acquisitionZONE Products for the week of May 24, 2010

National Semiconductor Says…

LMH6629: Low Noise, Ultra-High-Speed Amplifier
Delivers 0.69 nV/sqrt Hz Voltage Noise with 900 MHz Bandwidth at Gain of 10

A new ultra-high-speed operational amplifier (op amp) from National Semiconductor Corp. delivers the industry’s lowest noise (0.69 nV/sqrt Hz) with -3 dB bandwidth of 900 MHz at a gain of 10. The PowerWise LMH6629 provides designers a unique mix of wide bandwidth, high gain and precision amplification for systems where minimal noise addition is a key performance requirement. These include 16-bit systems for communications, test and measurement, medical imaging, industrial, and light detecting and ranging (LIDAR) applications.

The LMH6629 also provides low input current noise (2.6 pA/sqrt Hz) and second and third harmonic distortion (f = 1 MHz) performance of -90 dBc and -94 dBc, respectively. The op amp’s low input noise, low distortion and high speed combined with ultra-low DC errors -- 780 uV input offset voltage max at 25 degrees C and /- 0.45 uV per degree C of TCVos -- enable precision operation in both AC- and DC-coupled applications.

The LMH6629’s input common-mode range extends below ground and the output swings to within 0.8V of either rail with a linear output current of greater than /- 250 mA. The PowerWise op amp consumes 15.5 mA and features a supply voltage range from 2.7V to 5.5V. Its user selectable internal compensation eliminates the external compensation components and additional design time required with other amplifiers. With the LMH6629, designers can set a minimum gain of 4 or 10 by pulling the minimum gain select pin low or high.

EN-Genius Says…

The need for operational amplifiers that are capable of driving, properly, 16-bit ADC applications has never been greater. National appears to have broken the bogey of noise numbers with the LMH6629.

This part offers input referred noise voltage (>1 MHz) of typically 0.69 nv/rtHz and input referred noise current of 2.6 pA/rtHz with either a single 5 V or 3.3 V rail. At both rails the noise figure is a typical 8.0 dB. The voltage noise stays at about 0.7 nV/rtHz down to the 1/f noise corner that starts below 10 kHz. In particular the number is just about 0.9 nV/rtHz at 1 kHz.

It should be noted that the 8-pin package does not use the standard single op amp layout. The inputs are on pins 3 (inverting) and 4 (non-inverting) with the output on pin 7 and the rail applied across pins 8 and 5. The output is also taken back across the package to pin 2, to act as the feedback pin, while pin 1 is used for power-down and pin 6 is used for compensation. The separate feedback pin notion was first engineered, I believe, by Analog Devices and allows for rather simpler connection of feedback components without routing them under the package on the PCB.

Distortion numbers vary between rail voltages, of course, but at 5 V HD2 is a typical -90 dBc (at 1 MHz with a 2 V output) while HD3 is -94 dBc. Slew rate at 5 V is 1600 V/µs at a gain of 10 while small-signal bandwidth is 900 MHz and the 0.1 dB maximally flat bandwidth is 330 MHz. The device is neither rail-to-rail in or out. Two choices of internal compensation circuit are provided which affect, of course, bandwidths.

The supply current is a maximum of 18.2 mA while shutdown is a typical 1.1 mA.

National has broken the noise barrier for 16-bit operation with the LMH6629 although it will be up to the designer to come up with circuit layouts that allow full use of the performance. Manufactured on National’s CBiCMOS8 SiGe process (capable of producing both npn and pnp devices – as well as CMOS) the LMH6629 is taking full advantage of both the high speed and low noise characteristics that are offered. The part is under priced for the noise premium that is available. Layout issues will keep National’s applications engineers busy, as will stability issues with higher source/sink currents.

The LMH6629 is in production in thermally-enhanced LLP-8 operating over the extended industrial temperature range of -40ºC to 125ºC. It is priced at $1.88 in 1000-piece lots. An evaluation board is available.

Data Sheet