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Amplifiers promise gigahertz speed with industry’s best large signal bandwidth

Santa Clara, Calif. — National Semiconductor Corp. expanded its portfolio of high-performance amplifiers by introducing two new products that enable high signal fidelity with excellent stability at high speeds.

This results in improved performance for a variety of high-speed markets, including video, test and measurement, and other industrial markets.

Excellent bandwidth and slew rate specifications allow these devices to drive high-resolution red, green, blue (RGB) video in an array of applications. Additionally, the excellent dynamic performance makes these devices the ideal solution for driving single-ended, high-speed analog-to-digital converters (A/D converters).

“Today we're introducing a true gigahertz op amp,” said Erroll Dietz, vice president of National Semiconductor's amplifier products group. “National's LMH6703 provides a flatter, more stable frequency response at high speeds. The LMH6704, also introduced today, provides gain accuracy within one percent, for demanding high-performance applications.”

National's LMH6703 is the industry's most stable 1.2-GHz amplifier for high-speed signals, making it less pc board-layout dependent compared to most solutions currently on the market. It features the industry's best large-signal (2-V peak to peak) bandwidth at 750 MHz and 4500 V/μs slew rate for better signal fidelity, resulting in improved performance in high-end video applications with UXGA (1600 x 1200, 75 Hz) resolutions and beyond.

Additionally, the LMH6703 has a shutdown feature, provides 0.1-dB gain flatness out to 150 MHz, and low second/third harmonic distortion of -87/-100 dBc at 5 MHz, making it an ideal driver for single-ended high-speed A/D converters.

National's LMH6704 is a 650-MHz programmable gain buffer with internal gain-setting resistors, allowing the user to set the gain at -1, +1, or +2. It provides a shutdown feature, 0.1 dB gain flatness out to 200 MHz, and low second/third harmonic distortion of -62/-78 dBc at 10 MHz.

The LMH6704's low differential phase and gain specifications provide less luminance and chrominance distortion when driving composite video signals. The high output current (90 mA) allows both the LMH6703 and LMH6704 to drive low-impedance, high-capacitive loads in any application.


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National's Large Signal Bandwidth Comparison Chart

National's LMH6703 is available now in SOIC-8 and SOT23-6 packages, and is priced at $1.49.Click here for the LMH6703 data sheet. The LMH6704 is available now in SOIC-8 and SOT23-6 packages and is priced at $1.49. Click here for the LMH6704 data sheet. Both prices are for 1,000-unit quantities.

National Semiconductor , 1-408-721-5000, www.national.com.

Generally, you see many amplifiers touting GHz speeds, but you won't see an amplifier touting these speeds at large signals. In fact, most vendors will highlight their small signal bandwidth instead; for obvious reasons, which weren't so obvious to me until recently.

National Semiconductor's amplifier, however, offers solid bandwidth performance at both the small and large signal bandwidths, said Tushar Patel, National Semiconductor's product marketing engineer.

Apparently, it looks a lot better on paper to look at the smaller bandwidth specification — and this is where National Semiconductor's very wideband DC coupled monolithic operational amplifier separates from the pack. (See the enclosed large signal bandwidth comparison diagram)

It's difficult to maintain a high large signal bandwidth and to keep the power at a reasonable low level. “We have managed to keep the large signal bandwidth up, without adding the extra power. There isn't an op amp out there at 750 MHz, 2-V peak to peak with only 11mA of current,” Patel said.

It's also important to look at the large signal bandwidth in terms of a gain of two instead of one. For example, as you will see in the enclosed large signal bandwidth comparison chart, a competing device may provide 1.4-GHz speed at the small signal bandwidth, but the signal drops off to 520 MHz at a gain of two with a large signal, Patel said.

Basically, the LMH6703 is the latest addition to National's existing portfolio of high-speed amplifiers. The LMH6703 is a follow on product to the 1.7-GHz amp (LMH6702) that National developed two years ago.
“We were able to make some significant performance improvements. Besides the bandwidth increase, we reduced the current, put in the shutdown pin, and made the frequency response smoother, cleaner and more stable,” Patel said.

National was able to boost the large signal bandwidth at 2-V peak to peak on the newer device to 750 MHz, versus 720 MHz on the LMH6702.

Meanwhile, the large signal bandwidth is crucial to video applications since these applications mainly deal with a 1-V peak to peak signal. “Most manufacturers usually characterize their op amps at roughly 200 mV or 500-mV peak to peak,” Patel said. In fact, some companies don't even include the large signal bandwidth spec on the data sheet. “Video applications deal with large signals, not 200 mV signals,” he added.

One of the problems associated with all high-speed op-amps is that they become difficult to use and can be pc board-layout dependent. The LMH6703 was designed to help minimize the PC board layout dependency, and provide a nice, clean frequency response without peaking, Patel said.

The amplifier also offers lower distortion than competing parts, which is key when driving A/D converters. The second and third harmonic distortion is -75 dB and -103 dB, respectively, measured at 10 MHz, compared to similar parts with the best third harmonic distortion available in the market currently at about -86 dB also at 10 MHz, according to Patel.

National Semiconductor will also be releasing the LMH6704, which is a programmable gain buffer. The buffer is similar to the LHM6703, except the feedback resistors are on the chip. “For the LMH6704 we have brought the feedback resistors on chip for setting the gain at -1, +1, or +2. This saves board space, and makes gain matching easier for engineers who don't want to fiddle around with resistors,” Patel said. Therefore, the LMH6703 offers the user the flexibility of choosing the gain, while the LMH6704 has the gain internally.

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