REV. C
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., f
H
) 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., V
PEAK
/V
rms) that this bandlimited signal will encounter.
MAGNITUDE – dBm
FREQUENCY – MHz
–10
–70
–110
2.19 2.812.25 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
VOLTS
–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.,
f
CLOCK
/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
