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Op-amps’ response to data-acquisition is fast and furious

With performance requirements in data acquisition going through the roof, there's overnight demand on the system front end, i.e., the operational amplifier (op-amp), to do much more in a smaller package. And chip makers are responding with innovative chips as quickly as the need for better devices comes down the pike. Indeed, the front-end is the weakest part of the signal chain. Op-amps are now approaching the point where they can get totally “out of the way” of the analog-to-digital converter (ADC) so as to not limit its performance. To this end, the latest op-amp arrivals have significantly improved their AC and DC accuracy, provide higher gain at lower noise, and conserve more power while doing it all at lower cost— fairly universal goals despite the op-amp's very wide breadth of applications.

The major activity focuses in three prime areas: analog, industrial (medical, instrumentation and automotive, including substantial grey areas in between), and video, with the casual user able to roughly correlate the application to the op-amp's unity-gain bandwidth or gain-bandwidth specs and slew rate. In a few more cases, IC makers are bringing attention to technology differentiators (e.g., bipolar versus CMOS versus J-FET). Thus far, though, those selling points have met with a largely neutral reaction as designers look for the best specs any way they can be had&#8212ultimately the same response they had to such marketing strategies for power MOSFETs and switching power supplies in their heyday.



Audio and low frequency
National's LM4562 dual op-amp, particularly for high-end audio applications (hi-fi preamps, phono preamps, professional audio, equalization and crossover networks, active filters, line drivers, line receivers), claims benchmark performance for its combination of high-speed, low distortion, wide operating voltage, and high output drive. The amp, with a unity-gain bandwidth of 6 MHz and gain-bandwidth of 56 MHz, can deliver up to 45 mA and is unity-gain stable over a supply range of ±2.5 to ±17 volts. Common-mode and power-supply rejection (PSRR) is more than 108 dB. The op amp's output can swing to within 1 volt of the supply-voltage rails when driving a 2-kilohm load, or to within 1.5 volts when driving a 600-ohm load. The op-amp features a no-click, no-pop mute function, thermal shutdown, and output short-circuit protection.

Linear Technology's LT6003 (single), LT6004 (dual), and LT6005 (quad) rail-to-rail op-amps draw less than 1 microamp in operation at 1.8 volts, a new benchmark for precision low-voltage devices and tiny footprint, according to the company. The LT6003 in particular, in a 2-by-2 DFN package that's suited to active filters and handheld instrumentation such as gas monitors, has an input-offset voltage of 500 microvolts maximum at +25°C and maximum drift of 5 microvolts/°C, which reportedly makes the device the most precise op-amp and the smallest of its kind. As such, the current draw for these amps, which generally go up by a factor of three as the output approaches the rails, isn't compromised.

Analog Devices' AD866x portfolio, using the company's new industrial CMOS (iCMOS) technology, brings together high-voltage silicon processor and complementary bipolar technologies that, along with the company's Digitrim technology, is designed to provide the optimum price-performance match for a variety of instrumentation applications. The 4 MHz devices include the single AD8661, dual AD8662, and quad AD8664 parts, which with a voltage offset of 100 microvolts, tout the industry's highest precision for applications requiring the highest levels of DC accuracy (motor control, medical instrumentation, automotive sensors). DigiTrim is a digital trimming process that allows production of precision amplifier components at costs that are up to 30 percent less than competitive solutions. Other devices include Analog Devices' rail-to-rail AD8500, which reportedly consumes 30 percent less power than similar devices for battery-powered consumer and medical applications. Maximum supply current is 1 microamp.

Cirrus Logic's single- and dual-channel CS3003/13 and CS3004/14 claims the industry's most stable amps for low-signal-level thermocouple and gas detector applications. These next generation parts cut power consumption from 1,400 to 500 microamps at a slight tradeoff in noise performance.

Particularly suited to transducers, Texas Instruments' OPA333, with a gain-bandwidth of 350 kHz, bolsters its high-precision status with a proprietary auto-calibrated circuit for zero-drift offset.

Industrial and instrumentation
Op-amps for the higher-end industrial sector continue to attract the most popular attention, no doubt owing to the breadth of applications. Indeed, the op-amps applied in this area often share the same general specs. Clearly there's a blurring, with op-amps suited for instrumentation usually lower frequency devices, and industrial/medical applications often implying small, fast-changing signals that need a high-gain, very low noise op-amp.

Thus, the range of applications extend from very high speed DSP processing required by the most advanced industrial and medical imaging systems, to extremely precise low-signal-level work required at the other (but no less important) end of the medical spectrum, such as in-the-field instrumentation. The highest end products include Texas Instruments' fully differential THS4520, with a bandwidth of 600 MHz, suited to high-speed 12- and 16-bit ADCs for 3.3- and 5-volt data acquisition systems, test and measurement, and medical imaging applications. To this end the op- amp, optimized for the company's dual 12-bit, 65 MSPS ADS5232, and 16-bit 4 MSPS ADS8422, boasts the industry's lowest harmonic distortion through 100 kHz.

STMicroelectronics' TSH300 for high-end industrial, medical, and instrumentation, with a gain-bandwidth product of 1 GHz (i.e, bandwidth of 200 MHz at a gain of 5), uses a 0.25 micron complementary bipolar process to claim the lowest noise in the industry (0.65 nV/Hz1/2 ) for a wideband amp in a SOT23-5L package.



Improving harmonic performance, TI's OPA365, utilizing an improved charge pump over the company's previous-generation OPA364, implements a zero-crossover input architecture to provide glitch-free rail-to-rail performance for audio through portable medical systems. The device, with a gain-bandwidth of 50 MHz, is optimized to work with the company's TMS320 DSP platforms. “We replace the op amp's complementary stage with a charge pump that biases Vin above the Vin using the traditional topology,” said Tadija Janjic, product line manager for Precision Linear products. “So the input stage is constantly on, and there's no change in offest voltage.” As a result, the OPA365 largely eliminates the harmonics that would be generated during crossover in an op-amp using the typical two-stage topology.

The test specialists
Moving to mid-range industrial applications, we find more specialized devices more attuned to low-level signals and test equipment. NEC Electronics' µPC835 dual-op amp, for example, touts an edge for test equipment applications. “We recognized the need for designers to have an op-amp designed specifically to meet the unique needs of semiconductor test equipment,” said Bart Ladd, general manager of the Standard Solutions Strategic Business Unit. As such, the J-FET op amp has a slew rate of 9 V/microsecond to facilitate precision measurements, capacitive load capability of 4000 pf (four times higher than competing products), and a small package (3-by-3 mm TSOP) that's suited for system-on-chip (SOC) and logic test equipment applications. Input offset is ±3 mV maximum.

Also claiming an edge in instrumentation applications on another level, Linear Technology's LTC6244, a CMOS op-amp touting a gain-bandwidth of 50 MHz, is challenging bipolar amplifiers for high precision and low noise. “This breakthrough performance, previously unattainable from a CMOS amplifier, delivers maximum signal resolution in a variety of applications such as large- and small-area photodiodes, transimpedance and charge-coupled amplifiers, and precision integrators and filters,” said Dan Tran, design manager. This rail-to-rail amp has an offset voltage of just 300 microvolts maximum over the full industrial temperature range, reportedly rivaling the +25°C spec for other amplifiers in its class. The LTC6244 boasts low noise—8 nV/Hz1/2 . In addition to its low voltage gain error, the LTC6244 boasts high DC precision. Input offset voltage is less than 100 microvolts at +25°C.



Other specialized chips include Analog Devices' AD8336, a DC-coupled variable-gain amplifier that touts the industry's highest gain and bandwidth—100 MHz bandwidth over a gain range of 60 dB (-14 to +46 dB). Basically an op-amp with attenuator, it's the only such amplifier, according to the company, that's rated for operation over -55 to +125°C and a wide supply input (±3 to ±12 volts). “Designers of single-ended industrial and instrumentation systems who need variable-gain amps require good AC and DC performance over temperature and wide supply ranges,” said Steve Sockolov, product line director for the Precision Linear Products Group. “However, most amps are designed with RF applications in mind and operate on single supplies. The AD8336 is the first variable-gain amp capable of processing high-frequency bipolar signals at high gain.”

Basic test and measurement
Among the so-called conventional instrumentation amplifiers, Analog Devices' AD8250, a 12 MHz precision op amp, provides a settable gain of 1, 2, 5, and 10, and the AD8251 can be set for a gain of 1, 2, 4, and 8 for test and data-acquisition applications. As for DC performance, these devices have low noise (below 15 nV/Hz1/2 at a gain of 10), gain drift better than 10 ppm/°C, and input offset below 100 microvolts. Similarly, the company's AD8231 claims near zero drift and input noise that closely approximates the ideal instrumentation op-amp. The device, with a gain-bandwidth of 1 MHz, features programmable gain from 1 to 128, which is set by a three-pin interface or is hardware-settable via pin strapping. The device also touts an uncommitted op-amp that can be used to facilitate various auxiliary functions.

National Semiconductor's 9 new op-amps in the company's LPV , LMV and LMP series, with bandwidths extending from 4 to 17 MHz, serve industrial, medical, and automotive applications. National characterizes the LPV/LMV series of low-voltage, low-power op-amps as more suited to general purpose instrumentation. The single, dual and quad LMP amps, on the other hand, provide precision performance. In addition, the company's LM6211, a CMOS design with a bandwidth of 20 MHz, features a wide operating range (up to 24 volts).

Video vistas
Applications for largely consumer systems continue to grow. ON Semiconductor's NCS25xx family of 500 MHz to 1.4 GHz devices are designed for high definition video applications (set-top boxes, security video cameras, video switching). The NCS2510 and NCS2511 are current-feedback op-amps that work at 1.4, and 1 GHz, respectively. The NCS2535 (1.4 GHz) and NCS2540 (750 MHz) are triple op-amps suited for driving component video. The NCS2550 and NCS2552 are voltage-feedback op-amps working at 750 MHz, and the NCS2551 is a voltage-feedback op-amp working at 500 MHz.



Fairchild Semiconductor's five new devices in the company's 210-MHz FHP3x50 and 50-MHz FHP3x30 familes for high definition (HD) and standard definition (SD) video are the company's first entries into the high-performance op-amp market, yet designed to compete with the best. “The FHP3x30 family, for example, provides the lowest available power consumption combined with 33 percent higher bandwidth, 33 percent more output current and 55 percent better DC performance than comparable devices,” said Debbie Brandenburg, marketing engineer. “And our FHP3x50 family offers best-in-class input bias current, which is an order of magnitude better than any other device in this performance range.”

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