While conventional op amps minimize distortion with conservative designs that keep signals well within a device's voltage range, rail-to-rail models allow signals to swing within millivolts of either supply rail. With the number of low-voltage applications growing steadily, the need becomes even greater for devices that can use their full voltage range on output, input or both.

"There is a major trend in the industry toward lower-supply voltages, but with lower supply voltages come limits on signal ranges and resolution," said Bob Briano, product manager for high-speed amplifiers at Analog Devices (Norwood, Mass.). "To make the most of smaller signals, it's important for amplifiers to be able to swing within a few hundred millivolts of either rail. An increasing number of op amps for these kinds of applications are offering rail-to-rail input and output."

In addition to minimizing distortion in low-voltage applications by swinging between supply rails, thus creating more signal "head-room," rail-to-rail op amps bring the industry a step closer to universal op amps that can fit multiple applications.

"Some applications need rail-to-rail inputs, some need rail-to-rail outputs and some don't need either one," said Tadija Janjic, strategic-development engineer for high-performance linear products with TI's Burr-Brown product operation (Tucson, Ariz.). "So customers are asking for op amps with rail-to-rail I/O to cover all of those bases."

Among the most aggressive vendors is Intersil's Elantec operation (Milpitas, Calif.). The company last week launched 16 op amps with rail-to-rail output. There are single, dual, triple and quad versions, with and without a disable feature, at -3-dB gain bandwidths of 200 MHz and 600 MHz. All of the new devices operate with a single 5-V supply. Sameer Vuyyuru, Elantec's director of marketing, said the 200-MHz devices offer low power (2 milliamps) at a low cost (69 cents in 1,000 units). The 600-MHz amps, priced at $1.60, offer a bandwidth two to three times higher than competing devices, he said, with a slew rate of 100 V/microsecond.

At Linear Technology (Milpitas), the new LTC6800 precision rail-to-rail I/O instrumentation amplifier hits what product-marketing manager Eric Soule called an unequaled price/performance point. Priced at $1.55 in quantities of 1,000 and incorporating a zero-drift, the op amp offers maximum offset voltage of 100 microvolts, maximum offset voltage drift of 250 nV/ degrees C and common-mode rejection ratio (CMRR) of 116 dB with a single 5-V supply, independent of gain. It operates from 2.7 to 5.5 V.

"Designers want to use as few supplies as possible and want the lowest voltage possible," said Soule. "That presents a problem for analog designers, because it gives them less margin. Dynamic range is being squashed, and that's catapulted the trend toward rail-to-rail. Analog designers need to squeeze the last ounce out of an input signal."

Going below 2.7 V
"Op amps are operating at 3 V or 2.7 V, and will be going even lower, to 1.8 V and 1.25 V," said Carlos Sanchez, business manager at Maxim Integrated Products (Sunnyvale, Calif.). "It's almost mandatory to have rail-to-rail inputs at voltage levels below 2.7 V." Selecting an op amp typically requires that customers balance benefits against penalties, depending on what performance characteristics are most important for a particular application.

To make product selection easier, Maxim recently launched a Web-based service that lets customers enter their desired specifications and, usually within 24 hours, receive a specific recommendation via e-mail from a Maxim engineer. Maxim's op amp portfolio includes the MAX424x family, which can operate from a single supply from 1.5 to 5.5 V, or dual supplies from plus/minus 0.9 V to plus/minus 2.75 V. With "beyond-the-rails" inputs and rail-to-rail outputs, which allow the full power supply voltage to be used for signal range, the devices consume 10 microamps of supply current per amplifier and provide a 90-kHz gain-bandwidth product. A low-power shutdown mode reduces supply current to less than 1 microamps and forces the output into a high-impedance state.

Also beefing up its line is Analog Devices, which recently launched its AD8027, a rail-to-rail input op amp that can operate with supply voltages from 12 to 2.7 V. Bandwidth is 180 MHz, G = +1, and the slew rate is 100 V/microseconds (see Feb. 3, page 64). A selectable crossover feature allows designers to minimize the effects of crossover distortion by specifying a threshold voltage close to either the positive or the negative rail.

Taking a different approach to a similar end, TI's Burr-Brown product line in December introduced the OPA363 and OPA364. These 1.8-V op amps offer rail-to-rail input with no crossover between input stages and, as a result, reduced distortion and a better CMRR. Janjik said the devices have a single input stage and an internal charge pump in a design that eliminates the need for a second input stage and meets the needs of customers that would otherwise require two power supplies.

Last week TI introduced a power op amp, OPA569, featuring a 2.4-A output current and an output swing within 200 mV of the rails. The device is optimized for high-current applications in optical networking, test and instrumentation, industrial control and medical equipment. It drives resistive, inductive and capacitive loads while operating on a single 2.7- to 5.5-V supply, and Janjik said its rail-to-rail output swing minimizes the amount of heat dissipated in the output stage, thereby improving amplifier efficiency and system performance.

Other features include a user-selectable current limit, a shutdown function with output disable and output current monitoring without an external shunt resistor. The device is unity-gain stable, has low dc errors and is said to be free from the phase inversion problems sometimes found in power amplifiers. Packaged in TI's SO-20 PowerPad and specified for operation from -40 degrees C to 85 degrees C, the OPA569 is priced from $2.90 in 1,000-piece quantities.

Other companies touting their rail-to-rail op amps include ST-Microelectronics Inc. (Lexington, Mass.), National Semiconductor Corp. (Santa Clara, Calif.) and Micrel Semiconductor (San Jose, Calif.).

STMicro's lineup includes single, dual and quad devices in the TS185x and TS187x families, which operate from 1.8 to 6 V. The TS185x amps consume 120 microamps and offer a 480-kHz gain bandwidth, while the TS187x devices consume 400 microamps and have a gain bandwidth of 1.6 MHz.

Nicola Siciliano, an application engineer in ST's Analog and Logic Standard ICs group, said the TS185x and TS187x families maintain good dynamic stability over the full output current drive range, even with high-capacitance loads. Three gain stages are used to combine high open-loop gain performance with high output-current capacity and low power consumption.

Among some 30 op amps with rail-to-rail I/O, National Semiconductor offers the single LMV981 and dual LMV982, both of which operate from 1.8- to 5-V supply voltages. Their input common-mode voltage extends 200 mV beyond the supplies, while their output can swing rail-to-rail unloaded and within 105 mV from the rail with a 600-ohm load at 1.8-V supply.

Both devices are optimized to work at 1.8 V, which suits them for portable two-cell and single-cell lithium ion battery-powered systems, the company said. A shutdown pin can be used to disable the devices and reduce supply current. The amps achieve a 1.4-MHz gain-bandwidth product at 1.8-V supply voltage with low supply current. They can drive a 600- ohms load, and a capacitive load of up to 1,000 picofarads, with minimal ringing. Dc gain of 101 dB suits them for low-frequency applications.

Two dual op amps with rail-to-rail I/O from Micrel offer one-tenth the power consumption of other devices, the company said. The MIC862 provides a 5-MHz -3-dB band-width and a 3-MHz gain-bandwidth product while consuming 31 microamps of current per channel. The MIC863 provides an 800-kHz -3dB bandwidth, a 450-kHz gain-bandwidth product and consumes 4.2 microamps.

Target applications include PDAs, cell phones and laptop PCs. Both operate from 5 V to 2 V and are unity-gain stable. Typical input offset voltage is 0.1 mV, large-signal voltage gain is 96 dB, power supply rejection ratio is 92 dB and CMRR is 87 dB.

Company Contacts

Analog Devices Inc.
(800) ANALOGD
(262-5643)
www.analog.com

Intersil Corp.
Elantec Product Group
(408) 945-1323
www.intersil.com

Linear Technology Corp.
(408) 432-1900
www.linear.com

Maxim Integrated Products
(800) 998-8800
www.maxim-ic.com

National Semiconductor Corp.
(408) 721-5000
www.nsc.com

STMicroelectronics Inc.
(781) 861-2650
www.st.com

Texas Instruments Inc.
(800) 477-8924, ext. 4500
www.ti.com