ADL5385

Rev. 0 | Page 12 of 24

CIRCUIT DESCRIPTION

OVERVIEW

The ADL5385 can be divided into five sections: the local

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

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

amplifier, and the bias circuit. A detailed block diagram of the

device is shown in Figure 26.

TEMPERATURE

SENSOR

BIAS

VOUT

TEMP

ENBL

IBBP

IBBN

QBBP

QBBN

LOIP

LOIN

DIVIDE-BY-2

QUADRATURE

PHASE

SPLITTER

Figure 26. ADL5385 Block Diagram

The LO interface generates two LO signals at 90° of phase

difference to drive two mixers in quadrature. Baseband signals

are converted into currents by the V-to-I converters that feed

into the two mixers. The outputs of the mixers are combined in

the differential-to-single-ended amplifier, which provides a 50 Ω

output interface. Reference currents to each section are

generated by the bias circuit. A detailed description of each

section follows.

LO INTERFACE

The LO interface consists of a buffer amplifier followed by a

pair of frequency dividers that generate two carriers at half the

input frequency and in quadrature with each other. Each carrier

is then amplified and amplitude-limited to drive the double-

balanced mixers.

V-TO-I CONVERTER

The differential baseband input voltages that are applied to the

baseband input pins are fed to a pair of common-emitter,

voltage-to-current converters. The output currents then

modulate the two half-frequency LO carriers in the mixer stage.

MIXERS

The ADL5385 has two double-balanced mixers: one for the in-

phase channel (I channel) and one for the quadrature channel

(Q channel). These mixers are based on the Gilbert cell design

of four cross-connected transistors. The output currents from

the two mixers are summed together in the resistor-inductor

(RL) loads in the D-to-S amplifier.

D-TO-S AMPLIFIER

The output D-to-S amplifier consists of two emitter followers

driving a totem-pole output stage. Output impedance is

established by the emitter resistors in the output transistors.

The output of this stage connects to the output (VOUT) pin.

BIAS CIRCUIT

A band gap reference circuit generates the proportional-to-

absolute-temperature (PTAT) as well as temperature-independ-

ent reference currents used by different sections. The band-gap

circuit is turned on by a logic HIGH at the ENBL pin, which in

turn powers up the whole device. A PTAT voltage output is

available at the TEMP pin, which can be used for temperature

monitoring as well as for temperature compensation purposes.