a
FEATURES
Complete Multistage Logarithmic Amplifier
92 dB Dynamic Range: –75 dBm to +17 dBm
to –90 dBm Using Matching Network
Single Supply of 2.7 V Min at 7.5 mA Typical
DC-500 MHz Operation, 1 dB Linearity
Slope of 25 mV/dB, Intercept of –84 dBm
Highly Stable Scaling Over Temperature
Fully Differential DC-Coupled Signal Path
100 ns Power-Up Time, 150 A Sleep Current
APPLICATIONS
Conversion of Signal Level to Decibel Form
Transmitter Antenna Power Measurement
Receiver Signal Strength Indication (RSSI)
Low Cost Radar and Sonar Signal Processing
Network and Spectrum Analyzers (to 120 dB)
Signal Level Determination Down to 20 Hz
True Decibel AC Mode for Multimeters
Low Cost DC-500 MHz, 92 dB
Logarithmic Amplifier
AD8307
FUNCTIONAL BLOCK DIAGRAM
AD8307
VPS
SUPPLY
7.5mA
BANDGAP REFERENCE
AND BIASING
SIX 14.3dB 900MHz
AMPLIFIER STAGES
MIRROR
3
NINE DETECTOR CELLS
SPACED 14.3dB
COMMON
COM
INPUT-OFFSET
COMPENSATION LOOP
2
12.5k
COM
OFS
OFS. ADJ.
2 A
/dB
OUT
OUTPUT
ENB
ENABLE
INT
+INPUT
–INPUT
INP
1.15k
INM
INT. ADJ
PRODUCT DESCRIPTION
The AD8307 is the first logarithmic amplifier in an 8-lead (SO-8)
package. It is a complete 500 MHz monolithic demodulating
logarithmic amplifier based on the progressive compression
(successive detection) technique, providing a dynamic range of
92 dB to
±
3 dB law-conformance and 88 dB to a tight
±
1 dB
error bound at all frequencies up to 100 MHz. It is extremely
stable and easy to use, requiring no significant external compo-
nents. A single supply voltage of 2.7 V to 5.5 V at 7.5 mA is
needed, corresponding to an unprecedented power consumption
of only 22.5 mW at 3 V. A fast-acting CMOS-compatible con-
trol pin can disable the AD8307 to a standby current of under
150
µA.
Each of the cascaded amplifier/limiter cells has a small-signal
gain of 14.3 dB, with a –3 dB bandwidth of 900 MHz. The
input is fully differential and at a moderately high impedance
(1.1 kΩ in parallel with about 1.4 pF). The AD8307 provides a
basic dynamic range extending from approximately –75 dBm
(where dBm refers to a 50
Ω
source, that is, a sine amplitude of
about
±
56
µV)
up to +17 dBm (a sine amplitude of
±
2.2 V).
A simple input-matching network can lower this range to –88 dBm
to +3 dBm. The logarithmic linearity is typically within
±
0.3 dB
up to 100 MHz over the central portion of this range, and is
degraded only slightly at 500 MHz. There is no minimum
frequency limit; the AD8307 may be used at audio frequencies
(20 Hz) or even lower.
The output is a voltage scaled 25 mV/dB, generated by a current
of nominally 2
µA/dB
through an internal 12.5 kΩ resistor. This
voltage varies from 0.25 V at an input of –74 dBm (that is, the
ac intercept is at –84 dBm, a 20
µV
rms sine input), up to 2.5 V
for an input of +16 dBm. This slope and intercept can be trimmed
using external adjustments. Using a 2.7 V supply, the output
scaling may be lowered, for example to 15 mV/dB, to permit
utilization of the full dynamic range.
The AD8307 exhibits excellent supply insensitivity and tem-
perature stability of the scaling parameters. The unique combi-
nation of low cost, small size, low power consumption, high
accuracy and stability, very high dynamic range, and a frequency
range encompassing audio through IF to UHF, make this prod-
uct useful in numerous applications requiring the reduction of a
signal to its decibel equivalent.
The AD8307 is available in the industrial temperature range of
–40°C to +85°C, and in 8-lead SOIC and PDIP packages.
REV. A
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1999
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