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Zero-drift op amps pack precision in tiny packages

Dallas — Touting an unmatched combination of precision, micropower, and miniature packaging, Texas Instruments Inc. released a family of zero-drift operational amplifiers (op amps).

Featuring ultra-low offset (2 microvolts), ultra-low quiescent current (17 microamps), operation down to 1.8 volt and SC70 or SOT23 packages, the OPA333 is intended for power sensitive applications in medical instrumentation, temperature measurement, test equipment, security and consumer systems.

 
 
Single op amp bridge amplifier graphic

Fabricated using TI's high-performance, precision mixed-signal CMOS technology, the OPA333 uses patented auto-zeroing techniques to simultaneously provide very low offset voltage and near-zero drift over time and temperature, said Gregg Lowe, senior vice president of TI's high-performance analog business. The device offers high-impedance inputs that have a common-mode range of 100 millivolts beyond both rails, and rail-to-rail output that swings within 100 mV of the rail. Single or dual supplies as low as 1.8 volt and up to 5.5 V may be used.

The OPA333 features excellent common-mode rejection ratio (CMRR) without the crossover errors associated with traditional complementary input stages, Lowe said. “This design results in superior performance for driving analog-to-digital converters without degradation of differential linearity,” he said.

Both versions of the OPA333 family, which are available now, are specified for operation from -40°C to +125°C.

The OPA333 (single version) is packaged in the SC70, SOT23-5 and SO-8 and is priced at 95 cents in 1,000-piece quantities (suggested resale pricing). The OPA2333 (dual version) is offered in the DFN 3 x 3 mm and SO-8 package and is priced at $1.50 in like quantities. Click here for the OPA333/OPA2333 data sheet.

Texas Instruments Inc. , 1-800-477-8924, www.TI.com.

These high precision amplifiers are said to offer breakthrough performance in terms of offset voltage (2 uV), drift (0.02), and noise (1.1 uV peak to peak). The most distinguishing feature, however, is the mere 17 microamps of quiescent current. The closest competing device offers quiescent current of 180 uA, said Bruce Trump, strategic marketing engineer of TI's high performance linear products.

In addition, many zero-drift op amps don't offer this voltage range or rail-to-rail input, which makes the OPA333 suited for applications that require a wide input signal range. “There are no other zero drift-type amplifiers that run on 1.8 V, which allows designers to operate on a single lithium cell. The zero drift topologies just don't operate on 1.8 V with conventional cells. That is the difference between two and three cells. We get more battery life due to lower quiescent current and use fewer cells,” Trump said.

It's also the only zero drift op amp in a SC70 package, which makes it the smallest zero-drift op amp package in the world, according to Trump. “Currently, the smallest zero-drift op amps are housed in SOT23 packages. The SC70 package represents a 30 percent space savings,” he said.

TI's patent-pending notch filter is a new element in this type of design that is used to reduce the noise of the zeroing circuitry within the device. “We've done some clever things to keep the noise very low, while reducing the quiescent current by a factor of 10. The OPA333 delivers 50 percent less noise than other zero-drift op amps that use at least an order of magnitude more quiescent current,” Trump said.

The oscilloscope photo below illustrates the spectral density of the noise. The fact that the line in the oscilloscope photo is flat, as opposed to rising to left, shows that the device has very low (1/f or 0.1 to 10 Hz) noise.

 
 
Oscilloscope Graphic

Although the devices can be used in a variety of applications, Trump believes the amplifier's low quiescent current makes them especially suited for battery-powered devices like accessories for entertainment devices, smoke and gas detectors and portable instrumentation. “Op amps have been used previously in battery-powered devices at a penalty since higher quiescent current results in requiring larger batteries and poor battery life,” he said.

TI's previous generation op amp (OPA335), which is about two-years-old, didn't contain a notch filter. The OPA335 provided quiescent current of 350 uA.

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