Like others, Intersil's designers have taken the segmented current-source route and combined it with a 0.5-µm CMOS process to accomplish the desired specifications. According to the company, all CommLink DACs are pin-compatible for upward migration.
The 14-bit 130-Msample/s HI5960, the high-end member of this family, operates from a single 3- to 5-V supply. It achieves a typical SFDR of 63 dBc at a sampling rate of 100 Msamples/s and an output frequency of 20.2 MHz. Typical THD at a 5-V supply and a 100-MHz clock frequency is −71 dBc.
While the present generation of parts is limited to single-carrier, narrow-band types of applications, Intersil's designers would like to enter the multicarrier, multistandard fray. For that, Intersil plans to more than double the conversion speed of present converters, as it prepares its next-generation models with better SFDR and dynamic re-sponse over the cellular bandwidth. On-chip interpolation filters are on the drawing board too for these upcoming high-speed DACs. Intersil expects to launch the newcomers by the first half of next year. In addition, the company is pursuing the idea of packing matched DACs on the same chip as standalone devices for multichannel needs.
Multichannel Radio
For designers of digital modulation schemes in wireless systems, Intersil has combined a 12-bit dual-channel DAC with shaping and interpolation filters, a complex modulator, and numerically controlled oscillators (NCOs) on a single silicon substrate. The outcome of this integration is a wideband programmable modulator, the HSP50415, which supports a variety of modulation schemes (Fig. 3). It accepts an input quadrature programmable data stream at programmable symbol rates up to 25 Msamples/s and outputs a modulated quadrature data stream up to 100 Msamples/s. Essentially, it's a quadrature amplitude modulator (QAM) and an upconverter in a single package for wideband digital modulation applications.
Primarily employed for converting baseband to IF in multichannel radio applications, the HSP50415 supports QPSK, 16-256 QAM, and a quadrature PM format. The high-output sample rate generates a high-IF output from the 12-bit on-chip DACs.
According to the HSP50415's developer, the programmable carrier NCO and I and Q shaping FIR filters allow the input and output sample rate to have a noninteger or variable relationship. This makes the architecture flexible, thereby permitting communication systems to handle multiair interface standards. "In short, it's a complete modulation system that modulates the baseband data onto a programmable-carrier center frequency," says Glenn Oliver, CommLink product marketing manager at Intersil. "Consequently, it significantly cuts design time, system cost, and time-to-market."
The HSP50415 modulates the symbol data at the final sample rate onto a carrier signal that's tunable from 0.025 Hz to 50 MHz. To compensate for the (sin x)/x roll-off of the DACs, the wideband modulator provides optional x/(sin x) filters. Plus, the system- or DAC-induced gain imbalances between I and Q signals can be corrected prior to the output. Other key highlights include an SFDR of over 70 dB at an output sample rate of 100 Msamples/s, a constellation mapper, 20 mA of full-scale output current, and 3.3-V supply operation. The HSP50415 comes in a 100-lead MQFP.
Furthermore, Intersil has exploited its expertise in high-speed, high-resolution DACs to generate direct digital synthesizers (DDSs) required in a variety of wireless communications equipment. Integrating a 48-bit programmable carrier NCO, a 125-Msample/s 14-bit DAC, and a comparator on a monolithic CMOS die, the supplier has produced a complete, digitally controlled frequency synthesizer (Fig. 4). This standalone DDS, labeled ISL5314, presents an attractive alternative to analog voltage-controlled oscillators for agile-frequency synthesis applications. "A DDS system's output frequency and phase can be precisely and rapidly manipulated under digital process control," Oliver says.
The ISL5314 is accompanied by a parallel control interface for fast tuning, as well as a serial control interface. While the parallel processor interface is an 8-bit write only for center and offset frequency control registers, the serial interface loads the 40-bit frequency tuning word. The synthesizer is accurate to 0.4 µHz, Intersil claims. Its spurious performance is rated at an SFDR of 75 dBc at a 125-MHz clock frequency and output of 25 MHz. Power consumption is below 500 mW at a 3-V supply. The ISL5314 comes in a 48-pin LQFP package.
Emerging wideband multicarrier applications have also prompted Maxim Integrated Products to quickly move up the performance ladder. Maxim intends to leverage its high-speed expertise in data conversion to create faster high-resolution DACs capable of providing high-frequency output with the very low power re-quired in these applications.
Currently, Maxim is revamping the update rates of 8- and 10-bit 40-Msample/s DACs to 150 MHz. An 80-Msample/s version is in the making, but it's not expected to be released before the end of next year.
The existing MAX5180 series of monolithic CMOS DACs, with 40-MHz update rates, includes 8- and 10-bit models with voltage and current outputs. They consume only 18 mW from a 3-V supply. For I and Q baseband signal reconstruction, the series includes monolithic 10-bit dual DACs too.
As it prepares next-generation DACs, Maxim is exploring techniques to migrate to 0.35-µm CMOS. The supplier intends to increase the functionality on board while tailoring its next-generation wideband DACs for specific communications applications.
Using a segmented current-source architecture, Signal Processing Technologies has pushed the specifications for its DACs to 16 bits with an update rate of 200 MHz. The SPT5510 boasts an ultra-fast settling time of 25 ns to 14 bits, and 35 ns to 16-bit accuracy, with low glitch energy of 30 pV-s. It features true 16-bit linearity, with differential nonlinearity of ±0.6 LSB and integral nonlinearity of ±0.75 LSB.
To serve system designers, board maker Pentek Inc. has been tapping these advances to develop multichannel narrow/wideband digital receivers and transmitters for digital communication systems. To specifically link a DSP system to a radio transmitter, Pentek has developed a VIM module that translates digital baseband signals to IFs as high as 80 MHz. For that, its model 6229 combines a quadrature upconverter with built-in interpolation filters and dual 12-bit oversampling DACs on a single board. It attaches directly to VIM-compatible processor boards.
Thanks to wideband high-resolution and high-speed DACs, a single device can now generate multiple high-frequency carriers with minimal interference and noise. Infrastructure equipment designers are reaping those benefits. As a result, single basestation systems with the ability to handle multiple air-interface standards are a reality. The threat of discarding old equipment for newer standards is a thing of the past.