Electronic Design

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.

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.

The migration to lower power-supply voltages, along with the desire by many OEMs for a "universal" IC op amp, has led to the development of more op amps that operate from rail-to-rail supplies, either at the input, output, or both. This has challenged op-amp designers to maintain minimum distortion levels and sufficiently high resolution while operating within a smaller voltage swing.

In a conventional op-amp design, distortion is minimized by keeping the op amp's signal well within the device's operating voltage range. That's not difficult to do when the supply voltage swings between, say, +12 V and −12 V. But in newer designs that operate from a supply of 5 V or less (some down to 1 V), the signal swings within a few millivolts of the supply rail. Consequently, it's more difficult to maintain low distortion and high resolution.

Some op amps offer rail-to-rail outputs. Others offer rail-to-rail inputs, while still others offer rail-to-rail capability on both inputs and outputs. The choice of I/O rail-to-rail capability depends on design requirements. In general, an op amp with a rail-to-rail input suffers an order-of-magnitude decrease in precision performance. Still, some customers opt for both input and output rail-to-rail capability to cover different applications.

"All of our CMOS op amps have rail-to-rail capability for both input and output," says Art Eck, analog product marketing manager for Microchip Technology. "Performance constraints of rail-to-rail input and output op amps is an architectural problem that can be overcome with greater attention to the op amp's input circuit."

Last year, Texas Instruments introduced the 1.8-V OPA 363/364 IC op amps, whose rail-to-rail input design provides good performance while driving analog-to-digital converters (ADCs) like the company's 50-ksample/s 14-bit 1.8-V ADS8324 (Fig. 2). There's no crossover on the input side of the op amp, reducing distortion and improving common-mode rejection ratio (CMRR). The input makes use of a single stage and an internal charge pump.

One noteworthy single-supply op amp with both input and output rail-to-rail capability is the micropower 2.7-V Maxim MAX 4194/4197 precision op amp with 97 µA of supply current and 8 µA of shutdown current. This suits it well for instrumentation applications using a traditional three-op-amp configuration (Fig. 3).

The CMOS process is finding favor for good rail-to-rail input and output operation. Many such op amps are winding up in a host of video-output circuit applications for digital cameras, PDAs, and cell phones. Michael Steffes, TI's strategic marketing manager for the High-Speed Signal Processing Group, points to the company's 12-V OPA 725/726 IC op amp as a good example of a device made on a high-speed CMOS process with a low noise level of just 6 nV√Hz. In comparison, other op amps feature nominal 20 to 30 nV√Hz. CMOS also brings solid bandwidth performance, such as the 150-MHz bandwidth of TI's OPA300, a product of the company's HPA07 process.

Despite its dominance, the CMOS process has precision and voltage limitations that prevent its use in some applications. "About 70% of IC op amps with both rail-to-rail input and output are of the bipolar variety," says Bruce Trump, strategic marketing manager for TI's Precision Linear Group.

This is why some companies like Linear Technology use a complementary bipolar process to produce op amps with rail-to-rail output-only capability but with impressive performance like the company's LT6210 in a six-lead ThinSOT package. This 200-MHz op amp features adjustable supply current and bandwidth. The supply current is programmable from 300 µA to 6 mA, corresponding to a bandwidth ranging from 10 MHz to 200 MHz (Fig. 4). It features a high slew rate of 700 V/µs and high output drive current that ranges from 0.05 V to 2.85 V on a single 3-V power supply. Other features include low distortion of −75 dB at 1 MH and a fast settling time of 20 ns.

The need for smaller packages presents new design challenges. The trend now is toward surface-mount, chip-scale, and flip-chip technologies. As the package shrinks and performance rises, lead lengths and bond pads become critical elements that affect performance levels. As a result, some op-amp makers are turning to leadless packages.

How small can a high-performance op-amp package get? Take a look at Linear Technology's LT6011, a 150-µA precision IC op amp with a rail-to-rail output in a 3- by 3- by 0.8-mm-high dual fine no-lead (DFN) leadless package (Fig. 5). It has the same footprint as an SOT-23 and features a drift of less than 1 µV/C. The company is the first to offer IC op amps in a DFN package. Aside from its low profile and small footprint, the LT6011 provides low thermal resistance for space-constrained applications.

The most popular IC op-amp package is the SOIC-8. Other popular packages include the SOT-23 and the SC70. Analog Devices uses a lead-free chip-scale package (LFCSP) with a pinout that differs from traditional pinouts to maximize thermal and other op-amp performance parameters.

"Ultimately, the trend toward a smaller-footprint package will drive IC op-amp designs toward more functions being integrated on the chip, like data converters and other circuits," explains Microchip's Art Eck.

But it's not clear how much smaller the package can be made using the existing infrastructure of process equipment. "We see a limit on how small a practical op-amp die can be made before it becomes impractical to package it," says Analog Devices' Bob Esdale.

"Much of the trend toward smaller-footprint packaging is driven by lower power-supply voltages," says Erroll Dietz, product line director for National Semiconductor. The company was the first to introduce the popular SOT-23 package in 1995, followed by the SC70 (called "silicon dust") two years later, then flip-chip and smaller packages.

Last year, National Semiconductor unveiled the LMV1012/1014, the first "amps in a mic" (Fig. 6). These amplifier-in-a-microphone chips replace the usual JFET input in an op-amp IC with a CMOS device. This results in greater audio fidelity in mobile handsets, headset accessories, and other portable amplifier applications.

National Semiconductor credits the VIP10 10-GHz high-performance bipolar process for many of its op-amp advances. The process, which uses vertically integrated pnp transistors, was designed to produce high-performance op amps to drive lasers. National Semiconductor also developed a VIP10 plus CMOS process.

"The end user who isn't traditionally analog savvy needs a circuit that takes the transducer's output signal and processes it on the spot. This means that the circuit must have the op amp and the data converter on the same chip," adds Dietz.

Not everyone agrees that integrating sensors and transducers on the same chip is a good idea, since it takes away design flexibility for many applications. Some IC op-amp designers have taken another approach with custom linear designs that pack as many as 50 precision op amps on one chip for multichannel sensor applications.

But given the way op amps are achieving higher and higher integration levels, it's only a matter of time before the sensing element and all of its related signal-processing circuitry will be on the same chip as the op amp. It's a matter of economics. The monolithic op-amp manufacturer has to be able to deliver cost-effective data-acquisition and control building blocks, and the op amp is but one small part of such a block.

Need More Information?
Analog Devices
(800) 262-5643

Intersil, Elantec Business Group
(800) MCU-MCHP

Linear Technology
(408) 432-1900

Microchip Technology
(408) 945-1323

Maxim Integrated Products
(800) 998-9872

National Semiconductor
(408) 721-5000

Texas Instruments
(800) 477-8924, ext. 4500


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