AD8347ARU Analog Devices Inc, AD8347ARU Datasheet - Page 16

• Manufacturer Part Number: AD8347ARU
• Description: Quadrature Dmod 65MHz 28-Pin TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD8347ARU

• Package: 28TSSOP
• Device Type: Demodulator
• Maximum I/q Frequency: 65(Typ) MHz
• Modulation Type: Quadrature
• Typical Noise Figure: 11 dB
• Maximum Conversion Gain: 39.5(Typ) dB
• Rohs Status: RoHS non-compliant
• Function: Demodulator
• Lo Frequency: 800MHz ~ 2.7GHz
• Rf Frequency: 800MHz ~ 2.7GHz
• P1db: -30dBm
• Gain: 39.5dB
• Noise Figure: 11dB
• Current - Supply: 80mA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Package / Case: 28-TSSOP

AD8347
THEORY OF OPERATION
The AD8347 is a direct I/Q demodulator usable in digital
wireless communication systems including cellular, PCS, and
digital video receivers. An RF signal in the frequency range of
800 MHz to 2,700 MHz is directly downconverted to the I and
Q components at baseband using a local oscillator (LO) signal
at the same frequency as the RF signal.
The RF input signal goes through two stages of variable gain
amplifiers before splitting up to reach two Gilbert-cell mixers.
The mixers are driven by a pair of LO signals which are in
quadrature (90 degrees of phase difference). The outputs of the
mixers are applied to baseband I-channel and Q-channel
variable gain amplifiers. The outputs from these baseband
variable gain amplifiers are brought out to pins for external
filtering. The filter outputs are then applied to a pair of on-chip,
fixed gain, baseband amplifiers. These amplifiers gain up the
outputs from the external filters to a level compatible with most
A-to-D converters. A sum of squares detector is available for
use in an automatic gain control (AGC) loop to set the output
level. The RF and baseband amplifiers provide approximately
69.5 dB of gain control range. Additional on-chip circuits allow
the setting of the dc level at the I-channel and Q-channel
baseband outputs, as well as nulling the dc offset at each
channel.
RF VARIABLE GAIN AMPLIFIERS (VGA)
These amplifiers use the patented X-AMP® approach with NPN
differential pairs separated by sections of resistive attenuators.
The gain control is achieved through a gaussian interpolator
where the control voltage sets the tail currents supplied to the
various differential pairs according to the gain desired. In the
first amplifier, the combined output currents from the
transconductance cells go through a cascode stage to resistive
loads with inductive peaking. In the second amplifier, the
ENBL
VGIN
RFIN
RFIP
15
10
17
11
VPS1
2
INTERFACE
CONTROL
AD8347
GAIN
VPS2
12
CELL
BIAS
VPS3
DET 1
VDT1
21
20
VAGC
VREF
19
14
VREF
DET 2
VDT2
18
Figure 45. Block Diagram
Rev. A | Page 16 of 28
QMXO
IMXO
22
8
QOFS
IOFS
13
16
differential currents are split and fed to the two Gilbert-cell
mixers through separate cascode stages.
MIXERS
Two double balanced Gilbert-cell mixers, one for each channel,
perform the in-phase (I) and quadrature (Q) down conversion.
Each mixer has four cross-connected transistor pairs that are
terminated in resistive loads and feed the differential baseband
variable gain amplifiers for each channel. The quadrature LO
signals drive the bases of the mixer transistors.
BASEBAND VARIABLE GAIN AMPLIFIERS
The baseband VGAs also use the X-AMP approach with NPN
differential pairs separated by sections of resistive attenuators.
The same interpolator controlling the RF amplifiers controls the
tail currents of the differential pairs. The outputs of these amplifiers
are provided off chip for external filtering. Automatic offset
nulling minimizes the dc offsets at both I- and Q-channels. The
common-mode output voltage is set to the same level as the
reference voltage (1.0 V) generated in the Bias cell, also made
available at the VREF pin (see Figure 45).
OUTPUT AMPLIFIERS
The output amplifiers gain up the signal coming back from each of
the external filters to a level compatible with most high speed A-to-
D converters. These amplifiers are based on an active feedback
design to achieve high gain bandwidth with low distortion.
LO AND PHASE SPLITTERS
The incoming LO signal is applied to a polyphase phase splitter
to generate the LO signals for the I-channel and Q-channel
mixers. The polyphase phase splitters are RC networks
connected in a cyclical manner to achieve gain balance and
phase quadrature. The wide operating frequency range of these
phase splitters is achieved by cascading multiple sections of
VREF
VREF
SPLITTER
PHASE
QAIN
2
24
IAIN
6
VCMO
QOPP
IOPP
25
4
VCMO
SPLITTER
PHASE
QOPN
IOPN
1
26
3
28
23
27
5
1
7
9
VCMO
LOIN
LOIP
COM3
COM2
COM3
COM1