1.1.2 Common Types of Analog and Mixed-Signal Circuits


Analog circuits (also known as linear circuits) include operational amplifiers, active or passive filters, comparators, voltage regulators, analog mixed, analog switches, and other specialized functions such as Hall effect transistors. One of the very simplest circuits that can be considered to fall into the mixed-signal realm is the CMOS analog switch. In this circuit, the resistance of a CMOS transistor is varied between high impedance and low impedance under the control of a digital signal. The off-resistance may be as high as one megaohm or more, while the on-resistance may be 100 Ohm or less. Banks of analog switches can be interconnected in a variety of configurations, forming more complex circuits such as analog multiplexers and demultiplexers and analog switch matrices. 


Another simple type of mixed-signal circuit is the programmable gain amplifier (PGA). the PGA is often used in the front end of a mixed-signal circuit to allow a wider range of input signal amplitudes. Operating as a digitally adjusted volume control, the PGA is set to high gains for low-amplitude input signals and low gains for high-amplitude input signals. The next circuit following a PGA is thus provided with a consistent signal level. Many circuits require a consistent signal level to achieve optimum performance. These circuits therefore benefit from the use of PGA's.



PGAs and analog switches involve a trivial interaction between the analog and digital circuits. This is why they are not always considered to be mixed-signal circuits at all.  The most common circuits that can truly be considered mixed-signal devices are analog-to-digital converters (A/Ds or ADCs) and digital-to-analog converters (D/As or DACs). While the abbreviations A/D and ADC are used interchangeably in the electronics industry, this book will always use the term ADC for consistency. Similarly, the term DAC will be used throughout the book rather than D/A. An ADC is a circuit that samples a continuous analog signal at specific points in time and converts the sampled voltages (or currents) into a digital representation. Each digital representation is called a sample. Conversely, a DAC is a circuit that covert digital samples into analog voltages(or currents). ADCs and DSCs are the most common mixed-signal components in complex mixed-signal designs since they form the interface between the physical world and the world of digital logic.



Comprehensive testing of DACs and ADCs is an expansive topic, since there are a wide variety of ADC and DAC designs and a wide variety of techniques to test them. For example, an ADC which is only required to sample once per second may employ a dual slope conversion architecture, whereas a 100~MHz video ADC may have to employ a much faster flash conversion architecture. The weaknesses of these two architectures are totally different. Consequently, the testing of these two converter types is totally different. Similar differences exist between the various types of DACs.



Another common mixed-signal circuit is the phase-locked loop or PLL. PLLs are typically used to generate high-frequency reference clocks or to recover a synchronous clock from an asynchronous data stream. In the former case, the PLL is combined with a digital divider to construct a frequency multiplier. A relatively low-frequency clock, say, 50 MHz, is then multiplied by an integer value to produce a higher-frequency master clock, such as 1 GHz. In the latter case, the recovered clock from the PLL is used to latch the individual bits or bytes of the incoming data stream. Again, depending on the nature of the PLL design and its intended use, the design weaknesses and testing requirements can be very different from one PLL to the next.







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