Circuit Note

CN-0205

Rev. 0 | Page 3 of 9

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

Figure 3. Peak-to-Peak Differential Swing and the Swing Limiting

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

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.

RBIP

50Ω

RBIN

50Ω

67

66

21

22

IBBN

IBBP

AD9122

ADL5375-05

RBQN

50Ω

RBQP

50Ω

59

58

9

10

RSLI

100Ω

RSLQ

100Ω

OUT1_N

OUT1_P

OUT2_P

OUT2_N

QBBP

QBBN

LPI

771.1nH

LNI

771.1nH

53.62pF

C1I

350.1pF

C2I

LNQ

771.1nH

LPQ

771.1nH

53.62pF

C1Q

350.1pF

C2Q

Figure 4. DAC Modulator Interface with 10 MHz Third-Order, Bessel Filter

1

10

100

–60

–50

–40

–30

–20

–10

0

0

6

12

18

24

30

36

FREQUENCY (MHz)

MAGNITUDE

GROUP DELAY

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.

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

common-mode frequency with and without IF filter shown in

Figure 7.