Low Cost DC-500 MHz, 92 dB
Logarithmic Amplifier
a
AD8307
FEATURES
FUNCTIO NAL BLO CK D IAGRAM
Com plete Multistage Logarithm ic Am plifier
92 dB Dynam ic Range: –75 dBm to +17 dBm
to –90 dBm Using Matching Netw ork
Single Supply of 2.7 V Min at 7.5 m A Typical
DC-500 MHz Operation, ؎1 dB Linearity
Slope of 25 m V/ dB, Intercept of –84 dBm
Highly Stable Scaling Over Tem perature
Fully Differential DC-Coupled Signal Path
100 ns Pow er-Up Tim e, 150 A Sleep Current
AD8307
BANDGAP REFERENCE
AND BIASING
VPS
ENB
INT
SUPPLY
7.5mA
ENABLE
INT. ADJ
SIX 14.3dB 900MHz
AMPLIFIER STAGES
INP
1.15k⍀
+INPUT
–INPUT
MIRROR
INM
2A
/dB
3
OUT
OUTPUT
2
NINE DETECTOR CELLS
SPACED 14.3dB
12.5k⍀
APPLICATIONS
COMMON
COM
COM
Conversion of Signal Level to Decibel Form
Transm itter Antenna Pow er Measurem ent
Receiver Signal Strength Indication (RSSI)
Low Cost Radar and Sonar Signal Processing
Netw ork and Spectrum Analyzers (to 120 dB)
Signal Level Determ ination Dow n to 20 Hz
True Decibel AC Mode for Multim eters
OFS
INPUT-OFFSET
COMPENSATION LOOP
OFS. ADJ.
T he output is a voltage scaled 25 mV/dB, generated by a current
of nominally 2 µA/dB through an internal 12.5 kΩ resistor. T his
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.
P RO D UCT D ESCRIP TIO N
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.
T he AD8307 exhibits excellent supply insensitivity and tem-
perature stability of the scaling parameters. T he 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.
Each of the cascaded amplifier/limiter cells has a small-signal
gain of 14.3 dB, with a –3 dB bandwidth of 900 MHz. T he
input is fully differential and at a moderately high impedance
(1.1 kΩ in parallel with about 1.4 pF). T he 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. T he 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. T here is no minimum
frequency limit; the AD8307 may be used at audio frequencies
(20 Hz) or even lower.
T he AD8307 is available in the industrial temperature range of
–40°C to +85°C, and in 8-lead SOIC and PDIP packages.
REV. A
Inform ation furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assum ed by Analog Devices for its
use, nor for any infringem ents of patents or other rights of third parties
which m ay result from its use. No license is granted by im plication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norw ood, MA 02062-9106, U.S.A.
Tel: 781/ 329-4700
Fax: 781/ 326-8703
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© Analog Devices, Inc., 1999