[Technology Report]
Analog Front Ends Max Out Performance
These mixed-signal chips fit their target applications like a glove, speeding up system design and ultimately time-to-market.
Once regarded as more of a custom chip, the analog front end (AFE) has come into its own as a standard offering, whether the application is specific or general-purpose. Today, designers can select from multiple AFE configurations, satisfying a wide range of market requirements.
These versatile ICs accelerate system development by combining a certain amount of digital control and sometimes signal processing with data converters. Many AFE chips are application-specific, targeting wired and wireless communications, image processing, and industrial control, just to name a few.
In contrast, other AFEs are general-purpose building blocks. These may be "little-d, big-A" devices (some digital circuitry, much analog circuitry), with a simple state-machine-controlled multiplexer feeding one or more data converters. Or, they may be "big-D, little-a" chips (mostly digital with a smattering of analog) that include one or more data converters along with other microcontroller (MCU) peripherals.
Generally, the common functional denominator in all AFEs is their data converters—both digital-to-analog converters (DACs) and analog-to-digital converters (ADCs). The schemes for DACs don't differ greatly, but the architectures for ADCs may be delta-sigma, successive-approximation, or pipelined. Each architecture has limitations in terms of throughput, resolution, latency, filtering requirements, power consumption, and silicon footprint. Not surprisingly, the various converter architectures affect performance in the target application.
A BANNER YEAR... Over the past 12 months, a number of new exciting AFEs took aim at various applications, including both wired and wireless communications, industrial electronics, and consumer imaging. Several workhorse general-purpose AFEs have also arrived.
...for Wired Communications. DSL and other wireline communication modes constitute one of the larger markets for AFEs. Analog Devices, Texas Instruments, and STMicroelectronics, among others, have portfolios of parts that target commodity-volume applications.
Now, thanks to the promise of standards, powerline-based networking represents a new and challenging applications arena. Late in 2003, Analog Devices released the AD9865 (see "Taming the Wild Powerline," p. 56), which supports powerline networking, VDSL, and Home Phoneline Networking Alliance (HPNA) broadband modems.
Chip companies are forever driven to find the right combination of features to bundle in their AFEs. For instance, STMicro recently introduced a new AFE as part of a two-chip set for a USB-based, rate-adaptive ADSL modem. The MTC20154 consists of a 12-bit DAC and a 13-bit ADC, both running at 8.8 Msamples/s. In this chip set, almost all digital processing is handled by the companion chip, the MTC20455, a common functional partitioning among all AFE suppliers.
...for Wireless Communications. Bluetooth and the alphabet soup of IEEE 802.11 standards continue to propel the wireless AFE market. Earlier this year, STMicroelectronics introduced the STLC2150, a fully integrated Bluetooth single-chip radio transceiver intended to work with a variety of standard BlueRF-interface baseband processors, including STMicro's own STLC2410 (Fig. 1).
A more general-purpose AFE for the IF receive channel in software radios can be found with TI's AFE8201 (Fig. 2). It samples narrowband (2.5-MHz or less) IF signals and digitally mixes, filters, and decimates the signals to baseband.
...for Industrial Electronics. Aimed at the industrial market, Analog Devices' ADS7869 is a 12-channel, three-ADC motor-control front end. The AFE offers three fully differential inputs. Each input connects to a window comparator and a sign comparator. A digital interface on the chip's parallel port can be configured for different standards. Moreover, there's a serial peripheral interface (SPI) for control.
...for Consumer Imaging. Late last year, Philips Semiconductors introduced the TDA8754 triple 8-bit video data converter for liquid-crystal-display (LCD) monitors, projectors, and televisions. The IC accepts either analog RGB or YUV signals and converts them to digital output for use in either high-speed flat-panel displays with resolution up to QXGA (2048 by 1536 at 85 Hz) or in high-definition television receivers. At the input end of the imaging chain, Analog Devices maintains a robust portfolio of AFEs customized for a range of CCD and CMOS digital imagers.
...for General-Purpose AFEs. Several companies, including Analog Devices, Linear Technology, Maxim, and Silicon Labs, employ silicon CMOS to add programmability to instrumentation-type ADCs. The difference is in the style of programmability—state machine or MCU.
Todd Nelson, Linear Technology's product marketing manager, says the company's ADCs with pin-strap programmed multiplexers on their inputs are popular with engineers who design process-control systems or sensors. These engineers don't want to spend design time mating a separate multiplexer with their ADC.
Different vendors have different approaches to multiplexed-ADC AFEs. These include pin-strap or register-controlled (via a serial interface) programming and single or multiple data converters.
In Linear's LTC1850/51, an on-chip eight-channel multiplexer feeds a 10- or 12-bit, successive-approximation ADC (Fig. 3). The AFE features a scan mode that will repeatedly cycle through all eight multiplexer channels and is programmable with a sequence of up to 16 addresses and configurations that can be scanned in succession. It's also possible to read back the sequence memory. All of this is controlled by strapping the appropriate pins on the AFEs' packages.
The 8- and 10-bit members of the LTC1850 family each boast a pair of single, successive-approximation ADCs with a built-in 8-channel multiplexer. Absolute sampling rates are on the order of 1.25 Msamples/s. However, actual rates depend on how many inputs are being sampled. That is, if the design called for only two input channels, each would be connected to four of the multiplexer's inputs, and the effective sampling rate for each channel would be 625 ksamples/s. With a different input on each multiplexer channel, throughput would be 156 ksamples/s.
A similar design concept (with a specific application focus) lies behind Analog Devices' AD7266. The device integrates two separate 12-bit, successive-approximation ADCs, which allow for simultaneous sampling and conversion of two channels with throughput rates to 2 Msamples/s. Each ADC is preceded by a three-channel multiplexer, as well as a low-noise, wide-bandwidth track-and-hold amplifier that can handle input frequencies in excess of 10 MHz. Every ADC has two analog inputs, and there are three fully differential pairs or six single-ended channels that can be programmed. The conversion result of each channel can be read simultaneously on separate data lines or in succession on one data line.
Maxim's MAX1402 multiplexes fewer signals but exhibits greater resolution, with a delta-sigma modulator followed by a digital decimation filter to achieve 16-bit accuracy (Fig. 4). The digital filter's user-selectable decimation factor allows the conversion resolution to be reduced in exchange for a higher output data rate. True 16-bit performance is achieved at an output data rate of up to 480 sps.
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