The design of mixed-signal circuits has always been a bottleneck for hardware circuit designers in improving performance. As we all know, the real world is analog; only by converting analog signals into digital signals can further processing be facilitated. The real-time and accurate conversion between analog and digital signals is a crucial indicator in circuit design. Besides advancements in device technology and algorithms affecting the accuracy of analog-to-digital conversion, numerous interferences and noises in the real world are also major factors hindering the performance of mixed-signal circuits.
In mixed-signal circuit design, identifying the interference source, the object of interference, and the path of interference is fundamental to analyzing interference in mixed-signal designs. In typical circuits, analog signals contain continuously varying voltage and current components that change over time. During design and debugging, these two variables need to be controlled simultaneously, and analog signals are more sensitive to external interference, thus they are often analyzed as objects of interference. Digital signals, on the other hand, only contain time-varying, threshold-quantized voltage components. Compared to analog signals, digital signals have a higher tolerance for interference, but they change rapidly, especially with a fast edge rate, and contain high-frequency harmonic components that release energy externally, thus they are often considered interference sources.
I. Challenges Faced by Integrated Circuit Design Engineers The development of deep submicron technology has driven the shift in chip design and manufacturing from discrete ICs (ICs) to System-on-a-Chip (SoCs). Mixed-signal design can reduce costs, shrink circuit size and form factor, and provide better functionality. Chip verification accounts for 50% to 70% of the chip design workload, and as chip complexity increases, the verification workload and complexity increase exponentially. Therefore, verification technology is key to mixed-signal technology.
For most integrated circuit designers in China, the traditional approach is to write the digital part in HDL, perform simulation, synthesis, placement, and routing; and draw the analog part in circuit diagram, simulate using Spice, and perform layout. Then, the two parts are pieced together. However, true mixed-signal design requires a complete integration of digital and analog components, considering and verifying them holistically. This presents challenges such as: mixing behavioral-level digital with transistor-level analog; mixing HDL-driven digital with schematic logic-driven analog; and mixing top-down digital with bottom-up analog. It's no longer a simple superposition of digital and analog design. Mixed-signal design introduces new design concepts and necessitates a completely new design flow.
II. Mentor Graphics Mixed-Signal Design Solutions Designers have only recently begun to pay attention to and take mixed-signal design seriously, but EDA companies have been ahead of the curve for years. Before understanding ADMS, let's look at some other Mentor Graphics tools related to ADMS.
1. Proven and mature technologies
(1) HDL simulation tool ModelSim
ModelSim is currently the most popular digital simulator, and its mature technology is well-known, so it will not be described in detail here.
(2) SPICE simulation tool Eldo
The most crucial part of analog circuit design is circuit simulation, which almost exclusively uses the SPICE method. Currently popular versions of SPICE are all derived from UCB SPICE. Eldo, however, is a rising star in SPICE technology in recent years. Due to its advantages in accuracy, speed, capacity, and convergence, it has rapidly gained popularity among major design companies. The Eldo R&D team is located in France, and in Europe, Eldo is the most widely used SPICE in design. Recently, Eldo has begun to be promoted in North America and Asia.
2. ADMS's Revolutionary Impact on Traditional Design Methods ADMS is a mixed-signal verification platform that integrates the technologies of both tools mentioned above. For the analog circuit section, it uses Eldo's simulation algorithm; for the digital section, it uses ModelSim's simulation algorithm. However, ADMS is not simply a combination of these tools; it is a unified tool with only a single core engine. More importantly, the handling of the interface between digital and analog circuits is crucial to mixed-signal technology. ADMS automatically adds numerous ADCs or DACs between the digital and analog circuits according to the actual needs of the circuit. The traditional digital and analog design flows are broken down and recombined, allowing designers to verify the circuit at any stage. Digital and analog design are integrated into a unified whole through ADMS.
ADMS supports eight languages: VHDL, Verilog, SPICE, VHDL-AMS, Verilog-AMS, SystemVerilog, SystemC, and C. Regardless of whether the input file is in HDL, SPICE, or C, ADMS can read it, process it automatically, and provide simulation results. For example, when introducing an HDL-described IP into an analog circuit, or a VHDL-AMS behaviorally described op-amp unit from the tool's included unit library, the various languages can be combined seamlessly without noticeable differences.
III. Conclusion ADMS can be considered the only truly meaningful mixed-signal simulation tool. After years of development, ADMS has become a leader in mixed-signal simulation, with six of the top ten semiconductor companies using it. So, under what circumstances should one switch to mixed-signal design? Perhaps it can be simply described as follows: when digital circuit design engineers using HDL simulators face increasing analog components and analog circuit behavior, struggling with insufficient models and simulation accuracy; when analog circuit design engineers using SPICE or FastSPICE face increasing digital complexity and large size, struggling with slow simulation speeds. In these situations, adopting mixed-signal design can improve design speed and efficiency, enhance design quality, and reduce product costs.
