ADL5375-15ACPZ-R7 datasheet(25/36 Pages) AD

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Data Sheet

ADL5375

Rev. C | Page 25 of 36

The value of this ac voltage swing limiting resistor is chosen

based on the desired ac voltage swing. Figure 60 shows the

relationship between the swing-limiting resistor and the peak-

to-peak ac swing that it produces when 50 Ω bias-setting

resistors are used. The differential peak-to-peak swing at the

modulator input is

[

]

[

]

SIGNAL

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

100

1000

10000

RL (Ω)

Figure 60. Relationship Between the AC Swing-Limiting Resistor and the

Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors

Filtering

It is necessary to place an antialiasing filter between the DAC

and modulator to filter out Nyquist images, common mode

noise, and broadband DAC noise. The interface for setting up

the biasing and ac swing discussed in the Limiting the AC

Swing section lends itself well to the introduction of such a

filter. The filter can be inserted between the dc bias setting

resistors and the ac swing-limiting resistor. With this configuration,

the dc bias setting resistors and the signal scaling resistors

conveniently set the source and load resistances for the filter.

Figure 61 shows a third-order, Bessel low-pass filter with a 3 dB

frequency of 10 MHz. Matching input and output impedances

make the filter design easier, so 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 62.

IBBN

IBBP

AD9122

ADL5375-05

IOUT1N

IOUT1P

IOUT2

IOUT2N

QBBP

QBBN

RBIP

50Ω

RBIN

50Ω

RBQN

50Ω

RBQP

50Ω

RSLI

100Ω

RSLQ

100Ω

350.1pF

C2I

350.1pF

C2Q

53.62pF

C1I

53.62pF

C1Q

LPI

771.1nH

LNI

771.1nH

LNQ

771.1nH

LPQ

771.1nH

Figure 61. DAC Modulator Interface with

10 MHz Third-Order, Bessel Filter

100

–60

–50

–40

–30

–20

–10

FREQUENCY (MHz)

MAGNITUDE

GROUP DELAY

Figure 62. Frequency Response for DAC Modulator Interface with

10 MHz Third-Order Bessel Filter

Complex IF Operation

The ADL5375 can be used with a DAC, generating a complex-

IF (CIF), as well as a zero-IF signal (ZIF). The −1 dB bandwidth

of the ADL5375 is approximately more than 400 MHz

(Figure 63 and Figure 64 show the baseband frequency response

of the ADL5375, facilitating high CIF and providing sufficient

flat bandwidth for digital predistortion (DPD) algorithms).

Using a CIF places the LO leakage and the undesired sideband

outside the signal band at the modulator output where they can

be easily removed with a bandpass filter.

–1

–2

–3

–4

–5

–6

100

BASEBAND FREQUENCY (MHz)

Figure 63. ADL5375-05 Baseband Frequency Response Normalized to

Response for 1 MHz

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