Analog Layout Design Tips and Tricks
Analog Layout Design Tips and Tricks help to draw a good layout design. Analog design typically refers to circuit design. It comprises simulating, translating specifications into transistor-level circuits, and testing the design’s functioning and viability on silicon. Key points to remember In Analog Layout Design is:
The Analog design & layout is an iterative process!!! So Think before just jumping in it! If Design went wrong, Time will be wasted for both, your time and the time of others, and may delay the chip!!!
Rules for optimum matching in Analog Layout
- Minimum distance.
- Same temperature.
- Same shape and the same size.
- Use Common centroid geometries.
- Same structure and same orientation.
- Same surroundings(Dummy devices).
Best Layout Design practice
- Ask the Circuit Designer to give you a brief idea about how the circuit work. #
- Talk to the Top-level Layout guy and ask him about Input & Output Ports/Pins & Power Routing.
- See where your block/cell is going to fit in the Module, and based on that make floorplan.
- Please first do the Floorplan and Power plan, and review them with your senior, before review does not do the routing.
- Use Bindkeys for Your Layout design tool, it will increase your Layout Drawing speed.
- Check your Base-level DRC before you start routing. Make sure it is clean.
- In LVS, First, solve Open and Short related LVS, it will drastically reduce Your number of errors after solving it.
- Spend some time with Your senior and look at how they are doing layout.
- Try to learn Skill/TCL script, it will make your daily job Easy!!
(Any suggestions, please comment below, and I will add them to this article.)
Phases in an Analog Layout Design
There are main four phases in Analog Layout Design. #
- Architecture Phase
- Preliminary Design Phase
- Layout Phase
- Final Design Phase
The architecture Phase will determine what needs to be built, The Preliminary Design Phase will do the initial design & get ready for layout, The Layout Phase will help to create the preliminary layout, and last, the Final Design Phase will simulate parasitics & finish the design.
The Purpose of the Architecture Phase is to, Understand what you need to build to solve the problem. Find out how similar problems have been solved in the past. The Goals of the Architecture Phase are to Resolve any open issues that could affect your design and Decide exactly what is Your plan to design further. The following points are involved in this phase: #
- What is this circuit do?
- What is the aspect ratio?
- What specifications are important to consider?
- What parts of your design can be borrowed from previous work(Block reuse)?
- Decide exactly what it is that you plan to build in the Preliminary Design Phase
What an Architecture Review should cover?
Review again all required specifications Compare all architectures considered and Compare the pros & cons of each, and Explain which architecture was chosen and why. Collect Information about needed by other blocks and/or top-level I/O signals, clocks, and power supplies required, etc.
Preliminary Design Phase
The Purpose of the Preliminary Design Phase is to Design any new circuits required and verify the performance of any “reused” circuits in your application. Show that your design meets all specs, including an additional margin to account for expected layout parasitics, Get ready for the layout to start. The Goals of the Preliminary Design Phase are to complete your design to the point where it is ready to enter the layout, not to completely polish and finish the design. Develop a layout floor plan for your block. #
The Purpose of the Layout Phase is to Layout any new blocks being designed. Place and connect any existing blocks being re-used. The Goals of the Layout Phase are to create a layout that meets all requirements for your block, both in terms of how it fits into the overall chip layout and in terms of circuit performance. #
Final Design Phase
The Purpose of the Final Design Phase is to identify and correct any problems in the block’s layout which could cause performance problems like unwanted coupling between signals, too much capacitive loading on a node, and lack of symmetry on differential signals. The Goals of the Final Design Phase are to complete the circuit design & layout and ensure that the final design meets all specifications, including layout parasitics. #
Layout Techniques for Enhanced Matching in Analog Design
In the realm of analog circuit design, particularly for applications where precise matching is crucial, employing specific layout techniques becomes imperative. These techniques aim to minimize variations in device characteristics and enhance performance. Let’s delve into some of these layout strategies:
- Device Length Considerations: Unlike digital circuits where minimum-sized devices are common, analog circuits often require gate lengths longer than the minimum. This choice reduces the impact of channel-length modulation (λ), resulting in more predictable behavior. Longer devices are preferred in general analog design.
- Managing Implant Resistance: In analog MOSFET devices, the implant resistance of the source and drain regions can introduce parasitic resistances that affect device performance. To mitigate this, designers should add multiple contacts along the width of both the source and drain regions. This approach reduces resistance, increases current-carrying capacity, and ensures a more uniform distribution of current across the device.
- Parasitic Distribution with Parallel Devices: As device width increases, designers can employ a technique that involves splitting a single device into several parallel devices, each with a width one-fourth of the original. This technique serves multiple purposes:
- Capacitance Reduction: Parasitic capacitances associated with the reverse-biased implant substrate diode (Cdb and Csb) are proportional to device width (W). By splitting the device into multiple parallel units, these parasitic capacitances are significantly reduced. The reduction factor depends on the number of parallel devices (n), with odd values providing the most significant reduction.
- Resistance Reduction: Parasitic resistances that are in series with both the source and drain also get reduced when devices are placed in parallel. This reduction contributes to lower resistive losses.
In summary, layout techniques play a critical role in analog design to ensure matching and minimize parasitic effects. Longer gate lengths are preferred, and managing implant resistance through strategic contact placement is essential. Additionally, splitting devices into parallel units can significantly reduce parasitic capacitance and resistance, enhancing the overall performance and predictability of analog circuits. These techniques are invaluable in applications where precision and consistency are paramount.
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