14-Bit Converter Performs Direct Digital Synthesis For RF Signals To 3.6 Ghz

The new AD9789 and AD9739 TxDACs feature ADI’s proprietary Mix-Mode Super Nyquist Architecture, which supports high-fidelity digital synthesis of RF signals up to 3.6 GHz. The combination of best-in-class bandwidth and dynamic range with a direct-to-RF core allows broadband and next-generation-wireless equipment designers to use a single transmit-DAC architecture for multiple communications standards while eliminating an off-chip mixer and low-pass filter to reduce design complexity, cost, size, and power. The mix-mode capability was introduced two years ago with the release of ADI’s AD974x and AD978x TxDAC product series.



“Given the demands that carriers are placing on broadband-communication systems, the ability to maximise usable bandwidth and dynamic range creates a clear advantage,” said Dave Robertson, product line director for high-speed signal-processing, Analog Devices. “Whether the system in question is today’s latest cable-infrastructure equipment or tomorrow’s advanced digital-radio design, high-performance data conversion is the key to unlocking the advantages of a true direct-to-RF transmit architecture.”



The AD9789 14-bit TxDAC integrates a QAM encoder, interpolator, and digital up-converter that achieve a 2.4-GHz sample rate for cable infrastructure. The 2.5-GHz AD9739 exploits the same DAC core, features the industry’s widest useable input bandwidth, and is well suited for a broad range of applications including wireless-communications equipment, instrumentation, and defence electronics. Both devices provide multi-carrier capability up to the Nyquist frequency in baseband mode and use the mix-mode function to generate RF signals in the second and third Nyquist zones. This feature allows designers to eliminate a mixing stage, reducing component count and design complexity in direct-RF applications.



The AD9789 TxDAC shortens time-to-market for DOCSIS-III cable-infrastructure designs using low-cost FPGAs. The DAC includes a flexible digital interface that can accept up to four channels of complex data. The QAM encoder supports constellation sizes of 16, 32, 64, 128, and 256 with SRRC (square-root raised cosine) filter coefficients for all standards. The on-chip rate converter supports a wide range of baud rates with a fixed DAC clock. The digital up-converter can place the channels from 0 to 0.5 fDAC, to synthesize up to four contiguous DOCSIS channels anywhere in the DOCSIS band. The AD9789 also includes an SPI (serial peripheral interface) port for device configuration and status-register readback. The flexible digital interface accommodates data bus widths from 4 bits to 32 bits and can accept either real or complex data. Configuration options can set the data path to bypass the QAM encoder and SRRC filter to enable the DAC to operate within a broader range of applications such as wireless infrastructure. The nominal DAC output current is 20 mA, which produces a peak 0-dBm of power into a 50-ohm load. The AD9789 operates from 1.5-V, 1.8-V, and 3.3-V supplies for a total power consumption of 1.7 W–half of competing signal chains’ power dissipation.



With breakthrough AC performance, the AD9739 14-bit TxDAC provides output signals to 3.6 GHz. The DAC features a dual-port low-voltage differential signaling (LVDS) interface to support the high sample rate with existing FGPA and ASIC signal-processing technology. An SPI port is included for configuration and status-register read back. The output current is programmable from 8.7 mA to 31.7 mA. The AD9739 operates from 1.8-V and 3.3-V supplies for a total maximum power consumption of 1.1 W, which is a 25 percent reduction when compared with conventional transmit signal chains.