High-Speed ADCs and DACs Arm Broadband Communications

Oct. 15, 2008
Increasingly wide signal bandwidths and complex modulation schemes are driving data converter suppliers to push the performance levels on their products.

Data converters, while not directly involved in RF/ microwave communications, play critical roles in those and other high-frequency systems. Whether it is the digital-to-analog converter (DAC) used to create the complex modulation needed to compress voice, video, and data onto limited-bandwidth signals, or the fast analog-to-digital converters that translate received signals into highresolution digital code, data converters are as important to modern communications systems as the antennas, amplifiers, filters, and other RF/microwave components within the system. What follows is a short review of some recent additions to available high-speed, high-resolution ADCs and DACs.

Most suppliers differentiate their ADCs in terms of whether for low-power (portable) use, for high-precision (such as 16-b resolution), or for high-speed (100 MSamples/s or faster) applications. Some can combine several of these major traits into a single device. Analog Devices (www.analog.com), for example, offers its model AD9481 ADC that is optimized for high-speed operation but low power consumption. It delivers 8-b resolution and 250-MSamples/s sampling capability and can be used with single-ended or differential input signals. Suitable for use in digital sampling oscilloscopes (DSOs) and other measurement equipment as well as in communications systems, the ADC runs on a single +3.3-VDC supply with power dissipation of about 439 mW when sampling at 250 MSamples/s. The ADC handles a 1-V peak-to-peak (p-p) input range and incorporates its own on-chip 1-V voltage reference. For many applications, an external reference or driver components are not required.

The company also offers the model AD9214 with higher, 10-b resolution and selectable sampling capacities of 65, 80, and 105 MSamples/s. It consumes 190 mW for 65-MSamples/s operation and 285 mW power for 105-MSamples/s applications. The +3.3-VDC ADC features a 300-MHz input bandwidth for use in digitizing broadband communications channels. It does so with a signal-to-noise ratio (SNR) of 57 dB for an analog input bandwidth of 39 MHz. The single-channel ADC includes an on-chip track-andhold (T/H) amplifier and voltage reference. Standard operation is for an input range of 1 V p-p, with an option for 2-V p-p operation. The ADC has a power-down mode in which power consumption is reduced to a mere 30 mW.

Linear Technology (www.linear.com) supplies a number of high-speed ADCs, including the 16-b model LTC2209 ADC with sampling capability to 160 MSamples/s. With a full-scale noise floor of 77.3 dB and 100-dB SFDR, the LTC2209 offers a full-power input bandwidth of 700 MHz and integral S/H amplifier. It provides LVDS or CMOS outputs and operates from a single +3.3-VDC supply and 1.45-W power consumption.

The company also recently announced its model LTC6401-20, a low-noise differential ADC driver amplifier with 1.3-GHz 3-dB bandwidth and fixed gain of 10 V/V (20 dB). Suitable for use in systems with IFs to 140 MHz, the driver achieves thirdorder intermodulation distortion of -93 dBc for a 70-MHz input signal and -74 dBc for a 140-MHz input signal. The total input noise is 2.1 nV/(Hz)0.5 with a noise figure of 6.2 dB. It features an input impedance of 200 ohms and differential inputs and outputs.

MAXIM Integrated Products (www.maxim-ic.com) has developed its model MAX109 ADC for digitizing analog signals in RF and intermediate-frequency (IF) applications to 2.5 GHz. It provides 8-b resolution and sampling rate to 2.2 GSamples/s. Fabricated on an advanced silicon-germanium (SiGe) process, the MAX109 integrates a high-performance T/H amplifier, a quantizer, and a 1:4 demultiplexer on a single monolithic die. The ADC has a 2.8-GHz full-power input bandwidth. It can be used with single-ended and differential input signals and provides output data in standard low-voltage-differential-signal (LVDS) format.

The company's model MAX1213N is a monolithic, 12-b, 170-MSamples/s ADC optimized for outstanding dynamic performance at high-IFs beyond 300 MHz. At a sampling rate of 170 MSamples/s and with an input frequency of 100 MHz, it achieves a SFDR of -87 dBc with 67.2-dB SNR. This makes it ideal for wideband applications such as communications receivers, cablehead end receivers, and power-amplifier predistortion in cellular base-station transceivers. The power consumption at that sampling rate is 720 mW. The ADC operates from a single +1.8-VDC supply and is designed for single-ended or differential operation. It also features a selectable on-chip divide-by-2 clock circuit that accepts clock frequencies as high as 340 MHz, and provides standard LVDS outputs.

The firm's model MAX1215N is a monolithic ADC with 250-MSamples/s sampling rate and 12-b resolution. It can handle input IF bandwidths beyond 300 MHz and consumes only 886 mW power at rates to 250 MSamples/s. For an input frequency of 100 MHz, and operating at 250 MSamples/s, the SFDR is -84.7 dBc and the SNR is 66.7 dB, flat within 2 dB across the range. The MAX1215N operates from a single +1.8-VDC supply.

National Semiconductor (www.national.com) provides extensive lines of 8-b ADCs with high speed and low power consumption. The company's single-channel model ADC083000 consumes on 1.9 W at a sampling rate of 3 GSamples/s. Part of the company's PowerWise Family of ADCs, it operates on a +1.9-VDC supply and consumes only 25 mW power in a power-down mode. The ADC083000 can also be ordered in a buffered version, with a 4-kB buffer, as model ADC08B3000. The firm's dualinput model ADC08D1500 consumes only 1.8 W power for both channels while operating at 1.5 GSamples/s. The dual ADC can also be used for just one channel at sampling rates to 3 GSamples/s by interleaving the two internal channels. The typical SFDR is 56 dB for these ADCs, with SNR of typically 47 dB.

In terms of DACs, ADI offers the model AD9734 with 10-b resolution and 1.2 MSamples/s speed. The current-output, single-channel DAC features a 1.2-V on-chip reference and provides adjustable analog outputs from 8.66 to 31.66 mA. It operates on dual supplies of +1.8 and +3.3 VDC. The high-speed DAC features a parallel, LVDS input format.

For designs calling for a high amount of integration, Analog Devices also supplies the model ADV7123 high-speed device, which incorporates three 10-b 330-MHz DACs on a single device. Suitable for video processing, the DAC provides SFDR of 70 dB at 50 MSamples/s and an output frequency of 1 MHz. It operates from a single +3.3- or +5-VDC supply and provides output current from 2 to 26 mA. The CMOS DAC has TTLcompatible inputs and consumes only 30 mW power at +3.3 VDC.

Linear Technology (www.linear.com) has developed its LTC1666, LTC1667, and LTC1668 high-speed DACs for cellular base stations and other communications systems requiring 12-, 14-, and 16-b resolution, respectively, at update rates to 500 MSamples/s. Supplying differential current outputs, the DACs offer fast 20-ns settling time and high spectral purity with a SFDR of 87 dB for an output signal frequency of 1 MHz. The low-power DACs consume only 180 mW power from 5-VDC supplies. The high-performance DACs are fabricated with a silicon BiCMOS process using laser-trimmed, thin-film resistors. The DACs, which offer singleended or differential operation, can be configured to provide full-scale output currents to 10 mA. In addition to use in wireless communications systems, the DACs are well suited for direct-digital- synthesis (DDS) frequency sources, arbitrary waveform generation, and in measurement equipment.

Texas Instruments (www.ti.com) offers its model DAC5681 DAC with 16-b resolution and 1 GSamples/s processing speed. The single-channel device operates with a parallel, LVDS input signal format and current output format, and achieves impressive 81-dB typical SFDR and 79-dB typical SNR. The firm's model DAC5688 is a two-channel, 16-b device that operates at 800 MSamples/s with a parallel interface. It can achieve an SFDR of typically 80 dB and SNR of typically 79 dB.

For those seeking an introduction to data converters, many device suppliers offer excellent information sources on their web sites to help specifiers better understand the basic operating parameters and how to match products to applications. National Semiconductor (www.national.com), for example, shows the 64-page "ABCs of ADCs: Analog-to-Digital-Converter Basics" for free download in PDF file format. Texas Instruments (www.ti.com) has the 22-page Application Report, "Understanding Data Converters," for those who need a general overview of these critical system components.

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