LTC1668 LINER [Linear Technology], LTC1668 Datasheet - Page 8

• Manufacturer Part Number: LTC1668
• Description: 16-Bit, 50Msps DAC
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1668
accuracy improves rapidly, roughly in proportion to
1/I
reducing I
SFDR and THD due to increased INL and DNL.
Output Configurations
Based on the specific application requirements, the
LTC1668 allows a choice of the best of several output
configurations. Voltage outputs can be generated by ex-
ternal load resistors, transformer coupling or with an op
amp I-to-V converter. Single-ended DAC output configu-
rations use only one of the outputs, preferably I
produce a single-ended voltage output. Differential mode
configurations use the difference between I
I
equation 7. Differential mode gives much better accuracy
in most AC applications. Because the DAC chip is the point
of interface between the digital input signals and the
analog output, some small amount of noise coupling to
I
is common mode and is canceled by the differential mode
circuit. Other significant digital noise components can be
modeled as V
8
OUT B
OUT A
OUTFS
0.1 F
0.1 F
and I
to generate an output voltage, V
C1
. The AC performance tends to be less affected by
OUTFS
0.1 F
OUT B
R
C2
SET
REF
2k
, except for the unavoidable affects on
is unavoidable. Most of that digital noise
or I
CLOCK SOURCE
U
LOW JITTER
REFOUT
I
COMP1
COMP2
OUTFS
REFIN
V
– 5V
SS
U
noise. In single-ended mode,
CLK
IN
0.1 F
REFERENCE
PULSE GENERATOR
HP8110A DUAL
2.5V
–
+
AGND DGND
W
OUT 1 OUT 2
DIFF
, as shown in
Figure 2. AC Characterization Setup
V
5V
CLK
DD
OUT A
U
OUT A
DB15
0.1 F
HIGH SPEED
16-BIT
CLK
IN
and
DAC
, to
LOGIC ANALYZER WITH
DB0
DIGITAL
DATA
PATTERN GENERATOR
16
I
scale. In differential mode, I
midscale input, corresponding to zero analog output.
Many AC signals, including broadband and multitone
communications signals with high peak to average ratios,
stay mostly near midscale.
Differential transformer-coupled output configurations
usually give the best AC performance. An example is the
AC Characterization Setup circuit, Figure 2. The advan-
tages of transformer coupling include excellent rejection
of common mode distortion and noise over a broad
frequency range and convenient differential-to-single-
ended conversion with isolation or level shifting. Also, as
much as twice the power can be delivered to the load, and
impedance matching can be accomplished by selecting
the appropriate transformer turns ratio. The center tap on
the primary side of the transformer is tied to ground to
provide the DC current path for I
distortion, the DC average of the I
must be exactly equal to avoid biasing the core. This is
especially important for compact RF transformers with
small cores. The circuit in Figure 2 uses a Mini-Circuits
T1-1T RF transformer with a 1:1 turns ratio. The load
HP1663EA
OUTFS
LTC1668
LADCOM
I
I
OUT A
OUT B
noise is gone at zero scale and is fully present at full
1668 F02
50
50
110
OUTFS
MINI-CIRCUITS
OUT A
OUT A
T1–1T
noise is cancelled at
and I
and I
OUT B
OUT B
TO HP3589A
SPECTRUM
ANALYZER
50 INPUT
. For low
currents