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Manufacturers continue to develop the analogue ‘building blocks’

During New Electronics' Analogue Roundtable last month, the discussions focused on a number of areas. One was the increasing complexity of analogue components; another was their increasing functionality. But the participants also underlined the need for device developers to continue to develop what they called the 'building blocks' of analogue design.

The reason? While some engineers still like to use discrete components as part of their solutions, device developers are increasingly integrating these 'building blocks' into modular solutions. Without continuing to develop the building blocks, they can't create modules.

Bob Dobkin, pictured, Linear Technology's vp of engineering and chief technology officer, said: "When it comes to analogue building blocks, you would be hard pressed to name a device more basic than the operational amplifier. As a function, op amps are ubiquitous; they provide precise analogue functions without requiring the user to know precision transistor level design.

"The op amp and the components around the amplifier define the circuit characteristics. If precision low drift is needed, system designers don't need to know about precision low drift transistor design; they just need to pick the proper op amp. If wide bandwidths and low distortion are needed again, the proper op amp provides the solution."

Looking at the literature, it can be seen that devices which perform a function similar to today's op amps were being discussed at the beginning of the 20th Century. A device similar to an op amp featured in an artillery control system in the early years of World War II. But it wasn't until the development of semiconductor technology that the op amp came into its own.

Because the op amp is a building block, you might expect it to be integrated routinely in devices with more functionality – an approach being pursued by companies such as Linear Technology and Maxim. Yet that's not the case. Sales of discrete op amps continue to grow and the market is estimated to be worth some $3billion a year.

One reason for the op amp's continuing popularity is simply because they are good at dealing with 'real world' conditions. And, by specifying a discrete part, designers can have tighter control over the figures of merit – such as gain, noise and power – than may be available from a module.

An example of the respect shown to op amps came recently from Analog Devices, when it launched the ADA4528. Peter Real, vice president of the company's linear and rf products and technology group, said: "This new precision amplifier is a flagship product."

According to the company, a patented chopping technique allows the ADA4528 to achieve high levels of accuracy, making it suitable for low frequency sensor measurements in systems such as data acquisition, weigh scale and precision medical instrumentation.
A breakthrough is said to be a noise performance of 5.3nV/vHz at 1kHz. Elimination of unwanted 1/f noise is also said to improve the signal to noise ratio in low noise applications.

Drift with temperature and time is a concern in many designs. Looking to meet those needs, the ADA4528 employs patented technology to implement circuitry which continuously corrects the device, minimising drift over time.

Intersil has also recently added a high precision op amp to its portfolio. The ISL28028, a dual device, is aimed at industrial control applications. Manufacturing on Intersil's silicon on insulator bipolar process technology, the part has robust latch up immunity.

Operating from a 40V supply range, the device features wide common mode range inputs that function below ground. It also supports rail to rail output. Low offset is a design target and Intersil quotes a maximum offset of 230µV. Drawing just 2mW per channel, the part has a typical temperature drift of 0.1µV/°C.

Plessey Semiconductors is also focusing on the op amp market, recently announcing the PS13000 range of current feedback devices. The first parts will be quad configuration devices with four identical amplifiers integrated on one die.

Looking to combine high performance with low power consumption, the PS13004 features a small signal bandwidth of 280MHz and a slew rate of 1100V/µs. Capable of driving 65mA outputs, the part draws just 3.3mA of static supply current. Meanwhile, the PS13014 is optimised for a bandwidth of 450MHz and a slew rate of 1500V/µs, drawing a static current of 5.2mA.

Dr Keith Strickland, Plessey Semiconductors' technology director, said: "These features and performance add up to devices that are suited to video applications, where signals must be driven over significant cable lengths. Our op amps have a superior specification and the matching between amplifiers enables the implementation of high order active filters or twisted pair driver/receivers. The integration of four amplifiers in a single package is also proving to be attractive and cost effective for less demanding applications at audio frequencies."

Analogue specialist Maxim Integrated Products continues to make further additions to its op amp range. The latest are the MAX9636, 9637 and 9638. All rail to rail, low noise parts, they offer an input voltage noise density of 38nV/vHz, an input current noise density of 50fA/vHz and an input bias current of 0.1pA.

According to Maxim, the devices have been developed to ease problems associated with interfacing to high impedance sources, such as photodiode and piezoelectric sensors. Manufactured on its proprietary bicmos process, the devices are said to offer a good combination of size, low noise performance, bandwidth and supply current consumption. Maxim says the parts are suitable for use for sensor interfaces in portable applications that require low noise and low quiescent current.

Alastair Boyd, Linear Technology's UK managing director, told the New Electronics' Analogue Roundtable that the LT1001 – the company's first product – is still in production and still being designed in 29 years after it was introduced.

Why is the device still popular? Who better to answer than Dobkin? "Careful designs sometimes reach the limit of what is theoretically possible. Devices like LT1001 excel in offset and drift specifications to the extent that it cannot be done any better."

He said that, within the parameters of bipolar ic technology, the LT1001 achieves specifications at the limit of what is possible. This combination of specifications, reasonable price and availability is what brings longevity.

"If the specifications on new devices cannot significantly exceed existing devices, why bother to make a new device?," he asked. "As long as the existing devices behave consistently and predictably, it will be difficult to superede them with newer devices that provide any advantage."

Devices like the LT1001, said Dobkin, will continue to be used as long as engineers need very precise parameters. "While other devices may achieve better specifications in one particular area or another, or even equal the specifications of the LT1001, familiarity will keep this device available."

Concluding, Dobkin said: "Because the ease of designing high performance circuits with just a few external components is the forte of op amps, they will be around for a long time."

Author
Graham Pitcher

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