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Circuit-Level Considerations for Mixed-Signal Programmable Components

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Circuit-Level Considerations for Mixed-Signal Programmable Components Field-Programmable Mixed Systems Circuit-Level Considerations for Mixed-Signal Programmable Components Luigi Carro, Marcelo Negreiros, Gabriel Parmegiani Jahn, Adão Antônio de Souza Jr., and Denis Teixeira Franco Universidade Federal do Rio Grande do Sul tors in MOS technology take up too much The use of digital compensation algorithms eliminates the error introduced area. Moreover, programmability general- by switches and the nonlinear behavior of MOS transistors. This approach ly comes in the form of MOS switches. greatly reduces analog area and permits field-programmable mixed-signal systems built with entirely digital technologies. These transistors introduce some extra pole-zero pairs, and worse, they have non- linear voltage-current characteristics. IN THE PAST DECADE, researchers have paid signifi- We will show that, even when using analog tech- cant attention to the design of heterogeneous embedded nologies that allow components like linear capacitors systems. However, most of their work focused on digital or resistors, the resulting circuit will display nonlinear hardware and associated software,1,2 which remain firmly behavior because of the programmability requirement. rooted in an all-digital environment. In contrast, an embed- Moreover, this approach will still consume significant ded system must generally interface with the real world. At circuit area. this level, most signals are continuous time and can assume For these reasons, designers need a new paradigm only analog values. Most sensors (such as those for tem- for designing analog circuits. Here, we propose the use perature, pressure, humidity, resistance, and so on) gener- of nonlinear analog components with digital compen- ate small voltages or currents, and systems must preprocess sation. Such components can be area effective, allow- (amplify and filter) these signals before using them. ing designers to easily build important circuits. It is also Currently, many applications use digital signal pro- possible to directly develop this class of circuits in a dig- cessing because of its fast design cycle, available tools, ital technology. Moreover, the proposed circuit tech- and integration with current design methodologies. nique is easily programmable using switches and incurs Designers can use this technique in any problem that only a slight performance degradation. Some of our does not require a gigahertz frequency bandwidth or results, such as those for amplifiers and integrators, use extremely low power dissipation. Nevertheless, although only a small area and demonstrate a perfect balance digital processors can implement many analog functions, between the possibility of programming an analog or a some analog functions must be developed in the analog digital device in the same chip. This methodology domain. For example, amplifiers, low-pass antialias fil- allows the development of linear or sampled circuits ters, and an A/D converter can be developed only in the like switched capacitors. analog domain; the digital domain has no equivalent. So Although some work has addressed analog field- although most of a mixed-signal system might be digital, programmable gate arrays,3-5 most of it focuses on the such systems still require analog functions. prototyping of analog functions like filters and ampli- For linear behavior, a system should in principle use fiers. However, the embedded-systems market seems linear components. However, linear capacitors or resis- to require mixed-signal programmable systems, not 76 0740-7475/03/$17.00 © 2003 IEEE Published by the IEEE Computer Society IEEE Design & Test of Computers Continuous Antialias Amplifier Sampler A/D converter system filter Figure 1. Input path for the target applications of the proposed field-programmable mixed system (FPMS). only analog circuits. In a mixed-signal context, the process generally needs some amplification, because most important design parameter might be the trade- sensors rarely give the required voltage or current levels. off between speed and accuracy, as in the case of Because of present-day technology and supporting tool Σ-∆ converters. Most of the signal processing should sets, most signal processing occurs in the digital domain, occur in the digital domain, because it is much sim- so an antialias filter is absolutely required. Finally, pler to design and easier to prototype and to automate depending on the required converter’s speed and reso- than in the analog domain. Also, a mixed-signal chip lution, an application might need a sampler to maintain will include a microprocessor of some sort that a the resolution while working with fast signals. designer could use to develop the signal processing in Assuming that Figure 1 covers most applications that the digital domain. we want to address, several parameters must be subject to programmability. The signal from the continuous sys- Considerations for mixed-signal tem can be either a current or a voltage signal, which programmable systems means that the amplification stage should be able to Many electronic devices, such as current system- change its input impedance and gain. In addition to this on-a-chip (SoC) designs, contain components that requirement, the signal’s frequency response must be have increasing levels of integration and fit into a sin- treatable in a second stage that can implement filters. gle chip. However, when it comes to field configura- This, in turn, means that we must be able to place poles bility, just as for field-programmable gate arrays and zeros at certain frequencies. (FPGAs), the analog interface to the world—which Finally, analog sampling should be one of the pro- includes all conditioning and acquisition circuits— grammable features. You should be able to choose remains outside the IC. Maintaining this separation whether A/D and D/A converters are multiplexed requires external analog devices and increases the between channels and whether the system should trade probability of connection-related malfunctions. Such speed for resolution in any of the A/D or D/A converters. a situation is typical for the analog front end in systems that perform tasks such as data acquisition in control Analog front-end topology and measurement applications. Looking at Figure 1, we can divide the analog block One goal of the proposed programmable analog cir- into three main parts. The first part receives and ampli- cuit is to cover such needs in a SoC environment. For fies the analog signal; the second performs the antialias this environment, we would like to provide a front-end filtering on the signal; and the final block converts the analog subsystem that permits full development, proto- signal to the digital domain. In a design, we must con- typing, and testing of typical embedded applications. sider which topology to use in each of these stages to The entire field-programmable mixed system (FPMS) is achieve an optimal combination of programmability usable in the same way as purely digital FPGAs. and overall performance. Designers could then fully develop an acquisition device Our initial efforts to design and test the building composed of a sensor, its conditioning circuitry, and the blocks for the future FPMS’ logic cell focus on continu- digital data manipulation and storage block on the FPMS ous-time circuits. Other groups have already used device. They could also avoid testing delays by elimi- switched capacitor and switched current as the basic nating the need for designing a board and assembling technology for the programmable analog part.3-5 In con- all the necessary ICs and analog components. trast, we chose the continuous-time implementation to Figure 1 shows the typical front-end topology for a ensure that all the circuitry before the antialias filter desired target application. A real-world, continuous-time would not introduce spurious frequencies. January–February 2003 77 Field-Programmable Mixed Systems ment the programmable analog part. The Programmable first topology could be one presented ear- biquadratic lier, which offers an array of amplifiers, filters Programmable Time-discrete switches, and capacitors.3-5 This work uses amplifiers and processing instrumentation and A/D only linear devices, a distinct disadvantage amplifiers Routing Routing conversion in terms of area usage. The programmable Programmable integrators topology proposed by Pavan, Tsividis, and Nagaraj would be another possibility.6 This work addresses the problem of filter pro- Figure 2. FPMS macroarchitecture. grammability; its implementation uses only MOS transistors and gate-to-channel capacitors. Although this implementation First stage: Analog-signal amplification uses nonlinear capacitors, the capacitors are biased so To amplify the input signal, we must provide a block that they operate in their linear region. Our approach is that can implement voltage and current gain. We started quite different than all previous work because it does not with the well-known differential amplifier with three oper- cancel or mask nonlinearities in the analog domain, but ation amplifiers (op amps) and also provided shortcuts rather works within the digital domain. to allow one input to use only the final inverting stage. Thus, every two analog inputs are associated with three Final stage: Discrete-time and signal op amps, and are usable as differential voltage inputs. digitalization The first input of each pair can also bypass the input pair We will separate this stage from the filtering row by of op amps and use the third op amp as an inverting using a connection
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