CN-0205 Datasheet, PDF (3/9 Pages) Analog Devices – Interfacing the ADL5375 I/Q Modulator to the AD9122 Dual Channel, 1.2 GSPS High Speed DAC

Circuit Note
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
10
100
1k
10k
R_L (Ω)
Figure 3. Peak-to-Peak Differential Swing and the Swing Limiting
Resistor (RL) with 50 Ω Bias-Setting Resistors
Baseband Filtering
A filter must be inserted between the AD9122 and ADL5375 to
remove Nyquist images, spurs, and broadband noise originating
from the DAC. The filter should be placed between the dc bias
setting resistors and the ac swing-limiting resistor. With this
configuration, the dc bias setting resistors (RB in Figure 4) and
the signal scaling resistors (RL in Figure 4) conveniently set the
source and load resistances for the filter design.
Figure 4 shows a third-order Bessel low-pass filter with a −3 dB
frequency of 10 MHz. Matching input and output impedances of
the filter makes the filter design easy and results in better
passband flatness, which allows wide bandwidth filter designs.
In this example, the shunt resistor chosen is 100 Ω, producing
an ac swing of 1 V p-p differential. The frequency response of
this filter is shown in Figure 5.
AD9122
OUT1_P
OUT1_N
67
RBIP
50Ω
RBIN
66 50Ω
53.62pF
C1I
LPI
771.1nH
350.1pF
C2I
LNI
771.1nH
RSLI
100Ω
ADL5375-05
21
IBBP
22
IBBN
OUT2_N
OUT2_P
59
RBQN
50Ω
RBQP
58 ^50Ω
53.62pF
C1Q
LNQ
771.1nH
350.1pF
C2Q
LPQ
771.1nH
RSLQ
100Ω
9
QBBN
10
QBBP
Figure 4. DAC Modulator Interface with 10 MHz Third-Order, Bessel Filter
CN-0205
0
36
MAGNITUDE
–10
30
–20
24
GROUP DELAY
–30
18
–40
12
–50
6
–60
1
10
FREQUENCY (MHz)
0
100
Figure 5. Frequency Response for DAC Modulator Interface with
10 MHz Third-Order Bessel Filter
Filtering for Complex IF (CIF) Applications
Figure 6 shows the frequency response of the ADL5375
baseband I and Q inputs. Because this device has a wide and flat
frequency response (−3 dB point = 750 MHz), it is well suited to
complex IF (CIF) applications where the output signal from the
DAC has been digitally upconverted. In CIF applications, a low-
pass Nyquist filter is still desirable, primarily because the dc bias
level can be preserved from the DAC output to the modulator
input.
The filter topology shown in Figure 7 is a 5^th order Butterworth
filter with a 300 MHz corner frequency and is the
recommended filter topology. A purely differential filter can
reject differential-mode images, spurs, and noise from the DAC.
Using two capacitors with their common connection grounded
(C2 and C4 in Figure 7) diverts some of the common-mode
current to ground and results in better common-mode rejection
of high-frequency signals than would be obtained with a purely
differential filter.
The simulated and measured responses of this filter are shown
in Figure 8 and Figure 9. The measured flatness is ±0.6 dB from
dc to 250 MHz and ±0.4 dB from 125 MHz to 250 MHz. This
data was taken with the AD9122 inverse sinc function on. In
this configuration, Figure 10 shows the common-mode
rejection performance of the 2 × FDAC common-mode spur vs.
common-mode frequency with and without IF filter shown in
Figure 7.
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