ADL5370

Rev. 0 | Page 13 of 20

APPLICATIONS INFORMATION

DAC MODULATOR INTERFACING

The ADL5370 is designed to interface with minimal components

to members of the Analog Devices family of DACs. 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 ADL5370 with an Analog Devices TxDAC®

An example of the interface using the AD9779 TxDAC is shown

in Figure 31. The baseband inputs of the ADL5370 require a dc

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

outputs of the AD9779 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

ADL5370.

RBIP

50Ω

RBIN

50Ω

93

92

19

20

OUT1_N

OUT1_P

IBBN

IBBP

AD9779

ADL5370

RBQN

50Ω

RBQP

50Ω

84

83

23

24

OUT2_P

OUT2_N

QBBP

QBBN

Figure 29. Interface Between the AD9779 and ADL5370 with 50 Ω Resistors to

Ground to Establish the 500 mV DC Bias for the ADL5370 Baseband Inputs

The AD9779 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 ADL5370 baseband inputs ranges from 0 V to 1 V.

A full-scale sine wave out of the AD9779 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 shown in Figure 30. 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

93

92

19

20

IBBN

IBBP

AD9779

ADL5370

RBQN

50Ω

RBQP

50Ω

84

83

23

24

RSLI

100Ω

RSLQ

100Ω

OUT1_N

OUT1_P

OUT2_P

OUT2_N

QBBP

QBBN

Figure 30. AC Voltage Swing Reduction Through the Introduction

of a Shunt Resistor Between Differential Pair

The value of this ac voltage swing limiting resistor is chosen

based on the desired ac voltage swing. Figure 31 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 31. 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 low-pass filter the DAC outputs to remove

images when driving a modulator. The interface for setting up

the biasing and ac swing that was 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. Doing so establishes

the input and output impedances for the filter.

An example is shown in Figure 32 with a third-order elliptical

filter with a 3 dB frequency of 3 MHz. Matching input and output

impedances makes the filter design easier, so the shunt resistor

chosen is 100 Ω, producing an ac swing of 1 V p-p differential.