Easier Design-Ins Linear Technology isn't a slave to trends, insists CTO Robert Dobkin. He says the company sticks to pushing performance and offering a wide range of high-performance products that command decent margins. Even so, some of Linear's recent offerings combine performance with some simplification of the circuit designer's task. For example, the new design topology of the LT6411 differential amplifiers lets Linear integrate precision gain-setting resistors on-chip.
The LT6411 is intended for use as a high-speed analog-to-digital converter (ADC) driver, twisted-pair line driver, or single-ended to differential signal converter. Inside the chip is a pair of current-feedback amplifiers (CFAs) with matched 370Ω feedback and gain resistors. Using no external components, simply strapping the pins in different ways allows the amplifier to provide well-balanced differential signalling with gains of 1,–1, or 2 or singleended to differential conversion.
The performance story is a 650-MHz –3-dB small-signal bandwidth (it's flat, ±1 dB to 200 MHz), –77-dBc harmonic distortion at 30-MHz, and a slew rate of 3300 V/µs. The LT6411 can be used on split supplies as large as ±6.3 V or on a single supply as low as 4.5 V. Current draw for each amplifier is 8 mA when enabled. When disabled, either amp's output pins become high impedance, and its current draw drops to less than 350 µA.
Dynamic Range And Low Operating Voltages There's clearly a challenge in delivering performance in the face of shrinking supply voltages. National provides an example of an operational amplifier designed for rail-to-rail operation at both the input and output with power supplies from 3 V down to 1 V.
While 1-V operation is guaranteed, National recommends 3 V, but says the chip will operate even at 0.9 V. Typical specs include 2.7-MHz unity-gain bandwidth when driving a 500-pF capacitive load, 25-nV/√Hz voltage noise from –40°C to 125°C, and 80-dB commonmode and power-supply rejection ratios.
SiGe Delivers Performance At the other end of the voltage spectrum, the use of silicon-germanium (SiGe) process technologies is growing. Texas Instruments used its 36-V SiGe BiCom3HV bipolar process for the first time commercially to create the OPA211 and OPA827 op amps for driving precision ADCs in data acquisition systems for test and measurement, instrumentation, imaging, medical, audio, and processcontrol applications.
The rail-to-rail output OPA211 achieves an 80-MHz gain bandwidth product (GBW) with 1.1-nV/√Hz voltage noise with a 3.6-mA supply current. Offset voltage is 100 µV, with 0.2-µV/°C offset voltage drift and less than1-µs settling time.
The OPA827 is a JFET-input op amp for applications with a high source impedance. Its dc characteristics include 4.5nV/√Hz voltage noise, 250-µV offset voltage, 1-µV/°C offset voltage drift, and 400-nV p-p frequency noise. Its ac specifications include 18-MHz GBW, 22-V/µs slew rate, and 0.0004% total harmonic distortion (THD) at 1 kHz.
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