With the advent of synthesizable analog circuit design, the performance of digital circuits is getting stronger and stronger, and the analog circuits are more difficult to design due to the drop in power supply voltage. Sometimes, when you learn the circuit, you will be stupid and can't distinguish between analog and digital circuits.
1. Simulation and digital fit, what is the result?Analog and digital circuits are a classic classification of circuits. Digital circuits process discrete digital signals, and analog circuits process continuous analog signals. In the classic textbook, the analog circuit and digital circuit design and analysis method are very different. The analysis of analog circuit is like physics, the design is like an art; the analysis of digital circuit is like mathematics, and the design is one-on-one, based on Tight logic flow.
However, the spacing between analog and digital is not unbreakable. On the one hand, the nature of the signal is continuous. All digital circuits are only a special case of analog circuits. Even digital circuits can be used in analog circuits (for example, inverters can also be used as an amplifier, and current multiplexing). On the other hand, using digital circuits with good scalability and good controllability, we can move some signals processed in the analog domain to the digital domain for processing and get good results. The result of analog and digital fit is not just a mixed-signal circuit, but a completely new design concept.
Analog and digital fusion can be divided into two directions, namely, the use of digital methods to implement traditional analog circuits (digital-to-analog), and the use of analog methods to implement traditional digital circuits (analog-digital). In this issue we mainly introduce digital-analog circuits.
2. Digital - SimulationWith the development of semiconductor technology, more and more people are exploring to move the signals processed by the original analog domain to the digital domain for processing. This is not only unconventional, but also determined by the trend of semiconductor technology development. So, the fate of a circuit does not know the struggle of engineers, but also the historical process.
So, how does semiconductor technology make digital technology invade analog circuits? As we all know, with the development of semiconductor manufacturing, the feature size of the device becomes smaller, and the device speed becomes faster. This is a good thing for both digital circuits and analog circuits. However, the feature size is reduced to pay a corresponding price. The first cost is that as the gate oxide layer of the MOSFET becomes thinner, the breakdown voltage of the device decreases, so the power supply voltage also decreases. This is a problem that the performance impact on the digital circuit is offset by the faster the gate capacitance becomes smaller, so the problem is not too big for the analog circuit. First, the primary indicator of an analog circuit is the signal-to-noise ratio. Reducing the voltage means that the signal swing is reduced, but the noise does not decrease synchronously with the supply voltage, so low voltage has an adverse effect on the signal-to-noise ratio. Secondly, the threshold voltage of the device will not be reduced synchronously with the power supply voltage due to leakage, which makes the circuit structure that analog engineers like cascode and other engineers like can no longer be used at low voltage. Finally, because the threshold voltage is not synchronously reduced with the supply voltage, the operating range of the device is closer to the subthreshold region at low supply voltages, so the linearity of the analog circuit is worse.
In addition to the problems associated with reduced supply voltages, the price per unit area of ​​advanced semiconductor processes is high, but the overall size of analog circuits does not scale down as features shrink (especially passive devices such as inductors, their size and characteristics) The size doesn't matter at all, so the cost of analog circuits is actually rising in advanced semiconductor processes.
Contrary to analog circuits, feature size reduction is beneficial to digital circuit performance (except for increased leakage current, but this can be said to be no performance:)), so digital circuits can be used to replace analog circuits as much as possible to truly enjoy the benefits of semiconductor process evolution. .
From the track comparison of ISSCC in the past three years, we can see that the number slowly enters the analog field to replace the trend of classic analog circuits. In the ISSCC's 10 large tracks, the analog track includes traditional analog circuits and power management (Analog), and analog-to-digital converters (Data Converters). Digital tracks include architecture and systems (Digital Arch and Sys), and digital circuits (Digital Circuits). However, in the list below, there are also interesting sessions like "Digital PLL" and "Digital V-Regulator and Low Pwr".
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