AD9764ARZ datasheet(16/22 Pages) AD

16 / 22 page

REV. B

AD9764

–16–

MULTITONE PERFORMANCE CONSIDERATIONS AND

CHARACTERIZATION

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 dBFS), 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, this full-scale sine wave repre-

sents the lowest peak-to-rms ratio or crest factor (i.e., VPEAK/V

rms) that this bandlimited signal will encounter.

FREQUENCY – MHz

–10

–70

–110

2.19

2.81

2.25

2.31

2.38

2.44

2.50

2.56

2.63

2.69

2.75

–20

–60

–80

–100

–40

–50

–90

–30

Figure 39a. Multitone Spectral Plot

TIME

1.0000

0.8000

–1.0000

–0.2000

–0.4000

–0.6000

–0.8000

0.2000

0.0000

0.4000

0.6000

Figure 39b. Time Domain “Snapshot” of the Multitone

Waveform

However, the inherent nature of a multitone, spread spectrum,

or QAM waveform, in which the spectral energy of the wave-

form 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 39a is

just one example of a bandlimited multitone vector (i.e., eight

tones) centered around one-half the Nyquist bandwidth (i.e.,

fCLOCK/4). This particular multitone vector, has a peak-to-rms

ratio of 13.5 dB compared to a sine waves peak-to-rms ratio of

3 dB. A “snapshot” of this reconstructed multitone vector in the

time domain as shown in Figure 39b reveals the higher signal

content around the midscale value. As a result, a DAC’s

“small-scale” dynamic and static linearity becomes increas-

ingly critical in obtaining low intermodulation distortion and

maintaining sufficient carrier-to-noise ratios for a given modula-

tion scheme.

A DAC’s small-scale linearity performance is also an important

consideration in applications where additive dynamic range is

required for gain control purposes or “predistortion” signal

conditioning. For instance, a DAC with sufficient dynamic

range can be used to provide additional gain control of its

reconstructed signal. In fact, the gain can be controlled in

6 dB increments by simply performing a shift left or right on the

DAC’s digital input word. Other applications may intentionally

predistort a DAC’s digital input signal to compensate for

nonlinearities associated with the subsequent analog compo-

nents in the signal chain. For example, the signal compression

associated with a power amplifier can be compensated for by

predistorting the DAC’s digital input with the inverse nonlinear

transfer function of the power amplifier. In either case, the

DAC’s performance at reduced signal levels should be carefully

evaluated.

A full-scale single tone will induce all of the dynamic and static

nonlinearities present in a DAC that contribute to its distortion

and hence SFDR performance. Referring to Figure 3, as the

frequency of this reconstructed full-scale, single-tone waveform

increases, the dynamic nonlinearities of any DAC (i.e., AD9764)

tend to dominate thus contributing to the rolloff in its SFDR

performance. However, unlike most DACs, which employ an R-2R

ladder for the lower bit current segmentation, the AD9764 (as

well as other TxDAC members) exhibits an improvement in

distortion performance as the amplitude of a single tone is re-

duced from its full-scale level. This improvement in distortion

performance at reduced signal levels is evident if one compares

the SFDR performance vs. frequency at different amplitudes

(i.e., 0 dBFS, –6 dBFS and –12 dBFS) and sample rates as

shown in Figures 4 through 7. Maintaining decent “small-scale”

linearity across the full span of a DAC transfer function is also

critical in maintaining excellent multitone performance.

Although characterizing a DAC’s multitone performance tends

to be application-specific, much insight into the potential per-

formance of a DAC can also be gained by evaluating the DAC’s

swept power (i.e., amplitude) performance for single, dual and

multitone test vectors at different clock rates and carrier frequen-

cies. The DAC is evaluated at different clock rates when recon-

structing a specific waveform whose amplitude is decreased in

3 dB increments from full-scale (i.e., 0 dBFS). For each specific

waveform, a graph showing the SFDR (over Nyquist) perfor-

mance vs. amplitude can be generated at the different tested

clock rates as shown in Figures 9–11. Note that the carrier(s)-

to-clock ratio remains constant in each figure. In each case, an

improvement in SFDR performance is seen as the amplitude is

reduced from 0 dBFS to approximately –9.0 dBFS.

A multitone test vector may consist of several equal amplitude,

spaced carriers each representative of a channel within a defined

bandwidth as shown in Figure 39a. In many cases, one or more

tones are removed so the intermodulation distortion performance

AD9764ARZ Folha de dados(PDF) 16 Page - Analog Devices

AD9764ARZ Folha de dados(HTML) 16 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Folha de dados Download

Ligação URL