REV. 0

AD8345

–8–

CIRCUIT DESCRIPTION

Overview

The AD8345 can be divided into the following sections: Local

Oscillator (LO) Interface, Mixer, Differential Voltage-to-Cur-

rent (V-to-I) Converter, Differential-to-Single-Ended (D-to-S)

Converter, and Bias. A block diagram of the part is shown in

Figure 2.

OUT

LOIP

LOIN

IBBP

IBBN

QBBP

QBBN

PHASE

SPLITTER

Figure 2. AD8345 Block Diagram

The LO Interface generates two LO signals at 90 degrees of

phase difference with each other, to drive two mixers in quadra-

ture. Baseband signals are converted into current form in the

Differential V-to-I Converters, feeding into the two mixers. The

outputs of the mixers are combined to feed the Differential-to-

Single-Ended Converter, which provides a 50

Ω output interface.

Bias currents to each section are controlled by the Enable

(ENBL) signal. Detailed description of each section follows.

LO Interface

The LO Interface consists of interleaved stages of polyphase

phase-splitters and buffer amplifiers. The polyphase phase-splitter

contains resistors and capacitors connected in a circular manner

to split the LO signal into I and Q paths in precise quadra-

ture with each other. The signal on each path goes through a

buffer amplifier to make up for the loss and high frequency

roll-off. The two signals then go through another polyphase

network to enhance the quadrature accuracy. The broad oper-

ating frequency range (250 MHz to 1000 MHz) is achieved

by staggering the RC time constants of each stage of the phase-

splitters. The outputs of the second phase-splitter are fed into

the driver amplifiers for the mixers’ LO inputs.

Differential V-to-I Converter

In this circuit, each baseband input pin is connected to an op amp

driving a transistor connected as an emitter follower. A resistor

between the two emitters maintains a varying current propor-

tional to the differential input voltage through the transistor. These

currents are fed to the two mixers in differential form.

Mixers

There are two double-balanced mixers, one for the In-phase

Channel (I-Channel) and one for the Quadrature Channel (Q-

Channel). Each mixer uses the Gilbert-cell design with four

cross-connected transistors. The bases of the transistors are

driven by the LO signal of the corresponding channel. The

output currents from the two mixers are summed together in

two load resistors. The signal developed across the load resistors

is sent to the D-to-S stage.

Differential to Single-Ended Converter

The differential-to-single-ended converter consists of two emit-

ter followers driving a totem-pole output stage whose output

impedance is established by the emitter resistors in the output

transistors. The output of this stage is connected to the output

(VOUT) pin.

Bias

A bandgap reference circuit based on the

∆-VBE principle gen-

erates the Proportional-To-Absolute-Temperature (PTAT) as

well as temperature-stable currents used by the different sec-

tions as references. When the bandgap reference is disabled by

pulling down the voltage at the ENBL pin, all other sections are

shut off accordingly.

LO LEVEL – dBm

–150

–10

–152

–160

–151

–153

–154

–155

–156

–157

–158

–159

–9

–8

–7

–6

–5

–4

–3

–2

–1

012

0.7 V) Noise Measured at 20 MHz Offset from Carrier

LO LEVEL – dBm

–36

–10

–40

–50

–38

–42

–44

–46

–48

–9

–8

–7

–6

–5

–4

–3

–2

–1

012

°C,