AD9754

–17–

REV. A

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 34b reveals the higher signal

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

scale” dynamic and static linearity becomes increasingly criti-

cal in obtaining low intermodulation distortion and maintaining

sufficient carrier-to-noise ratios for a given modulation 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

TIME

1.0000

0.8000

–1.0000

–0.2000

–0.4000

–0.6000

–0.8000

0.2000

0.0000

0.4000

0.6000

Figure 34b. Time Domain “Snapshot” of the Multitone

Waveform

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., AD9754)

tend to dominate thus contributing to the roll-off in its SFDR

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

ladder for the lower bit current segmentation, the AD9754 (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 perfor-

mance 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 37a. In many cases, one or more

tones are removed so the intermodulation distortion performance

of the DAC can be evaluated. Nonlinearities associated with the

DAC will create spurious tones of which some may fall back into

the “empty” channel thus limiting a channel’s carrier-to-noise

ratio. Other spurious components falling outside the band of

interest may also be important, depending on the system’s spectral

mask and filtering requirements.

This particular test vector was centered around one-half the

would lead to an improvement in performance while centering

degradation in performance.