800 MHz, Linear-in-dB
VGA with AGC Detector
AD8368
Analog variable gain range: −12 dB to +22 dB
Linear-in-dB scaling: 37.5 dB/V
3 dB bandwidth: 800 MHz @ VGAIN = 0.5 V
Integrated rms detector
P1dB: 16 dBm @ 140 MHz
Output IP3: 33 dBm @ 140 MHz
Noise figure at maximum gain: 9.5 dB @ 140 MHz
Input and output impedances: 50 Ω
Single-supply voltage from 4.5 V to 5.5 V
RoHS-compliant, 24-lead LFCSP
FUNCTIONAL BLOCK DIAGRAM
VPSO
MODE
10
21
VPSO VPSI VPSI VPSI VPSI VPSI
9
11
12
22
ICOM 16
GAIN INTERPOLATOR
FIXED-GAIN
AMPLIFIER
OCOM 7
OUTPUT
BUFFER
0dB –2dB
–4dB
–36dB
REF
HPFL
4
DECL
14 DECL
50Ω
DECL
ATTENUATOR LADDER
–
X2
15 DECL
+
ICOM 20
2
5
DETO
APPLICATIONS
Complete IF AGC amplifiers
Gain trimming and leveling
Cellular base stations
Point-to-point radio links
RF instrumentation
OUTP
3
INPT 19
ICOM 18
24 ENBL
8
gm STAGES
ICOM 17
13
AD8368
OCOM 6
GAIN 1
23
DETI
05907-001
FEATURES
Figure 1.
GENERAL DESCRIPTION
The AD8368 is a variable gain amplifier (VGA) with analog
linear-in-dB gain control that can be used from low frequencies
to 800 MHz. Its excellent gain range, conformance, and flatness
are attributed to the Analog Devices, Inc., X-AMP® architecture,
an innovative technique for implementing high performance
variable gain control.
The gain range of −12 dB to +22 dB is scaled accurately to
37.5 dB/V with excellent conformance error. The AD8368 has
a 3 dB bandwidth of 800 MHz that is nominally independent
of gain setting. At 140 MHz, the OIP3 is 33 dBm at maximum
gain. The output noise floor is −143 dBm/Hz, which corresponds
to a 9.5 dB noise figure at maximum gain. The single-ended
input and output impedances are nominally 50 Ω.
The gain of the AD8368 can be configured to be an increasing
or decreasing function of the gain control voltage depending
on whether the MODE pin is pulled to the positive supply or
to ground, respectively. When MODE is pulled high, the
AD8368 operates as a typical VGA with increasing gain.
By connecting MODE to ground and using the on-board rms
detector, the AD8368 can be configured as a complete
automatic gain control (AGC) system with RSSI. The output
power is accurately leveled to the internal default setpoint of
63 mV rms (−11 dBm referenced to 50 Ω), independent of the
waveform crest factor. Because the uncommitted detector
input is available at DETI, the AGC loop can level the signal at
the AD8368 output or at any other point in the signal chain
over a maximum input power range of 34 dB. Furthermore, the
setpoint level can be raised by dividing down the output signal
before applying it to the detector.
The AD8368 operates from a supply voltage of 4.5 V to 5.5 V
and consumes 60 mA of current. It can be fully powered down
to <3 mA by grounding the ENBL pin. The AD8368 is fabricated
using the Analog Devices proprietary SiGe SOI complementary
bipolar IC process. It is available in a 24-lead LFCSP and
operates over the industrial temperature range of −40°C to
+85°C. Application boards are available upon request.
Rev. B
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rights of third parties that may result from its use. Specifications subject to change without notice. No
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