More recently, however, with low-cost CMOS technology muscling its way into the precision op amp arena, the benefits have been fine-line geometries, inherently high input impedance and the easy implementation of auto-zero techniques. Historically, designers of precision op amps have looked to bipolar process technology, with its fast, low-noise and well-matched transistors, adding JFET front ends if necessary, to raise the input impedance and reduce input bias currents. Medical gear, battery monitors, scales of all sizes and active automotive suspension systems are just some of the places precision op amps are likely to turn up. Typically, test and measurement, process monitoring and industrial equipment gauge real-world conditions through sensors and transducers that feed into precision op amps. Work for precision op amps tends to span a wide range of applications in instrumentation and data acquisition. According to press material, spikes from intermodulation, or aliasing, are also said to be eliminated in the AD857x family. In addition, the company said, the AD857x wields a spread-spectrum clocking technique to cut down clock spikes. For example, Analog Devices claims its AD855x and AD857x have the lowest voltage noise among auto-zero op amps and no measurable 1/f noise. Manufacturers have recently been touting noise-reduction schemes that claim to negate these problems. Even for low-frequency signals, the auto-zero approach typically calls for a low-pass filter on the input to block troublesome high-frequency components, thus inflating the die area. Worse, if relatively high-frequency signals or fast settling times are the object of attention, the switching action common to auto-zero techniques poses problems with aliases. On the other hand, when compared with bipolar technology, CMOS circuitry is a notorious source of a form of noise, called 1/f noise, that is inversely proportional to signal frequency. On the one hand, precision op amps are commonly used to carry dc and low-frequency signals. The Achilles' heel of CMOS-grown auto-zero circuitry is noise. “We found some new topologies that let us reduce the die size,” he said, which resulted in 20-bit accurate parts having offset voltages as low as 1 microvolt, as well as 3-V- to 5-V rail-to-rail operation, 50-microseconds typical overload settling time and starting prices of $1.14 in quantities of 1,000. The AD855x and AD857x single, dual and quad op amps represent significant enhancements to earlier auto-zero products, said Steve Sockolov, director of precision linear products at Analog Devices. (Norwood, Mass.) aimed two auto-zero op amp families at low-cost, low-voltage targets. Specifically, it is a natural foundation for building on-chip digital auto-zero circuitry-sometimes called chopping or zero-drift circuitry-that works to continuously nullify input offset voltage, the key specification for defining a precision op amp. But most important, CMOS paves a path to very high precision. Highly shrinkable, CMOS also helps to reduce die size, which is one way for manufacturers to cut prices and package size.
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For one thing, CMOS makes it easier to build op amps with the low supply and signal voltages-down to 5 V and even 3 V-that are common in logic systems. Bipolar technology has historically served operational amplifiers well, but vendors building precision op amp chips are turning to CMOS in growing numbers to solve their most pressing problems.