Analog Design

Design Procedure

Analog design nowadays involves very complex equations since circuitry has become more sophisticated. The calculations that need to be done to solve these equations are extremely time-consuming, so computers are often used to help do this work. A public-domain circuit-analysis program called SPICE (Simulation Program with Integrated Circuit Emphasis) is commonly used in the analog design industry. It has equations for various circuit components such as transistors, capacitors, and resistors and it can do circuit interaction calculations that would take months to do by hand in a matter of hours [3].

Some steps that are completed during the process of analog design include:

Design Specification

• • This is where the functions of the design and various features such as performance, power, and cost are laid out [2][5]. Functional models of the functions are created to help make decisions on other factors such as size, transistor selection, and usage of inductors or capacitors as well as to determine any constraints in the design [5].

Sub-Circuit design [5]

• • This is where device-level circuit descriptions are made using electrical components such as capacitors, resistors, and transistors [2]. Macro-level measurements are made to determine if there are any further constraints and to also gain an expectation of the performance of the circuit [5].

Simulation

• • The circuits must go through simulations and optimizations to ensure that it is meeting the specifications that were laid out [2][5]. Tools such as VHDL-AMS are used to perform high-level simulations and determine any constraints in the specification [5].

Physical Layout

• • The components of the circuit are laid out based on how they were defined during the simulations [2] and then design rule checks occur [5]. Various rules must be followed to ensure the circuit is optimized, for example the density of the layout is optimized so that costs can be minimized. Validation that these rules are followed is called physical verification [2]. 

Layout vs. Schematic (LVS) [2][5]

• • The equivalent circuit is extracted from the layout and compared to the original design to verify that the correct components were used and connected properly [1][2].

Post-Layout Simulation [5]

• • The design is re-simulated to make sure it still works with parasitic effects such as crosstalk and wiring resistance now present in the design [2]. Flaws may be revealed in the design, which may require the circuit to be redesigned, laid out, and simulated all over again [5].

Tape-Out [5]

• • The design is ready for manufacturing or to be integrated into a larger digital design [2].  

Best Practices

• When dealing with mixed-signal design (analog and digital signal), it is important to keep the analog and digital circuitry separate since this reduces noise in the analog circuitry [1][6]. This is especially true for high-speed signals such as SPI, I2C, CAN, and Ethernet [4].
• Proper ground separation must be provided in mixed-signal design [4]. Analog circuits should have a dedicated analog ground [6] and all of its signal should be referenced to this ground [4]. The analog ground can be connected to the digital ground at one single point [4][6], but they should be isolated otherwise [6]. Failure to do this may result in analog signal being corrupted by digital noise [1].  
• Another tip is to keep analog traces short [4], especially in those of higher frequencies [1][6]. This will reduce the chance of of electromagnetic interference from nearby sources [1][4][6].  
• Analog signal return paths should be direct, clear and as unobstructed as possible while also being the shortest connection back to their source [4][6].   
• The number of vias on analog traces should be minimized [4]. This can be done by keeping as many traces as possible on one layer [1][6].