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DAC with integrated mixing stage suits direct-RF applications

Weybridge, England – Developed with broadband and next-generation-wireless equipment designs in mind, two 14bit digital-to-analogue converters (DACs) have been introduced by Analog Devices that perform direct digital synthesis for signals up to 3.6GHz.

The AD9789 and AD9739 TxDACs are single transmit-DAC architecture devices that are part of ADI's TxDACs range, and feature multi-carrier capability up to the Nyquist frequency in baseband mode, plus use a mix-mode function to generate RF signals in the second and third Nyquist zones. This feature allows designers to eliminate an off-chip mixer and low-pass filter, thus reducing component count and design complexity.

The AD9789 TxDACs, designed to meet the needs of DOCSIS-III cable-infrastructure designs that use low-cost FPGAs, integrates a QAM encoder, interpolator, and digital up-converter to achieve a 2.4GHz sample rate. It has a flexible digital interface that accepts 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. An on-chip rate converter supports a range of baud rates with a fixed DAC clock. The device includes an SPI port for device configuration and status-register readback.

Notably, configuration options on the AD9789 allow the data path to be set to bypass the QAM encoder and SRRC filter to enable the DAC to operate within a broader range of applications such as wireless infrastructure. According to ADI, the nominal DAC output current is 20mA, producing a peak 0dBm of power into a 50ohm load. The AD9789 operates from 1.5V, 1.8V, and 3.3V supplies for a total power consumption of 1.7 W ” ADI claims this is half of competing signal chains' power dissipation.

Making use of the same DAC core, the 2.5GHz AD9739 is said to have the industry's widest useable input bandwidth, suiting it to a range of applications that include wireless-communications equipment, instrumentation, and defence electronics. The AD9739 operates from 1.8V and 3.3V supplies, consuming a maximum of 1.1W, a 25 percent reduction when compared with conventional transmit signal chains, says ADI.

Both devices are already in production in chip scale ball grid array packaging.

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