[Technology Report]
Monolithic IC Op Amps Keep Pace With Design Needs
Driven by architectural, process, circuit design, and packaging improvements, this low-cost building-block device is available in a plethora of performance choices to suit any design.
Ever since Robert Widlar designed the µA702 in 1963, the first monolithic op-amp IC from Fairchild Semiconductor (followed by the venerable µA709 in 1965), IC op amps have largely kept pace with design demands and sophisticated manufacturing processes. Besides the traditional bipolar process, multiple-process technologies now dominate, including CMOS, biCMOS, complementary bipolar (CB), silicon germanium (SiGe), and gallium arsenide (GaAs). CMOS is now the dominant process.
"The widely popular µA741 of the 1970s offered a 1-MHz bandwidth at 10 mA of current drain. In comparison, designers can now have a 350-MHz IC op amp, the AD8038/8039 (single/dual), with an exceptionally low quiescent current of 1 mA/op amp," says Bob Esdale, Analog Devices' product line director for high-speed linear products. "This is an example of how we've always looked at op-amp power efficiency, which is being driven by our XFCB process." The XFCB process allows low-noise operation (8 nV√Hz and 600 fA/Hz) at extremely low quiescent currents.
Indeed, IC op-amp performance continues to rise dramatically, to hundreds of megahertz and over a gigahertz, as exemplified by the Maxim SiGe 400-MHz to 2.5-GHz MAX 2640/2641 ultra-low-noise op amps with noise figures of 0.9 dB (at 900 MHz) and 1.2 dB (at 1.9 GHz), respectively. The same can be said for parameters like dynamic range, distortion, low noise, and low power. These continue to improve despite the need for decreasing power-supply voltages and smaller package footprints, driven by a vastly large portable electronics world powered by batteries.
Additionally, there's a critical need to operate faster without burning up lots of current. Exemplifying the latest in high-performance IC op-amp advances is the OPA683 from Texas Instruments, a low-cost 180-MHz current-feedback device with a 1-mA current drain.
Even the matter of specifying an IC op amp is becoming easier. Maxim Integrated Products recently launched a Web-based service that lets users enter their desired op-amp specifications. Within 24 hours, they'll receive a specific recommendation, via e-mail, from an applications engineer.
The trend is to make greater use of single-ended IC op amps that operate from one supply voltage. This dovetails nicely with the world of digital logic and memory devices. But it also comes at the expense of a smaller dynamic range, signal-to-noise ratio, and rail-to-rail voltage swing. On the low-speed end, digital trimming and offset adjustments, as well as improvements in low-voltage performance, are driving the trend toward greater levels of on-chip integration.
In addition, there's a move to greater digital control of the analog op amp. Just one example, the MCP6S21/22 family from Microchip Technology, can be controlled digitally via the serial peripheral interface bus. The company's 604x/614x represent the lowest-current IC op amps in the industry. They draw just 600 nA, an extremely important number for battery-powered devices and distributed systems.
Clever calibration and chopper-stabilized designs, both digital and analog, have evolved over the years to maintain IC op-amp stability over a wide bandwidth and operating-temperature range. And, digital techniques are becoming more common for controlling input and output voltages of variable-gain IC op amps.
BURSTING WITH PERFORMANCE There's no shortage of performance improvements in IC op amps, regardless of the process they're made on. Just a few mentions of worthy products offer a snapshot.
Low noise levels can be seen in Linear Technology's 623x rail-to-rail output IC op amps. These devices offer the lowest noise levels of 1 nV√Hz at 3 mA (the LT6230) and 2 nV√Hz at 1 mA (the LT6232) for baseband audio applications.
Low-noise performance combined with low-offset performance can be seen in National Semiconductor's LMV771. The op amp possesses a very low offset of 1 mV as well as guaranteed low noise of 7 to 8 nV√Hz, all without the need for trimming. An even lower offset of 5 µV is available from the company's LMV2011 housed in an SOT-23 package.
For driving video loads, Intersil's Elantec Unit claims the fastest fixed-gain IC op amps in the EL5106 and EL5108. These op amps with fixed gains of +1, −1, and +2 feature bandwidths of 350 and 450 MHz, as well as slew rates of 4000 and 6000 V/µs, respectively. Also, Linear Technology's LT6553, a 700-MHz IC op amp, suits video applications.
No op amp offers "ideal" performance, as some companies would have you believe. In general, op amps can be considered voltage-feedback and current-feedback (transimpedance) types. The former offers lower noise, better dc performance, and greater feedback flexibility. The latter has a wider bandwidth, faster slew rates, and lower distortion levels, but it's limited in terms of feedback flexibility. So which type you choose involves trading the performance parameters you need.
"Navigating through the jungle of IC op-amp architectures and performance specifications is a challenging task," says Erik Soule, general manager for the Signal Conditioning Business Unit of Linear Technology (see "Op-Amp Tradeoffs," p. 76).
One factor pushing op-amp performance is the development of higher-speed and higher-resolution data converters. For instance, there's Analog Devices' AD8099 (Fig. 1). This low-distortion and low-noise device (−90 dB at 10 MHz and 0.95 nV√Hz) is designed to drive 16-bit converters at a low price. Unlike other IC op amps that offer either low-distortion or low-noise performance, this one features both.
So far, analog op amps have taken advantage of being made on leading-edge, largely digital processes with line widths of 0.5 µm to 0.35 µm (and even down to 0.18 µm), even in the face of decreasing supply voltages. Designers have used multistage design architectures without cascoding, instead of the usual highly cascoded architectures. But further shrinking of process line widths, like the proposed 90-nm CMOS processes, are sure to tax the ingenuity of analog op-amp designers. A fundamental limitation is sampling, because thermal noise voltage increases with shrinking line widths.
Some experts propose the use of analog circuits that operate at higher supply voltages than what the digital core transistors require. In fact, some op-amp manufacturers are designing their products using I/O transistors, with gate lengths on the order of 0.35 µm (for 3.3-V I/Os) and 0.25 µm (for 2.5-V I/Os).
While precision performance has been the norm for low-speed op amps for a long time, that goal is now being directed at obtaining higher-accuracy junction field-effect transistor (JFET) op amps. This is important for certain high-speed applications like medical computerized-tomography (CT) scanners, wireless basestations, optical networks, and automatic test equipment (ATE), where offset voltages of less than 0.1 mV and drifts of 0.5 µV/°C are needed.
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