ADL5385ACPZ-R7 datasheet(17/24 Pages) AD

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Data Sheet

ADL5385

Rev. D | Page 17 of 24

APPLICATIONS INFORMATION

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 mA 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 33. 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

IBBN

IBBP

AD9777

ADL5385

RBQN

50Ω

RBQP

50Ω

IOUTB1

IOUTA1

IOUTA2

IOUTB2

QBBP

QBBN

Figure 33. 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 mA 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 34. 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

IOUTB1

IOUTA1

IOUTA2

IOUTB2

Figure 34. 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 35 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

100

1000

10000

RL (Ω)

Figure 35. 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 section and the CMTS Transmitter

Application section.

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