POWER AND LINEAR IC SOLUTIONS

Fully Differential Drivers Unlock High-Speed ADC Performance

Until recently, selecting an amplifier to drive a high-speed ADC has been a challenge. RF amplifiers are typically single-ended, large, power hungry, and require a 5V to 12V power supply. In contrast, fully differential amplifiers are often optimised for a narrow input-signal bandwidth, require a higher voltage supply, or limit performance. A new family of 3V amplifiers from Linear Technology helps designers maximize ADC performance, while at the same time simplifying high-frequency board design.

With a fixed gain of 20dB, the LTC6400-20 and LTC6401-20 achieve excellent performance working from a single 3V or 3.3V supply. The LTC6400-20 consumes only 90mA, yet achieves a -3dB bandwidth of 1.8GHz and 4500V/µs slew rate. The LTC6401-20 achieves roughly two-thirds the speed of the LTC6400-20 at half the power. Including the internal gain-setting resistors, total input-referred noise density is just 2.1nV/√Hz. The LTC6400 delivers excellent intermodulation distortion performance of -90dBc at up to 140MHz and -70dBc at up to 250MHz.

In addition to the amplifier, the devices integrate several other features including gain-setting resistors, output signal filtering and output common-mode circuitry. All this fits inside a tiny 16-lead QFN (Quad Flat No leads) package. This high level of integration means that fewer external resistors and capacitors are required, reducing system footprint and simplifying design.



Integration also delivers dramatic improvements in stability. By bringing the sensitive feedback loop inside the package, stray capacitances normally associated with routing external gain setting resistors are eliminated. Furthermore, the inductive-bond wires connecting the inputs and outputs to the outside world are moved outside the sensitive feedback loop.

A further benefit of these integrated devices is optimized speed and distortion. Tuning the internal compensation for a specific gain setting maximizes gain flatness over as wide a bandwidth as possible and group delay variation minimizes.

Two sets of differential outputs – filtered and unfiltered – are provided. The single-pole RC filter simplifies driving capacitive ADC loads. The corner frequency can be easily adjusted with the addition of a few external components. An output common-mode pin makes it easy to set the DC output signal level. The LTC6400-20 can be AC- or DC-coupled at the inputs and/or outputs.

Features:

• 20dB fixed gain amplifiers
• Single 3V or 3.3V supply
• Supply current as low as 50mA
• 3dB bandwidth of up to 1.8GHz
• Slew rate up to 4500V/µs
• Total input-referred noise density 2.1nV/√Hz
• Inter-modulation distortion: -90dBc at up to 140MHz, -70dBc at up to 250MHz
• Integrated gain-setting resistors
• Output signal filtering and output common-mode circuitry
• 3mm x 3mm 16-lead QFN package
• Output common-mode pin
• Choice of AC- or DC-coupling

Applications

• Differential ADC driver
• Differential driver/receiver
• Single-ended to differential conversion
• IF sampling receivers
• SAW filter interfacing

For further information, including the LTC6400-20 and LTC6401-20 Datasheets, please click here