ADL5375-15ACPZ-R71 datasheet(23/32 Pages) AD

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ADL5375

Rev. 0 | Page 23 of 32

APPLICATIONS INFORMATION

DAC MODULATOR INTERFACING

Driving the ADL5375-05 with a TXDAC®

The ADL5375-05 is designed to interface with minimal

components to members of the Analog Devices, Inc. TxDAC

families. These dual-channel differential current output DACs

feature an output current swing from 0 mA to 20 mA. The

interface described in this section can be used with any DAC

that has a similar output.

An example of an interface using the AD9779 TxDAC is shown

in Figure 57. The baseband inputs of the ADL5375-05 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 resis-

tor 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

ADL5375-05.

RBIP

50Ω

RBIN

50Ω

OUT1_N

OUT1_P

IBBN

IBBP

AD9779

ADL5375-05

RBQN

50Ω

RBQP

50Ω

OUT2_P

OUT2_N

QBBP

QBBN

Figure 57. Interface Between the AD9779 and ADL5375-05 with 50 Ω

Resistors to Ground to Establish the 500 mV DC Bias for the ADL5375-05

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 ADL5375-05 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 the shunt between each side of the

differential pair, as shown in Figure 58. 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

AD9779

ADL5375-05

RBQN

50Ω

RBQP

50Ω

RSLI

100Ω

RSLQ

100Ω

OUT1_N

OUT1_P

OUT2_P

OUT2_N

QBBP

QBBN

Figure 58. 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 59 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 59. 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 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. Doing so establishes the input and output

impedances for the filter.

Figure 60 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 61.

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