This article will give you some insight into my journey into the world of analog microelectronics, as told in my recent talk at ORConf 2024.

Back in 2020, inspired by Tim Ansell’s announcement of free tapeouts, I jumped headfirst into the world of open-source ASIC design. My first chip was a digitally focused project, reflecting my background in FPGA programming. It was amazing to see the power of digital abstraction—designing with ones and zeros, instantiating tons of transistors with a single line of code—but it also made me appreciate the incredible complexity hidden beneath the surface.

Tiny Tapeout 6 marked a significant milestone by introducing support for analog and mixed-signal ASIC designs. This innovation opened up a world of possibilities for open-source chip development, building on the project’s success in the digital realm.

Prior to Tiny Tapeout 6, only digital designs were supported. However, the demand for analog and mixed-signal capabilities was evident. In an interview, Carsten Wulff, Principal IC Scientist at Nordic Semiconductor, expressed his enthusiasm for the open-source ASIC movement, noting that a key strength of this approach lies in the mixed-signal domain. FPGAs and microcontrollers excel in digital applications, but mixed-signal designs on ASICs offer unique advantages and capabilities.

In this insightful interview, I talk with Carsten Wulff, Principal IC Scientist at Nordic Semiconductor, about all things analog ASIC design.

Carsten kicks things off by sharing a fascinating anecdote about how he ended up designing an analog SAR during his winter holiday in the Norwegian mountains, sparked by the opportunity to include analog designs on Tiny Tapeout 6.

The conversation then shifts to Carsten’s thoughts on the accessibility of 130nm technology for analog design, which he believes is “the last comfortable node”. While larger designs with millions of digital gates might not be suitable, 130nm shines for analog applications, offering a sweet spot between performance and cost-effectiveness.

Harald Pretl made a fascinating submission to Tiny Tapeout 3, an analog circuit made from digital standard cells.

It builds a DAC out of a lot of tristate inverters, and a big capacitor by ganging up a lot of NAND gates.

In this interview with Harald we discuss how it works and how he made it and simulated it.

You can check his design here, including the source and GDS.

Part 1: Schematic Capture with Xschem

While it’s possible to just draw transistors directly - for example in SiliWiz, this doesn’t scale well and gets confusing fast. We want to draw a circuit diagram first, with levels of hierarchy for more complex designs. These can then be simulated and checked that specifications are met before moving onto layout.

Lab 1.1: Try the Xschem demos

• Build Xschem familiarity.
• Experiment with some of the demos that come with the PDK.

Lab 1.2: Draw your own circuit

• Draw your own circuit and get it ready for simulation
• If you don’t have a circuit ready, you can use one of the demos
• Follow best practices

Part 2: Simulation

After you have a schematic and have exported the netlist to a spice file, we can use ngspice to simulate it. In this part of the course you will start to learn spice, using it to simulate demo designs and then your own.

Want to learn how to design analog and mixed signal chips?

The Zero to ASIC Analog course will guide you through the process of taping out analog integrated circuits using open-source tools. While digital design often relies on hardware description languages and automated synthesis, analog design involves more in depth simulation and drawing circuit layouts by hand. You’ll learn to use tools like Xschem for schematic capture, NGspice for simulation, and Magic for layout.