Fully Differential Amplifiers Enable High-Speed ADC Performance

(Product News, 30 May 2007)

Linear Technology Corp. has released its LTC6400-20 and LTC6401-20 ADC drivers that achieve unprecedented performance on a single 3V supply. These fully differential amplifiers include gain setting resistors, simplifying the challenge of driving the highest performance high-speed ADCs. The LTC6400-20 provides a fixed gain of 20dB with -93dBc third-order intermodulation distortion (IMD3) performance and noise figure of 6.5dB for a 140MHz input frequency. At 240MHz, IMD3 is better than -70dBc. Other family members with fixed gains ranging from 8dB to 26dB will follow.

The LTC6400-20 achieves this outstanding performance operating on a 3V supply voltage, providing a differential output voltage swing of 4.4Vp-p. In addition to minimizing power consumption, this eliminates the need for a separate 5V supply in applications using the latest 3V and 3.3V ADCs. To further simplify interfacing to high speed ADCs such as the LTC2208, a VOCM pin sets the output common mode voltage of the LTC6400-20 to match the input range of the ADC. An optional on-chip filter helps to handle charge injection common to the capacitive inputs characteristic of pipelined ADCs. Inputs may be AC- or DC-coupled. While fully differential, the LTC6400-20 can be used to easily convert single-ended inputs to differential outputs.

The LTC6401-20 is a lower-power part optimized for lower input frequencies. Consuming just 50mA, half that of the LTC6400-20, the LTC6401-20 maintains the same low noise performance and -90dBc distortion performance for input frequencies up to 70MHz.

"The LTC6400-20 is the first of a family of parts that achieves excellent noise and distortion performance from DC to hundreds of megahertz," says Kris Lokere, design manager. "In addition to driving ADCs in communications and instrumentation applications, it will find many applications as a high performance differential driver and receiver."

These parts are now available in a tiny 3x3mm 16-lead QFN package, starting at $3.68 each for the LTC6400-20 and $2.95 each for the LTC6401-20 in 1,000-piece quantities.

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