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Driving A/D converters is effortless with these amplifiers

Milpitas, Calif. — Linear Technology Corp. released a family of high-speed, fully-differential amplifiers for driving high-resolution, high-speed analog-to-digital converters.

The devices are the first fully-differential amplifiers that truly simplify the task of driving high speed A/D converters and reduce total solution size.

With fixed gain options of 2 (6 dB), 4 (12 dB) and 10 (20 dB) and a built-in user adjustable filter, the amplifier enables designers to directly drive A/D converters and achieve optimal performance without using complex support circuitry.

Ideally suited for use in wired and wireless communications, imaging systems and test and measurement equipment, the amplifier offers excellent noise and distortion performance. The low harmonic distortion of -70 dBc, combined with the unmatched 12.3 dB noise figure at 70 MHz, ensures system accuracy and high signal integrity.

Unlike existing differential amplifiers, the amp works everywhere with minimal support circuitry. The internal gain eliminates a handful of external gain setting resistors that are commonly required with other differential amplifiers. The built-in user-adjustable low- pass filter simplifies pre-A/D converter filtering often desired to provide either anti-aliasing or improve the signal-to-noise ratio. The adjustable output common mode voltage allows direct interfacing with A/D converters without the need for ac-coupling capacitors or transformers. The device can be dc-coupled in applications that have frequency ranges including dc. The combination of these features eliminates external support circuitry resulting in an integrated single-chip solution that is easier to use and simpler to lay out with minimal debug and tweak time.

“No other high speed differential amplifier offers the ease of use the LT1993,” said Erik Soule, general manager of Linear Technology's signal conditioning product line. “Designers of high speed systems are continuously striving to find new ways to simplify their designs, while achieving the optimal balance of low noise and low distortion. Our new amplifier family solves these design challenges by delivering the best possible performance levels within a highly integrated, yet easy-to-use solution.”

The LT1993-2 features fixed gain of 6 dB with 800 MHz -3dB bandwidth. The LT1993-4 has a 12 dB fixed gain with bandwidth reaching 900 MHz. The LT1993-10 offers a high gain of 20 dB with 700 MHz bandwidth. All gain options are available for operation over the commercial and industrial temperature ranges.


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Linear Technology's LTC1993-2

Offered in a 3mm x 3mm QFN package, pricing starts at $2.95 each in 1,000-piece quantities. Click here for the LT1993-2 data sheet.

Linear Technology , 1-800-454-6327, www.Linear.com.

These differential amps for driving A/D converters combine low distortion, low noise and ease of use.

The devices represent a new concept for Linear Technology. “It's really a unique topology for a high-speed A/D driver because it's more of a traditional differential op amp design that make the amp go really fast, as opposed to a narrow-banded, open loop RF-style amp commonly used in cellular basestations,” Soule said.

The amplifier's sole purpose in life is to condition a signal so that it can be easily digitized by the A/D converter. “Most of the competing parts out there in this frequency range usually require a lot of tricks, and they only work over a limited frequency range,” Soule said. Linear came up with a general-purpose amplifier that's optimized for input frequencies ranging from dc to 70 MHz. This feature provides many design advantages. For example, you can do dc coupling, which eliminates transformers, inductors and ac coupling capacitors, Soule explained.

Additionally, the amplifiers offer three different fixed gains, which provide key design advantages although some may argue that it also limits your options in terms of flexibility. Generally, you need external resistors to set the gain. With this part, the resistors are on-chip, greatly simplifying the board layout, Soule said. Passive components such as resistors and capacitors can alter the frequency response or distortion of the amplifier if careful attention is not paid to the board layout. Bringing the resistors on-chip takes some of the sensitive parts of the circuit that are normally on the printed-circuit board and puts them on the silicon. “The advantage to the system designer is that you put the amplifier down on the board and it works. Essentially, you get excellent specs — right out of the box,” Soule said.

The amplifiers can be used in oversampling applications with converters that operate over 100 Msamples/s or 200 Msamples/s, or they can be used in undersampling applications with converters that operate down to 20 Msamples/s to 50 Msamples/s. Typically, these would be 12-bit A/D converters with at least 70 MHz of input signal bandwidth, Soule said.

Although these amplifiers are intended for a wide variety of applications, they are especially suited for RF applications. In a wideband CDMA basestation, for example, a typical basestation signal chain may include an antenna, a low noise amp, mixers, filters and sometimes multiple gain stages. The signal levels go up and down during the signal pass and there are tradeoffs that the designer makes during this process. Having different gain options allows designers to take different amounts of gain before driving the A/D converter for added flexibility.

In addition to the fixed gain aspect, a filtered output also makes the amplifiers easy to work with. You can take the LT1993 and put it in a circuit with a minimal amount of external circuitry. “With other differential amps, you would have to pay close attention to the circuit layout and use many more external components to make it work,” Soule said.

Most A/D converters are nasty to drive. Loads change when sampling and you have to be able to respond quickly. These differential amps contain a built-in single-pole filter — it's simple but cleans up a lot of wideband noise before it goes into the A/D converter to simplify filtering in the DSP, Soule said. It has two sets of outputs — one that's filtered and one that's straight out in case you don't want to use the filter, he added.


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4-Channel WCDMA Receive Channel

Evaluation boards are free to qualified customers.

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