DACs Ready To Tackle New Challenges
Digital-to-analog converters (DACs) have made significant strides in the last few decades and have spurred the use of digital techniques in a multitude of applications. The performance from a single chip only seems to get better every day as processes continue to advance and novel topologies continue to emerge. Hybrids and boards are relegated to niche applications where users are looking for a total solution with specific features and interfaces from a single module or board. Meanwhile, monolithics have taken control of the majority of applications.
Today, 24-bit resolution accompanied by excellent performance from a single low-cost miniature package is standard. CMOS reigns in areas where density must accompany low power and low supply voltages. BiCMOS takes precedence where speed and resolution must be combined with superior ac and dc performance. Progress in process technologies is letting designers pack multiple high-performance, high-resolution DACs on one die. In addition, more functions and features are being integrated without compromising performance, while keeping the power budget under control and the cost attractive.
Driven by some emerging applications, integration is on the rise. To serve optical switching requirements, some suppliers are attempting to pack 128 DACs in one package. Others are integrating high-resolution devices with flash memory to store linearization curves, as well as adding the latest I2C interface. Furthermore, high-performance DACs are being merged with powerful DSP cores on the same piece of silicon.
The future even looks brighter as self-adaptive silicon techniques are ready to tackle new challenges and bring themselves on a par with mainstream digital circuits. Both precision analog and faster digital circuits can reside side by side on a single SoC solution using the lowest-cost, highest-density standard CMOS process available from any foundry.
The delta-sigma (Δ-∑)architecture continues to dominate high-resolution DACs that run at slow to moderate speeds. In this sector, designers combine a multibit second-order and higher architecture with a deep-submicron CMOS process to push resolution to 24 bits with high dynamic performance and low noise at 3- or 5-V single-supply voltages. As they exploit the state-of-the-art CMOS technology, these converters permit more functionality on-chip. They also allow multiple such units to be integrated on the same die for multichannel applications.
To meet the stringent dynamic requirements of 3G wideband, multicarrier cellular basestations, and broadband software radios, suppliers are readying segmented-current source DACs with 14-bit and better resolution with input data rates of 160 Msamples/s and output rates of 400 Msamples/s. Based on an interpolating architecture, they implement on-chip selectable 2x/4x/8x digital interpolation filters to provide high image rejection and low intermodulation distortion of 80 dBc up to 30 MHz. These converters, especially those from Analog Devices, also achieve adjacent-channel power ratio (ACPR) of 71 dBc at 71 MHz. In its latest incarnation, Texas Instruments (TI) has boosted the ACPR and third-order intermodulation distortion performance of its 14-bit 400-Msample/s offering to support IF frequencies above 150 MHz. Plus, it includes on-chip a low-voltage differential signaling interface (LVDS) to ASIC and FPGA.
For PC graphics, set-top boxes, and other imaging applications, the trend is toward monolithic 10- and 12-bit triple DAC chips with very good linearity specifications. For single-channel video DACs, the trend is toward 14 bits at a 20-Msample/s sampling rate, with 16-bit, 20-Msample/s versions on the drawing board. Signal Processing Technology (www.spt.com) has added such correction in its latest 14-bit solution to offer INL of 0.5 LSB and DNL close to 1 LSB.
Today, 5-V DACs based on 0.35-µm CMOS are prevalent. Some 3-V high-resolution, high-performance versions have hit the market too. Now, the makers are prepping high-resolution and high-speed versions with excellent ac and dc performance to operate at 2.5 V. For instance, Signal Processing Technology (www.spt.com) is readying a 14-bit, 40-Msample/s unit to operate at 3 V, and a 10-bit, 70-Msample/s device to perform at 2.5 V.