Difficulties in design and simulation of digital-analog hybrid circuits
In the design of digital-analog hybrid circuits, the identification of interference sources, interference objects, and interference paths is the basis for analyzing the interference of digital-analog hybrid designs. In normal circuits, due to the continuous variation of the effective components of voltage and current on the analog signal over time, during the design and debugging process, these two variables need to be controlled at the same time, and they are more sensitive to external interference, so they are usually used as The interfered object is analyzed; the digital signal only has a time-limited threshold voltage component, which has a higher tolerance to interference than the analog signal, but this type of signal changes quickly, especially the rapid change of the edge speed. It has high high-frequency harmonic components and releases energy to the outside. It is usually used as a source of interference.
The digital circuit part as the interference source mostly adopts the CMOS process, which results in extremely high input resistance at the digital signal input end, which is usually in the range of tens of kohms to megaohms. Such high internal resistance causes the current on the digital signal to be very weak, so only the voltage effective signal is active. In the digital-analog mixed interference analysis, this type of signal can be used as a voltage-type interference source, such as CLK signal, Reset and other signals. In addition to fast alternating digital signals, the synchronous switching noise (SSN) caused by the pin inductance and mutual inductance on the power pins of digital signals is also an important class of voltage-type interference sources in digital-analog mixed circuits. In addition, there are some current signals in the circuit, especially there is a large current on the power supply signal between the DC power supply and the device load. According to the right-hand spiral theorem, a magnetic field is induced around the current signal, which causes a changing electric field. At this time, the DC power supply is used as a current source of interference.
Whether it is a voltage or current type interference source, when it is coupled to the interfered object, it may be coupled through circuit conduction, space electromagnetic coupling, or both. However, the general simulation analysis tools can only analyze one of them due to their limited functions. For example, in traditional SPICE circuit simulation tools, only circuit-conducted interference is considered, and the coupling of spatial electromagnetic fields is not considered; while general PCB signal integrity (SI) analysis tools only consider the coupling of spatial electromagnetic fields. The ground is regarded as the ideal DC direct current, and will not be analyzed and considered. The incomplete extraction of the coupling path is also an important cause of the digital-analog mixed noise analysis.
In digital-analog hybrid design, the division of power and ground is the focus of debate in the industry. In the traditional design, the digital and analog parts are strictly separated; however, as the system becomes more and more complex, the integration of digital and analog circuits continues to increase, and the segmentation will cause the digital signal to cross-segment, and the signal backflow will be incomplete, which will affect the signal integrity. The division of the power supply also causes the impedance of the power distribution system to be too high; Some people have proposed a "single point connection": still do the division, but a single point connection under the signal across the division to avoid cross division problems; but if there are a lot of signals between digital and analog It is difficult to separate. This kind of "single point connection" also has difficulties, so some people have proposed not to divide, but to keep the digital and analog parts do not cross; there are also some materials, including ground wires or parallel capacitors next to the signals across the division, Used to provide a complete return path. No matter which method is used, there seems to be some truth and there are precedents for success. However, the effectiveness and possible problems of all these segmentation schemes have not been tested.
The simulation of digital-analog hybrid circuits also has model problems. The analog circuit simulation model generally accepted in the industry is still the SPICE model, and the digital circuit signal integrity analysis uses the IBIS model. A number of EDA companies' simulation software have launched hybrid model simulators that support multiple models. However, the main difficulty on the designer's desk is the device model, especially the analog device model is difficult to obtain. From the perspective of digital design, the transient analysis in the time domain, that is, the voltage value determined at a certain point in time, is the main means of simulation, as intuitive as an oscilloscope in debugging. Without an accurate model, transient analysis cannot be achieved. However, for analog design, especially noise analysis, the excitation source is difficult to describe or difficult to predict on the time axis. Only the frequency bandwidth range and approximate amplitude are known. At this time, we usually introduce a frequency domain sweep analysis to examine the sweep signal. Changes in focus, like the role of a spectrum analyzer. Or simply investigate the frequency domain SYZ parameters of the channel through which the signal or noise passes, as in a network analyzer (NA), and then predict the frequency and amplitude of the interference. It can be seen that digital-analog mixed noise analysis requires both a simulator supporting a mixed model and a simulator that supports both time-domain analysis and frequency-domain analysis.