AD8345
Rev. B | Page 11 of 20
CIRCUIT DESCRIPTION
OVERVIEW
The AD8345 can be divided into the following sections: local
oscillator (LO) interface, mixer, differential voltage-to-current
(V-to-I) converter, differential-to-single-ended (D-to-S)
converter, and bias. A block diagram of the part is shown in
Figure 27.
OUT
LOIP
LOIN
IBBP
IBBN
QBBP
QBBN
PHASE
SPLITTER
Σ
00932-027
Figure 27. AD8345 Block Diagram
The LO interface generates two LO signals at 90° of phase
difference with each other, to drive two mixers in quadrature.
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. A 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
quadrature 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 operating frequency range (140 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 VOLTAGE-TO-CURRENT
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 proportional
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 differential-to-single-ended converter.
DIFFERENTIAL-TO-SINGLE-ENDED CONVERTER
The differential-to-single-ended converter consists of two
emitter 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 pin (VOUT).
BIAS
A band gap reference circuit based on the Δ-VBE principle
generates the proportional-to-absolute temperature (PTAT) as
well as temperature-stable currents used by the different
sections as references. When the band gap reference is disabled
by pulling down the voltage at the ENBL pin, all other sections
are shut off accordingly.
