The project aims to design a 10-bit Potentiometric Digital to Analog Converter using end-to-end Open-source EDA tools. The target is to design 10-bit potentiometric DAC with 3.3v analog voltage, 1.8v digital voltage and 1 off-chip external voltage reference using sky130nm technology node.

• 1.Purpose of Digital to Analog Converter (DAC)
• 2.IP Block Design Specifications
  • IP Block Diagram
  • Terminal Functions
• 3.EDA tools used to implement Potentiometric DAC
• 4.Implementation of 10Bit Potentiometric DAC
• 5.Pre-layout Designs and Simulations
  • Switch design and simulation
  • 2-Bit DAC design and simulation
  • 3-Bit DAC design and simulation
  • 4-Bit DAC design and simulation
  • 5-Bit DAC design and simulation
  • 6-Bit DAC design and simulation
  • 7-Bit DAC design and simulation
  • 8-Bit DAC design and simulation
  • 9-Bit DAC design and simulation
  • 10-Bit DAC design and simulation
  • INL and DNL Waveforms
• 6.Post-Layout Designs and Simulations
  • Resistor 250 layout and value
  • Resistor 500 layout and value
  • Inverter layout
  • Switch layout and simulation
  • 2-Bit DAC layout and simulation
  • 3-Bit DAC layout and simulation
  • 4-Bit DAC layout and simulation
  • 5-Bit DAC layout and simulation
  • 6-Bit DAC layout and simulation
  • 7-Bit DAC layout and simulation
  • 8-Bit DAC layout and simulation
  • 9-Bit DAC layout and simulation
  • 10-Bit DAC layout and simulation
  • INL and DNL Waveforms
• 7.Instructions to get started with the design
  • Spice simulation speed improvement (Multi threading)
  • Pre-layout Simulation commands
  • Post-layout Simulation commands
• 8.Future Works
• 9.Contributors
• 10.Acknowledgments

In real world, most of the data available is in the form of analog in nature. We have two types of converters analog to digital converter and digital to analog converter. These two converting interfaces are essential to obtain the required operations of a processor to manipulate the data of digital electronic equipment and an analog electric equipment. Digital to Analog Converter (DAC) is a device that transforms digital data into an analog signal in order to interact with the real world. The digital signal is represented with a binary code, which is a combination of bits 0’s and 1’s. The digital data can be produced from a microprocessor, Field Programmable Gate Array (FPGA), or Application Specified Integrated Circuit (ASIC). There are two commonly used DAC conversions – Weighed resistors method and R-2R ladder network method. Applications of a DAC: audio amplifier, video encoder, display electronics, data acquisition systems, calibration, Digital potentiometer.

The design has been built using open-source EDA tools. The library used is sky130. This design is implemented using xschem, and ngspice is used to run the simulations & verify the circuitry. For circuit layout implementation, Magic will be used. The step to install xschem with sky130 and ngspice can be found here.

The basic idea is to divide the voltage into N different voltage values in the range of VREFH and VREFL- 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 10-Bit potentiometric DAC – Switch, 2-bit, 3-bit, 4-bit, 5-bit, 6-bit, 7-bit, 8-bit, 9-bit and 10-bit.

Basic Architecture of Potentiometric DAC

Switch design implementation and respective waveform are shown below

To see this waveform run switch.spice file

2Bit DAC is implemented using 3 switch instances. 2-Bit circuitry and waveform are shown below

To see this waveform run my_2bitdac.spice file

3Bit DAC is implemented using 2 2-Bit DACs and 1 switch instances. 3-Bit circuitry and waveform are shown below

To see this waveform run my_3bitdac.spice file

4Bit DAC is implemented using 2 3-Bit DACs and 1 switch instances. 4-Bit circuitry and waveform are shown below

To see this waveform run my_4bitdac.spice file

5Bit DAC is implemented using 2 4-Bit DACs and 1 switch instances. 5-Bit circuitry and waveform are shown below

To see this waveform run my_5bitdac.spice file

6Bit DAC is implemented using 2 5-Bit DACs and 1 switch instances. 6-Bit circuitry and waveform are shown below

To see this waveform run my_6bitdac.spice file

7Bit DAC is implemented using 2 6-Bit DACs and 1 switch instances. 7-Bit circuitry and waveform are shown below

To see this waveform run my_7bitdac.spice file

8Bit DAC is implemented using 2 7-Bit DACs and 1 switch instances. 8-Bit circuitry and waveform are shown below

To see this waveform run my_8bitdac.spice file

9Bit DAC is implemented using 2 8-Bit DACs and 1 switch instances. 9-Bit circuitry and waveform are shown below

To see the waveform run my_9bitdac.spice file.

10Bit DAC is implemented using 2 9-Bit DACs and 1 switch instances. 10-Bit circuitry and waveform are shown below

To see the wavefrom run my_10bitdac.spice file.

Every block of the circuit until 10 bit DAC are tested and spice models until 10 bit dac are included in specified folders.

R = 249.8ohms

R = 497.2ohms

Inverter Implementation is shown below

Switch layout implementation and its respective waveform are shown below

To see this waveform run switch_layout_test.spice file

2Bit DAC is implemented using 3 switch instances. 2-Bit layout and waveform are shown below

To see this waveform run 2bitdac_layout_test.spice file

3Bit DAC is implemented using 2 2-Bit DACs and 1 switch instances. 3-Bit layout and waveform are shown below

To see the wavefrom run 3bitdac_layout_test.spice file.

4Bit DAC is implemented using 2 3-Bit DACs and 1 switch instances. 4-Bit layout and waveform are shown below

To see the wavefrom run 4bitdac_layout_test.spice file.

5Bit DAC is implemented using 2 4-Bit DACs and 1 switch instances. 5-Bit layout and waveform are shown below

To see the wavefrom run 5bitdac_layout_test.spice file.

6Bit DAC is implemented using 2 5-Bit DACs and 1 switch instances. 6-Bit layout and waveform are shown below

To see the wavefrom run 6bitdac_layout_test.spice file.

7Bit DAC is implemented using 2 6-Bit DACs and 1 switch instances. 7-Bit layout and waveform are shown below

To see the wavefrom run 7bitdac_layout_test.spice file.

8Bit DAC is implemented using 2 7-Bit DACs and 1 switch instances. 8-Bit layout and waveform are shown below

To see the wavefrom run 8bitdac_layout_test.spice file.

9Bit DAC is implemented using 2 8-Bit DACs and 1 switch instances. 9-Bit layout and waveform are shown below

To see the wavefrom run 9bitdac_layout_test.spice file.

10Bit DAC is implemented using 2 9-Bit DACs and 1 switch instances. 10-Bit layout and waveform are shown below

To see the wavefrom run 10bitdac_layout_test.spice file.

Ngspice provides multithreading options to improve the simulation time. To enable multithreading following steps are to be followed:

o Install ngspice from tarball

o sudo apt-get install -y autoconf

o sudo apt-get install -y libtool

o tar -zxvf ngspice

o cd ngspice

o ./autogen.sh

o ./configure --enable-xspice --enable-openmp --disable-debug --with-readline=yes

o make clean

o make

o sudo make install

Then in the netlist's control section, add the following: set num_threads=4 (or more)

However, multithreading option is effective if the major part of the circuit are MOSFETs (BSIM 3V8 or BSIM4V5),since the DAC consits of more number of resistors, multithreading option was not helpful to increase the simulation speed.

o Clone the git repo with following command

    git clone https://github.com/vsdip/avsddac_3v3_sky130_v1.git

o Open the terminal from the cloned folder or run below command after cloning while in the same path

        cd avsddac_3v3_sky130_v1/

o Command to simulate .Spice files of conventional design

        cd Prelayout/

        ngspice my_nbitdac.spice  

o Clone the git repo with following command (if you haven't cloned for pre-layout simulation)

        git clone https://github.com/vsdip/avsddac_3v3_sky130_v1.git

o Open the terminal from the cloned folder or run below command after cloning while in the same path

    cd avsddac_3v3_sky130_v1/

o Command to simulate .Spice files of conventional design layout

   cd Postlayout/
   
   ngspice nbitdac_layout_test.spice

o type y when simulation asks for

Runtime for layout can be reduced further.

Shalini Kanna, Master of Science in Computer Engineering, University of Massachusetts Lowell, Lowell,MA, USA; Contact: [email protected], LinkedIn

Harshitha Basavaraju, PhD Scholar @ University of Bundeswehr, Munich, Germany; Contact: [email protected], LinkedIn

Skandha Deepsita S, PhD Scholar @ IIITDM Kancheepuram; Contact: [email protected], LinkedIn

Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - [email protected]

Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - [email protected]