Objective of Page

This page provides a jumpstart on how to use different open-source tools and the Skywater open-source PDK (Process Design Kit) to perform analog IC (Integrated Circuit) design. Please go here to either download the pre-configured VirtualBox image or follow the steps there to create your own virtual machine for your analog IC design.

Introduction

To do analog IC design, at least we need four items:

• Device Models
  • Used to describe the electrical behaviour of devices (e.g. resistors, capacitors, transistors, etc.) commonly used in analog IC design
  • Device models from Skywater open-source PDK is used
• Schematic Editor
  • Used to create the schematic of your analog circuit and generate the coressponding netlist for simulation
  • XSCHEM is used
• Simulator
  • Used to simulate the electrical behaviour of your analog circuit. The netlist and device models mentioned above are used by the simulator to calculate the results.
  • NGSPICE is used
• Waveform Viewer
  • Used to view the simulation results such as bode plots for stability analysis or time-domain waveforms (aka transient waveforms)
  • Modified GAW is used

Setting Up the Tools

1. Once you login to the Ubuntu OS, press "Ctrl"+"Alt"+"t" to launch the Terminal Window.
2. Copy the following into the Terminal Window, and hit "Enter":

git clone https://github.com/eescottie/opensrc_analog.git

3. Again, copy the following into the Terminal Window, and hit "Enter":

cd opensrc_analog
chmod +x setup.sh
./setup.sh
cd ../

4. The setup of tools is completed.

Use of Open-Source Tools to Perform Analog Circuit Design

A two-stage miller-compensated Operational Transconductance Amplifier (OTA) is used to illustrate how analog circuit design can be done with the open-source tools mentioned above.

Simulating DC Operating Point (OP)

First, we need to launch the XSCHEM such that we can view the schematic of the miller-compensated two-stage OTA. Same as above, press "Ctrl"+"Alt"+"t" to launch the Terminal Window. Once the Terminal Window is up, copy the following and hit "Enter":

cd ExampleAnalogIC

Right now you are at the folder containing all three schematics, let's launch the schematic about DC OP by copying the following into the Terminal and hit "Enter"

./run_xschem opamp_dc.sch

The DC OP schematic window pops up. There are three important buttons at the top right corner of the schematic window (click the picture if it is too small for you):

• Nelist: To generate the netlist for later simulation
• Simulate: Start simulation
• Waves: Launch waveform viewer

At the top left corner of the schematic, there are two elements:

• TT_MODEL: Specifing which device models to use
• DC_SIM: Specifing which simulations to run and what data to save

In order to check the details, use the mouse to left click the element then it's color is changed from green to grey. Then press "q" to launch a window where shows you all the details. The figure below shows you the corresponding window of TT_MODEL.

Before performing any simulation, netlist must be generated. To do so, click the top right corner button "Netlist". Then, simulation can be started by clicking the top right corner button "Simulate". Then, just be patient as once the simulation is completed, a window pops up. If no errors, you can dismiss the window.

There are two arrows on the schematic:

• Annotate: Shows DC OP (e.g. voltage, current, transconductance) on the schematic
• View Output File: Shows the simulation output file

Press the "Ctrl" button and left click the arrow to activate the corresponding funtion.
After the "Annotate" arrow is clicked, the DC OP is printed on the schematic. This is also called back-annotation.
When the "View Output File" arrow is clicked, the simulation output file is displayed. It is configured to show the Vgs, Vth, Vds, Vdsat, gm, gds, of different MOS transistors. Please see the figure below for details.

Simulating AC Performance (e.g. Stability)

We have to open another schematic. Press Ctrl"+"Alt"+"t" to launch the Terminal Window, then copy the following and hit "Enter":

cd ExampleAnalogIC

let's launch the schematic about AC Performance by copying the following into the Terminal and hit "Enter"

./run_xschem opamp_ac.sch

Let's now click the the top right corner "Nelist" button and then the "Simulate" button. Once the simulation is completed, let's now click the top right corner "Waves" button to launch the wavefrom viewer. As shown in the below figure, there are two windows. The left hand side one is for you to select which signal to be plotted. The right hand side one is to show you the waveform you want.

First, we have to click the upper Panel of the right hand side window to tell which Panel we want to show the waveform. Then, let's select a signal called "db_out" from the left hand side window and click it. You should see the following:

Second, click the lower Panel of the right hand side window. Then, select a signal called "p_out" from the left hand side window and click it. You may need to scroll down at the left hand side window to find out the signal "p_out". You should now see the following:

To show a Bode plot, we have to change the x-axis from linear scale to log scale. Click the pull down menu "Preferences" and click "Log X scale". You should now see the following:

From the cursor, we can estimate the phase margin. However, it is not accurate enough. Let's go back to the schematic window, press "Ctrl" and left click the arrow "View Output File". It is configured to measure both the phase margin and gain margin. Please see the below figure for details:

Simulating Transient Performance

Press Ctrl"+"Alt"+"t" to launch the Terminal Window, then copy the following and hit "Enter":

cd ExampleAnalogIC

let's launch the schematic about transient performance by copying the following into the Terminal and hit "Enter"

./run_xschem opamp_tran.sch

Let's now click the the top right corner "Nelist" button and then the "Simulate" button. Transient simulation takes longer time than the above DC and AC simulations. Once the simulation is completed, let's now click the top right corner "Waves" button to launch the wavefrom viewer. This time we will use another method to plot the transient waveforms.

Let's go back to the schematic window, and left click the signal we want to plot the waveform. For example, we left click the "vp" net and then press "k". This will highlight the "vp" net as shown below:

Then, we left click the "out" net and press "k" to highlight the "out". As shown below, both the "vp" net and the "out" net are now highlighted.

Now, we press "Shift" + "j", the tranisent waveforms of "vp" net and "out" net are shown in the waveform viewer automatically.

Conclusion

This page basically gives you some working examples to jumpstart on how to use open-source tools and open-source PDK to design analog circuit. Enjoy !