ADL5375-15ACPZ-R7 datasheet(19/36 Pages) AD

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Data Sheet

ADL5375

Rev. C | Page 19 of 36

THEORY OF OPERATION

CIRCUIT DESCRIPTION

The ADL5375 can be divided into five circuit blocks: the LO

interface, the baseband voltage-to-current (V-to-I) converter,

the mixers, the differential-to-single-ended (D-to-S) stage,

and the bias circuit. A block diagram of the device is shown in

Figure 53.

PHASE

SPLITTER

LOIP

LOIN

IBBP

IBBN

QBBP

QBBN

RFOUT

DSOP

Figure 53. Block Diagram

The LO interface generates two LO signals in quadrature.

These signals are used to drive the mixers. The I/Q baseband

input signals are converted to currents by the V-to-I stages,

which then drive the two mixers. The outputs of these mixers

combine to feed the output balun, which provides a single-

ended output. The bias cell generates reference currents for

the V-to-I stage.

LO Interface

The LO interface consists of a polyphase quadrature splitter

and a limiting amplifier. The LO input impedance is set by

the polyphase splitter. Each quadrature LO signal then passes

through a limiting amplifier that provides the mixer with a

limited drive signal.

The LO input can be driven single-ended or differentially.

For applications above 3 GHz, improved OIP2 and LO leakage

may result from driving the LO input differentially.

V-to-I Converter

The differential baseband inputs (QBBP, QBBN, IBBN, and

IBBP) present a high impedance. The voltages applied to these

pins drive the V-to-I stage that converts baseband voltages into

currents. The differential output currents of the V-to-I stages

feed each of their respective mixers. The dc common-mode

voltage at the baseband inputs sets the currents in the two

mixer cores. Varying the baseband common-mode voltage

influences the current in the mixer and affects overall modula-

tor performance. The recommended dc voltage for the baseband

common-mode voltage is 500 mV dc for the ADL5375-05 and

1500 mV for the ADL5375-15.

Mixers

The ADL5375 has two double-balanced mixers: one for the

in-phase channel (I channel) and one for the quadrature chan-

nel (Q-channel). The output currents from the two mixers sum

together into an internal load. The signal developed across this

load is used to drive the D-to-S stage.

D-to-S Stage

The output D-to-S stage consists of an on-chip active balun

that converts the differential signal to a single-ended signal.

The balun presents 50 Ω impedance to the output (VOUT).

Therefore, no matching network is needed at the RF output

for optimal power transfer in a 50 Ω environment.

Bias Circuit

An on-chip band gap reference circuit is used to generate a

proportional-to-absolute temperature (PTAT) reference current

for the V-to-I stage.

DSOP

The DSOP pin can be used to disable the output stage of the

modulator. If the DSOP pin is connected to ground or left

unconnected, the part operates normally. If the DSOP pin is

connected to the positive voltage supply, the output stage is

disabled and the LO leakage is also reduced.

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