Linear Technology’s High-Speed DACsGenerate Signals with Industry-Leading Spectral Purity

Linear Technology has been introducing industry-leading data converters for over 20 years. Starting with industry’s first serial data output ADC, the LTC1290, and followed by the LTC2175 that consumed one-third the power of its competing ADCs, Linear Technology has continued to revolutionize the data converter business by coming up with superior products that push performance boundaries.Linear’s commitment to innovation has brought about modern data converters that deliver unprecedented performance and speed to help find solutions across a multitude of applications. Linear Technology’s ADC portfolio includes precision SAR, high speed pipeline and no latency delta sigma ADCs. Its DAC portfolio comprises voltage and current output precision and high speed DACs.

Delivering one of its latest innovations, Linear Technology, home of the Analog Gurus, has expanded its expertise in linear IC products to create the industry’s highest performancedigital-to-analog converter(DAC), the LTC2000.  End applications ranging from cable modems and other communications-based applications to test equipment and instrumentation will benefit from this leap in performance. The key to LTC2000’s leadership is in the generation of high frequency and broadbandsignals with a low noise floor and high spectral purity.  As shown in figure 1, the spectral purity, as measured by the spurious-free dynamic range (SFDR), exceeds the competition by over 10dB at almost every frequency point.

Fig. 1: Graph Showing LTC2000 SFDR vs Output Frequency

The LTC2000 has a uniquely low additive phase noise and its datasheet explicitly shows the additive phase noise performance as depicted in figure 2. Maintaining a low phase noise is necessary in multi-channel communications systems where it limits channel-to-channel interference, increasing the data transfer efficiency of the system. Also, low phase noise helps resolve close-in information in radar and automatic test equipment (ATE) applications. Doppler radar, for instance, relies on deciphering close-in information. An incident tone gets hugely attenuated before getting reflected at a small frequency shift. Low phase noise helps pick up crucial information in the reflected tone.

Fig. 2: Linear Plot of Addictive Phase Noise with Offset Frequency

Communicationsis evolving at an astronomical rate.  Customers need to keep pace withnew industry advancements, developing increasingly higher performance products as well as the test equipment and instrumentation needed to evaluate these products.  As the industry attempts to transport more data than ever over widening bandwidths, DACs must keep pace with the demand for faster transmission rates with better dynamic range.  High spectral purity is vital for maintaining good signal integrity in applications such asDOCSIS, and for transmission and reception across other communications channels. 

The LTC2000 family comprises of 16-bit, 14-bit, and 11-bit 2.5Gsps and 2.7 GspsDACs.  These advanced devices offer exceptional spectral purity, high full-scale output current, wide analog output bandwidth and low latency.  This family of products features better than 78dBc 2-tone intermodulation distortion (IMD) from DC up to 1080MHz output frequency, low phase noise and a wide 2.1GHz 3dB output bandwidth.  The LTC2000A family accepts up to 635MHz DDR LVDS or DHSTL interface at 2.7 Gspswhen using 32 lanes (dual port mode) or 1.35Gsps when using 16 lanes (single port mode).  In single port mode, the latencyis only 7.5 clock cycles, while in dual port mode it is still a low 11 clock cycles. Maintaining low latency is important in applications such as RADAR or in solutions involving feedback loops.

Rounding out the package is an SPI interface that offers flexibility in control and communications.  In addition to programmability, junction temperature sensing monitors the device in order to optimize the output signal while avoiding excessive heating.  An internal pattern generator to simplify system development and debug and an adjustable 1.25V reference are also included. 

The LTC2000 family of DACs enables the direct synthesis of high frequency or broadband RF signals with exceptional spectral purity from DC to 1 GHz.  The LTC2000 has unmatched analog outputs including:

·   Noise spectral density is better than −158dBc/MHz up to 500MHz

·   Spurious free dynamic range (SFDR) better than 74dBc up to 500MHz

·   Spurious free dynamic range (SFDR) better than 68dBc up to 1GHz

The full scale output current of the LTC2000 enables it to generate signals with excellent signal strength achieving a high compliance level of ±1V.  The output signal current is programmable over a wide range of 10mA to 60mA via an external resistor.  A gain adjustment register that can be accessed via software control enables the output current to be adjusted from 89.2% to 114.3%of the resistor programmed value in 0.4% increments.  The nominal output current is 40mA,yielding high performance SFDR levels over a wide range of frequencies as shown in Figure 3.

Fig. 3: Graph Depicting High-Performance Levels by LTC2000 at 40mA Output Current

Fig. 4: 

Design Support

In order to test run the LTC2000 family, Linear Technology has created a series of demonstration boards to accommodate 16-bit, 14-bit and 11-bit FPGA applications.    Each of the boards accepts DDR LVDS inputs while operating at 2.5Gsps (LTC2000 family) or 2.7Gsps (LTC2000A family) rate.

Fig.5: Table Showing Various Demo Boards of LTC2000 Family

These demo boards offer quick start capability making signal generation achievable in a short amount of time while minimizing the required test equipment.  Onboard voltage regulators provide the voltage levels needed by the LTC2000 from an input of 5V thus limiting the demonstration board’s requirements to a single supply.  The user has the option to provide individual input voltages to the 1.8V and 3.3V digital and analog supplies as well as the SPI supply.  Individual test points are broken out for measuring signals.  A USB interface allows communication via the SPI interface and LTDACGensoftware [4]. The LTC2000 demo board,DC2085, is depicted in Figure 5.

Fig. 6: 2-D view of DC2085 Series Demo Boards Offer Quick Start Capability by Limiting the Inputs to Essential Functions

Linear Technology offers free, downloadable software for programming the LTC2000.  Signal generation can be arranged using the LTDACGen Software which simplifies the evaluation of the LTC2000 [4].    Users select the clock frequency and number of samples to create graphical and tabular output signal data as depicted in Figure 6.

Fig. 7: Image Showing LTDACGen Software that Simplifies Evaluation of LTC2000

The demo manuals that come with the demo boards include complete parts lists (bill of materials) for each demonstration board.  Downloadable schematic [8] diagrams are available via the Linear Technology website along with the Verilog code [9].

The generation of a signal with high spectral purity is best accomplished with a low phase noise, high speed clock.  Linear Technology offers two solutions for clocking the LTC2000 DC2085 family of demonstration boards.  The LTC6946 high frequency synthesizer provides a clean, flexible clock solution from 370 MHz to 6.39 GHz using an internal VCO.  This product has its own demonstration board, the DC1705C [6].  For even lower clock phase noise, the LTC6945 combined with a VCO could be used instead.  The LTC2000 performs automatic internal synchronization to the applied external clock signal. 

Digital signal input from an FPGA is created using the AlteraStratix® IV GX FPGA Development Kit[7].  This kit, along with the provided reference software, has customers up and running quickly in order to transition digital data into high spectral purity analog signals.

Summary

Linear Technology offers an advanced line of DACs in the form of the LTC2000 family of 2.5Gsps and 2.7Gsps products for 16-bit, 14-bit, and 11-bit applications.  This family of advanced solutions enables customers to develop state of the art products for cable modems as well as radar, broadband wireless, instrumentation and automatic test equipment applications.  Along with the demonstration boards and programmable software, customers can be on their way to generating analog signals with high spectral purity in a short amount of time. 

References

1. LTC2000-HS_DAC_FamAd, Linear Technology LTC2000 Family Advertisement http://cds.linear.com/docs/en/ad/LTC2000-HS_DAC_FamAd.pdf

2.“Voltage Output DACs” Linear Technology selector chart http://cds.linear.com/docs/en/product-selector-card/2PB_dacsfb.pdf

3.“2.5 Gbsps 16-/ 14- /11- bit  2000f LTC2000A-16  data sheet,” http://cds.linear.com/docs/en/datasheet/2000f.pdf

4.“LTDDACGen,” software for programming the LTC2000 family of 2.7 Gsps DACs http://ltspice.linear.com/software/LTDACGenInstall.zip

5.“Demo Manual DC2085,” LTC2000 DC2085 Demo Board manual, http://cds.linear.com/docs/en/demo-board-manual/DC2085AFA.PDFi

6.“DC1705B,” LTC6946IUFD clock demoboard manual, http://cds.linear.com/docs/en/demo-board-manual/DC1705bf.pdf

7.“Stratix® IV GX FPGA Development Kit” Altera FPGA programming kit http://www.digikey.com/en/product-highlight/a/altera/stratix-iv-gx-fpga-dev-kit

8.DC2085A Demo Board schematic http://cds.linear.com/docs/en/demo-board-schematic/DC2085A3-SCH.PDF

9. Verilog code for the LTC2000 http://cds.linear.com/docs/en/software-and-simulation/ltc2000_align_all_DACs.v

10.“High Speed DAC for Wide Bandwidth Signal Generation,”  Linear Technology LTC2000 family product video http://www.linear.com/solutions/5091

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