(LSB) v/s Digital Code Graph for 10-Bit DAC

For post-layout simulation please follow this.

7. Instructions to get started with the design

For Pre-Layout Simulation -

1. Download eSim from the given website.
2. Download this folder.
3. Keep the contents of subcircuits folder inside the eSim subcircuits folder (C:\FOSSEE\eSim\library\SubcircuitLibrary).
4. Store the libary files in the eSim User Library section (C:\FOSSEE\eSim\library\deviceModelLibrary\User Libraries).
5. Open the 10_bit_dac project in eSim kept in Pre-Layout Simulation folder .
6. Run the schematic, extract spice netlist, and simulate the design using ngSpice to view the output. You should get the output as shown above in the figure.

The simulation may take more than an hour to complete. Kindly keep patience.

For Post-Layout Simulation -

1. Download Magic from the given website for Linux and Mac. For windows, you will have to install Cygwin Terminal and then Magic can run on the Windows platform also. Kindly look here for Windows operating system.

2. Download this folder or complete repository.

3. Go to the specific directory to run the Magic file.

4. To open the 10BitDac.mag with osu180nm.tech using terminal type - magic -T osu180nm.tech 10BitDac

5. To extract the spice netlist type in tkcon window -

   extract all
   ext2spice 10BitDac.ext
   

6. After you get the extracted netlist, to add the contents of this file at the beginning of the spice file, and also add the contents of this file at the end of the spice file to append the pmos,nmos libraries, and simulation parameters.

7. Now, to simulate the layout, run ngspice in another terminal using -

   ngspice 10BitDac.spice
   

8. You should get the output as shown above in the figure.

8. Author

• Ashutosh Sharma, B.Tech, Electronics & Communication Engineering, IIITD&M Kurnool

9. Acknowledgments

• Kunal Ghosh, Co-founder VSD Corp. Pvt. Ltd.
• Philipp Gühring, Software Architect, LibreSilicon Association.

I would also like to thank research fellows for extending their help and guidance during the research internship program. Many Thanks to

• Ankur Sah, M.tech Embedded Systems, NIT Jamshedpur, ankursah5@gmail.com
• Nikhil Shinde, B.E., KJSIEIT Mumbai, shinde.nv@somaiya.edu
• Shubham Negi, B.Tech. Computer Engineering, IIITD&M Kurnool, coe17b030@iiitk.ac.in
• Sheryl Serrao, Undergraduate Student, Mumbai University, sherylcorina@gmail.com

10. Contact Information -

• Ashutosh Sharma, B.Tech, Electronics, and Communication Engineering, IIITD&M Kurnool, xzlashutosh@gmail.com
• Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - kunalghosh@gmail.com
• Philipp Gühring, Software Architect, LibreSilicon Association - pg@futureware.at