ADL5375ACPZ-R7 datasheet(24/36 Pages) AD

24 / 36 page

ADL5375

Data Sheet

Rev. C | Page 24 of 36

The two pull-up inductors of the Zbias provide two 50 Ω source

impedances in combination with R1 resistor in parallel for the

filter. While the ADL5375 is specified to be driven by a single-

ended LO, the LOIP and LOIN input pins are naturally

differential. Therefore, the differential LO drive from the

ADF4350 is more desirable.

The output power from the ADF4350 can be set to −4 dBm,

−1 dBm,+2 dBm, and +5 dBm using Register 4 Bits[D2:D1] and

−6 dBm to +7 dBm LO drive level for ADL5375 is recommended.

If the physical distance between the PLL and the IQ modulator

is significant, the filter should be placed adjacent to the IQ

modulator, and two 50 Ω traces should be run between the

devices (since there is a 50 Ω impedance looking from each of

the filter inputs back to each of the PLL outputs).

The ADL5375 evaluation board can be reconfigured for

differential drive and also includes component pads in its LO

path to accommodate a harmonic filter. The ADF4350 evaluation

board can also be configured to provide a differential output and

can be connected directly to the ADL5375 evaluation board.

Optimizing the interface between a PLL LO and I/Q modulator

is discussed further in CN-0134 Broadband Low EVM Direct

Conversion Transmitter: How to Optimize the Interface

Between a PLL LO and I/Q Modulator.

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 AD9122 TxDAC is shown

in Figure 58. The baseband inputs of the ADL5375-05 require a

dc bias of 500 mV. The nominal midscale current on each of the

outputs of the AD9122 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.

IBBN

IBBP

AD9122

ADL5375-05

IOUT1N

IOUT1P

IOUT2

IOUT2N

QBBP

QBBN

RBIP

50Ω

RBIN

50Ω

RBQN

50Ω

RBQP

50Ω

Figure 58. Interface Between the AD9122 and ADL5375-05 with 50 Ω

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

Baseband Inputs

The AD9122 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 AD9122 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 59. It has the effect of

reducing the ac swing without changing the dc bias already

established by the 50 Ω resistors.

IBBN

IBBP

AD9122

ADL5375-05

IOUT1N

IOUT1P

IOUT2

IOUT2N

QBBP

QBBN

RBIP

50Ω

RBIN

50Ω

RBQN

50Ω

RBQP

50Ω

RLI

100Ω

RLQ

100Ω

Figure 59. AC Voltage Swing Reduction Through the Introduction

of a Shunt Resistor Between Differential Pair

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