ADL5385

Rev. 0 | Page 15 of 24

APPLICATIONS

DAC MODULATOR INTERFACING

The ADL5385 is designed to interface with minimal components

to members of the Analog Devices family of digital-to-analog

converters (DAC). These DACs feature an output current swing

from 0 to 20 mA, and the interface described in this section can

be used with any DAC that has a similar output.

Driving the ADL5385 with an Analog Devices TxDAC®

An example of the interface using the AD9777 TxDAC is shown

in Figure 31. The baseband inputs of the ADL5385 require a dc

bias of 500 mV. The average output current on each of the

outputs of the AD9777 is 10 mA. Therefore, a single 50 Ω

resistor to ground from each of the DAC outputs results in an

average current of 10 mA flowing through each of the resistors,

thus producing the desired 500 mV dc bias for the inputs to the

ADL5385.

RBIP

50Ω

RBIN

50Ω

73

72

13

14

IBBN

IBBP

AD9777

ADL5385

RBQN

50Ω

RBQP

50Ω

69

68

17

18

QBBP

QBBN

Figure 31. Interface Between AD9777 and ADL5385 with 50 Ω Resistors to

Ground to Establish the 500 mV DC Bias for the ADL5385 Baseband Inputs

The AD9777 output currents have a swing that ranges from

0 to 20 mA. With the 50 Ω resistors in place, the ac voltage

swing going into the ADL5385 baseband inputs ranges from

0 V to 1 V. A full-scale sine wave out of the AD9777 can be

described as a 1 V p-p single-ended (or 2 V p-p differential)

sine wave with a 500 mV dc bias.

Limiting the AC Swing

There are situations in which it is desirable to reduce the

ac voltage swing for a given DAC output current. This can be

achieved through the addition of another resistor to the

interface. This resistor is placed in shunt between each side of

the differential pair, as illustrated in Figure 32. It has the effect

of reducing the ac swing without changing the dc bias already

established by the 50 Ω resistors.

RBIP

50Ω

RBIN

50Ω

IBBN

IBBP

AD9777

ADL5385

RBQN

50Ω

RBQP

50Ω

RSLI

100Ω

RSLQ

100Ω

QBBP

QBBN

73

72

13

14

69

68

17

18

Figure 32. AC Voltage Swing Reduction Through Introduction of Shunt

Resistor Between Differential Pair

The value of this ac voltage swing-limiting resistor is chosen

based on the desired ac voltage swing. Figure 33 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.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

10

100

1000

10000

Figure 33. Relationship Between AC Swing-Limiting Resistor and

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

Filtering

When driving a modulator from a DAC, it is necessary to

introduce a low-pass filter between the DAC and the modulator

to reduce the DAC images. The interface for setting up the

biasing and ac swing lends itself well to the introduction of such

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

resistors and the ac swing-limiting resistor, thus establishing the

input and output impedances for the filter.

Examples of filters are discussed in the 155 MBPS (STM-1) 128

QAM Transmitter and the CMTS Transmitter Application

sections.