AD9754
–16–
REV. A
An example serves to illustrate the effect of supply noise on the
analog supply. Suppose a switching regulator with a switching
frequency of 250 kHz produces 10 mV rms of noise and for
simplicity sake (i.e., ignore harmonics), all of this noise is con-
centrated at 250 kHz. To calculate how much of this undesired
noise will appear as current noise super imposed on the DAC’s
full-scale current, I
OUTFS
, one must determine the PSRR in dB
using Figure 32 at 250 kHz. To calculate the PSRR for a given
R
LOAD
, such that the units of PSRR are converted from A/V to
V/V, adjust the curve in Figure 32 by the scaling factor 20 × Log
(R
LOAD
). For instance, if R
LOAD
is 50 Ω, the PSRR is reduced
by 34 dB (i.e., PSRR of the DAC at 1 MHz which is 74 dB in
Figure 32 becomes 40 dB V
OUT
/V
IN
).
Proper grounding and decoupling should be a primary objective
in any high speed, high resolution system. The AD9754 features
separate analog and digital supply and ground pins to optimize
the management of analog and digital ground currents in a
system. In general, AVDD, the analog supply, should be decoupled
to ACOM, the analog common, as close to the chip as physi-
cally possible. Similarly, DVDD, the digital supply, should be
decoupled to DCOM as close as physically as possible.
For those applications requiring a single +5 V or +3 V supply
for both the analog and digital supply, a clean analog supply
may be generated using the circuit shown in Figure 33. The
circuit consists of a differential LC filter with separate power
supply and return lines. Lower noise can be attained using low
ESR type electrolytic and tantalum capacitors.
100mF
ELECT.
10-22mF
TANT.
0.1mF
CER.
TTL/CMOS
LOGIC
CIRCUITS
+5V OR +3V
POWER SUPPLY
FERRITE
BEADS
AVDD
ACOM
Figure 33. Differential LC Filter for Single +5 V or +3 V
Applications
Maintaining low noise on power supplies and ground is critical
to obtain optimum results from the AD9754. If properly
implemented, ground planes can perform a host of functions on
high speed circuit boards: bypassing, shielding current trans-
port, etc. In mixed signal design, the analog and digital portions
of the board should be distinct from each other, with the analog
ground plane confined to the areas covering the analog signal
traces, and the digital ground plane confined to areas covering
the digital interconnects.
All analog ground pins of the DAC, reference and other analog
components should be tied directly to the analog ground plane.
The two ground planes should be connected by a path 1/8 to
1/4 inch wide underneath or within 1/2 inch of the DAC to
maintain optimum performance. Care should be taken to ensure
that the ground plane is uninterrupted over crucial signal paths.
On the digital side, this includes the digital input lines running
to the DAC as well as any clock signals. On the analog side, this
includes the DAC output signal, reference signal and the supply
feeders.
The use of wide runs or planes in the routing of power lines is
also recommended. This serves the dual role of providing a low
series impedance power supply to the part, as well as providing
some “free” capacitive decoupling to the appropriate ground
plane. It is essential that care be taken in the layout of signal and
power ground interconnects to avoid inducing extraneous volt-
age drops in the signal ground paths. It is recommended that all
connections be short, direct and as physically close to the pack-
age as possible in order to minimize the sharing of conduction
paths between different currents. When runs exceed an inch in
length, strip line techniques with proper termination resistors
should be considered. The necessity and value of this resistor
will be dependent upon the logic family used.
For a more detailed discussion of the implementation and
construction of high speed, mixed signal printed circuit boards,
refer to Analog Devices’ application notes AN-280 and AN-333.
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 represents
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 34a. Multitone Spectral Plot
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 34a is
just one example of a bandlimited multitone vector (i.e., eight
tones) centered around one-half the Nyquist bandwidth (i.e.,
