Rainbow Electronics MAX555 - Manual (Page 8)

MAX555
300Msps, 12-Bit DAC with
Complementary Voltage Outputs
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possible latchup. Appropriate decoupling is needed to
prevent digital-section current spikes from affecting the
analog section (Figure 4).
It is recommended that a multilayer PC board be used,
containing a solid ground and power planes. All analog
and digital ground pins must be connected directly to
the analog ground plane at the MAX555, preferably with
a “star connection” at the LGND pins (15 and 16).
High-speed ECL inputs, as well as the output from the
MAX555, should employ good transmission-line tech-
niques, with terminations close to the device pins.
Separate power-supply buses for analog and digital
power supplies are recommended as good general
practice. Best results will be achieved by bypassing
the device pins with high-quality ceramic chip capaci-
tors connected physically close to the pins.
Applications Information
Reference Input
The MAX555 uses an internal op-amp circuit to buffer the
reference current. The input to the op amp may be driv-
en with an external current source of 1.25mA or a 1V
external voltage reference. The reference input is the REF
pin. The input impedance to the op amp is 800
Ω. As
shown in Figure 1, REF/2 is brought out externally with
400
Ω of impedance to the op amp. These reference
inputs can be used to vary the full-scale output for high-
speed multiplying applications. ROFFSET must be con-
nected to analog ground. In addition, a 0.1µF capacitor
should be connected from REF/2 to analog ground to
reduce reference current noise.
Outputs
The analog outputs are laser trimmed to 50 Ω. They can
be used either as a voltage drive with 50
Ω impedance, or
to drive into a virtual null using a transimpedance amplifi-
er. Greater speed is achieved driving into 50
Ω loads.
The differential outputs of the MAX555 may be used to
drive a balun for conversion to a single-ended output,
while at the same time greatly reducing the second-har-
monic content of the output.
Dynamic Performance
The Typical Operating Characteristics graphs show the
MAX555’s performance when used in direct digital synthe-
sis (DDS) applications for generating RF sine waves. The
first six graphs show the MAX555’s spurious-free dynam-
ic range (SFDR) for clock frequencies of 50MHz to
300MHz at various output frequencies. The seventh
graph displays the SFDR for clock frequencies from
50MHz to 350MHz while producing an output frequen-
cy of about 1/16 the clock frequency.
The last two graphs show the MAX555’s third and sec-
ond harmonic distortion while producing an output fre-
quency of about 1/5 f
CLK for clock frequencies from
100MHz to 300MHz as a function of the reference volt-
age. The third harmonic content of the output can be
reduced at clock frequencies below about 200MHz by
reducing the reference voltage from its 1.000V nominal
value. At clock frequencies above about 200MHz, the
output’s third harmonic content is dominated by cou-
pling from the high-speed digital inputs to the output.
Reducing the reference voltage at these high clock
rates increases the third harmonic distortion in the out-
put, since the carrier amplitude drops but the third har-
monic level remains relatively constant.
The second harmonic distortion of the outputs is shown
as a function of clock frequency and reference voltage.
It is relatively constant for clock frequencies below
about 200MHz at different V
REF values. As with the
third harmonic distortion, however, the second harm-
onic distortion also increases at clock frequencies over
200MHz for lower V
REF values. Reducing the swing of
the input logic levels and/or decreasing the rise time of
the digital signals can improve the output’s harmonic
content. Some experimentation may be required to
optimize the MAX555’s performance for a particular
application.
Figure 3 shows the spectrum analyzer plots of the
MAX555 when used in DDS applications. These plots
show the MAX555’s output spectrum at clock frequen-
cies from 50MHz to 300MHz while producing various
output frequencies. Observing the output spectrum
while adjusting the reference voltage or varying the
logic levels is a sensitive method of optimizing MAX555
performance. The plots shown were obtained with a
0.75V reference voltage level and 500mV ECL logic
swings.
Typical Application
Figure 4 shows a typical connection. With VOUT used
to drive a 50
Ω line, the unused complementary output,
VOUT, should also be terminated to 50 Ω. A 1V refer-
ence voltage at REF gives a -0.5V full-scale voltage at
VOUT (when doubly terminated with 50
Ω on the out-
put). Because some loads may represent a complex
impedance, be sure to match the output impedance
with the load. Mismatching the impedances can cause
reflections that will affect AC-performance parameters.
In all applications, the LOOPCRNT pin is always con-
nected to AGND, and compensation capacitors are
connected to pins ALTCOMPC, ALTCOMPIB, and
LBIAS. The LBIAS compensation is recommended for
non-multiplying applications.