0.1 GHz to 2.5 GHz 70 dB
Logarithmic Detector/Controller
AD8313
FEATURES
Wide bandwidth: 0.1 GHz to 2.5 GHz min
High dynamic range: 70 dB to ±3.0 dB
High accuracy: ±1.0 dB over 65 dB range (@ 1.9 GHz)
Fast response: 40 ns full-scale typical
Controller mode with error output
Scaling stable over supply and temperature
Wide supply range: 2.7 V to 5.5 V
Low power: 40 mW at 3 V
Power-down feature: 60 mW at 3 V
Complete and easy to use
FUNCTIONAL BLOCK DIAGRAM
+
VPOS
1
C
INT
INHI
2
8dB
INLO
3
EIGHT 8dB 3.5GHz AMPLIFIER STAGES
INTERCEPT
CONTROL
BAND GAP
REFERENCE
GAIN
BIAS
6
NINE DETECTOR CELLS
+
+
+
+
I→V
8
VOUT
8dB
8dB
8dB
LP
V→I
7
VSET
AD8313
VPOS
4
SLOPE
CONTROL
COMM
5
PWDN
APPLICATIONS
RF transmitter power amplifier setpoint control and
level monitoring
Logarithmic amplifier for RSSI measurement cellular
base stations, radio link, radar
Figure 1.
GENERAL DESCRIPTION
The AD8313 is a complete multistage demodulating logarithmic
amplifier that can accurately convert an RF signal at its differ-
ential input to an equivalent decibel-scaled value at its dc output.
The AD8313 maintains a high degree of log conformance for
signal frequencies from 0.1 GHz to 2.5 GHz and is useful over
the range of 10 MHz to 3.5 GHz. The nominal input dynamic
range is –65 dBm to 0 dBm (re: 50 Ω), and the sensitivity can be
increased by 6 dB or more with a narrow-band input impedance
matching network or a balun. Application is straightforward,
requiring only a single supply of 2.7 V to 5.5 V and the addition
of a suitable input and supply decoupling. Operating on a 3 V
supply, its 13.7 mA consumption (for T
A
= 25°C) is only 41 mW.
A power-down feature is provided; the input is taken high to
initiate a low current (20 µA) sleep mode, with a threshold at
half the supply voltage.
The AD8313 uses a cascade of eight amplifier/limiter cells, each
having a nominal gain of 8 dB and a −3 dB bandwidth of
3.5 GHz. This produces a total midband gain of 64 dB. At each
amplifier output, a detector (rectifier) cell is used to convert the
RF signal to baseband form; a ninth detector cell is placed
directly at the input of the AD8313. The current-mode outputs
of these cells are summed to generate a piecewise linear approxi-
mation to the logarithmic function. They are converted to a low
impedance voltage-mode output by a transresistance stage, which
also acts as a low-pass filter.
When used as a log amplifier, scaling is determined by a separate
feedback interface (a transconductance stage) that sets the slope
to approximately 18 mV/dB; used as a controller, this stage
accepts the setpoint input. The logarithmic intercept is positioned
to nearly −100 dBm, and the output runs from about 0.45 V dc
at −73 dBm input to 1.75 V dc at 0 dBm input. The scale and
intercept are supply- and temperature-stable.
The AD8313 is fabricated on Analog Devices’ advanced 25 GHz
silicon bipolar IC process and is available in an 8-lead MSOP
package. The operating temperature range is −40°C to +85°C.
An evaluation board is available.
2.0
FREQUENCY = 1.9GHz
1.8
1.6
OUTPUT VOLTAGE (V DC)
5
4
3
2
1
0
–1
–2
–3
–4
–70
–60
–50
–40
–30
INPUT AMPLITUDE (dBm)
–20
–10
0
–5
01085-C-002
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
–80
Figure 2. Typical Logarithmic Response and Error vs. Input Amplitude
Rev. D
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.326.8703
© 2004 Analog Devices, Inc. All rights reserved.
OUTPUT ERROR (dB)
01085-C-001
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