Volume 33, Number 4, April, 1999|
Download this article in PDF format. (58 KB)
New TxDAC® Generation
Fig. 1 AD9754 functional block diagram.
These DACs are current-output; they have a nominal 20-mA full-scale output with a 100-kW source impedance and 1.25-V compliance range. The outputs are differential to support either differential or single-ended applications. The devices include an internal 1.20-V reference and a control amplifier which can set output full-scale values from 2 mA for power conservation to 20 mA, providing 20 dB of output flexibility. They have low dissipation of 190 mW, which can be reduced to 65 mW at the 2-mA end of the output scale, and just 20 mW in power-down mode. They are equipped with edge-triggered latches, and the digital interface is CMOS compatible (+2.7 to 5.5 V)
Emerging wireless/wireline communications standards (Wideband, Multicarrier) are demanding lower distortion and noise to improve system capacity and signal quality. The D/A converter in a modem/transceiver transmitting chain (TxDAC) is the basic analog signal generator; it defines the ultimate performance available in a communications system. Everything after it-power amplifier, antenna, transmission medium, and receiver front end-can only contribute to the degradation of a digitally defined signal, and the end result can never be better than what is at first available from the TxDAC.
The AD975x family is specifically designed for applications such as wideband CDMA, software/multi-carrier base stations, wireless local-loop RIUs, wireless LAN, broadband set-top boxes, and xDSL modems that utilize wide signal bandwidths to transmit high volumes of data over a desired medium. Key specifications over a 25-MHz band, such as 68-dBc SFDR (spurious-free dynamic range) and -65 dBc THD (total harmonic distortion), are improved over those of the AD976x family by 8 to 10 dB, accompanied by a 9-dB lower integrated noise floor at -109 dB.
The AD975x DACs operate over the industrial temperature range from -40 to +85°C. Evaluation boards are available. Prices* (1000s) for 14/12/10 bits are $21.09/$16.70/9.45.
What's special about DACs for multitone?
The frequency-domain performance of high-speed DACs has traditionally been characterized by analyzing the spectral output of a reconstructed full-scale (i.e., 0 dB FS) single-tone sine wave at a particular output frequency and update rate. Although this characterization data is useful, it is often insufficient to reflect a DAC's performance for a reconstructed multitone or spread-spectrum waveform. In fact, evaluating a DAC's spectral performance using a full-scale single-tone at the highest specified frequency (i.e., fH) of a bandlimited waveform is typically indicative of a DAC's "worst-case" performance for that given waveform. In the time domain, the full-scale sine wave represents the lowest peak-to-rms crest factor (i.e., VPEAK/VRMS) that the bandlimited signal will encounter.
However, the inherent nature of a multitone, spread-spectrum, or QAM waveform, in which the spectral energy of the waveform is spread over a designated bandwidth, will result in a higher peak-to-rms ratio when compared to the case of a simple sine wave. As the reconstructed waveform's peak-to-average ratio increases, an increasing amount of the signal energy is concentrated around the DAC's midscale value. Figure 2 is just one example of a bandlimited multitone vector (i.e., eight tones) centered on one-half the Nyquist bandwidth (i.e., fCLOCK / 4).
Figure 2. Multitone spectral plot.
Figure 3. Time-domain "snapshot" of the multitone waveform.
A full-scale single tone will induce all of the dynamic and static nonlinearities present in a DAC that contribute to its distortion; the distortion components will reduce its SFDR (Spurious-free dynamic range). Thus, in Figure 4 as the frequency of the digitally generated single-tone waveform increases, the dynamic linearities of a DAC (here the AD9754) tend to dominate, thus contributing to the substantial reduction in SFDR with increased frequency.
Figure 4. AD9754 SFDR vs. fOUT @ 0 dB FS.
Figure 5. AD9754 SFDR vs. fOUT at 65 MSPS.
*Prices indicated here are recommended resale prices (U.S. Dollars) FOB U.S.A. Prices are subject to change without notice. For specific price quotations, get in touch with our sales offices or distributors.