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.
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.
| Name | Pin No. | I/O | Description |
|---|---|---|---|
| D [0:9] | 1-10 | I | Digital inputs |
| EN | 11 | I | Enable pin |
| VDD | 12 | I | Digital power supply (1.8) |
| VSS | 13 | I | Digital ground |
| OUT | 14 | O | DAC analog voltage output |
| VDDA | 15 | I | Analog voltage supply (3.3) |
| VSSA | 16 | I | Analog ground |
| VREFH | 17 | I | Reference voltage high for DAC(3.3) |
| VREFL | 18 | I | Reference voltage low for DAC |
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: kannashalini97@gmail.com, LinkedIn
Harshitha Basavaraju, PhD Scholar @ University of Bundeswehr, Munich, Germany; Contact: harshithab0707@gmail.com, LinkedIn
Skandha Deepsita S, PhD Scholar @ IIITDM Kancheepuram; Contact: skandha.deepsita5@gmail.com, LinkedIn
Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - kunalghosh@gmail.com
Kunal Ghosh, Co-founder, VSD Corp. Pvt. Ltd. - kunalghosh@gmail.com