DC-Coupled Demodulating
a
120 MHz Logarithmic Amplifier
AD640*
FEATURES
signal output at +50 dB (referred to input) is provided to operate
AD640s in cascade.
Complete, Fully Calibrated Monolithic System
Five Stages, Each Having 10 dB Gain, 350 MHz BW
Direct Coupled Fully Differential Signal Path
Logarithmic Slope, Intercept and AC Response are
Stable Over Full Military Temperature Range
Dual Polarity Current Outputs Scaled 1 mA/Decade
Voltage Slope Options (1 V/Decade, 100 mV/dB, etc.)
Low Power Operation (Typically 220 mW at ؎5 V)
Low Cost Plastic Packages Also Available
The logarithmic response is absolutely calibrated to within ±1 dB
for dc or square wave inputs from ±0.75 mV to ±200 mV, with
an intercept (logarithmic offset) at 1 mV dc. An integral X10
attenuator provides an alternative input range of ±7.5 mV to
±2 V dc. Scaling is also guaranteed for sinusoidal inputs.
The AD640B is specified for the industrial temperature range of
–40°C to +85°C and the AD640T, available processed to MIL-
STD-883B, for the military range of –55°C to +125°C. Both are
available in 20-lead side-brazed ceramic DIPs or leadless chip
carriers (LCC). The AD640J is specified for the commercial
temperature range of 0°C to +70°C, and is available in both
20-lead plastic DIP (N) and PLCC (P) packages.
APPLICATIONS
Radar, Sonar, Ultrasonic and Audio Systems
Precision Instrumentation from DC to 120 MHz
Power Measurement with Absolute Calibration
Wide Range High Accuracy Signal Compression
Alternative to Discrete and Hybrid IF Strips
Replaces Several Discrete Log Amp ICs
This device is now available to Standard Military Drawing
(DESC) number 5962-9095501MRA and 5962-9095501M2A.
PRODUCT HIGHLIGHTS
PRODUCT DESCRIPTION
1. Absolute calibration of a wideband logarithmic amplifier is
unique. The AD640 is a high accuracy measurement device,
not simply a logarithmic building block.
2. Advanced design results in unprecedented stability over the
full military temperature range.
3. The fully differential signal path greatly reduces the risk of
instability due to inadequate power supply decoupling and
shared ground connections, a serious problem with com-
monly used unbalanced designs.
4. Differential interfaces also ensure that the appropriate ground
connection can be chosen for each signal port. They further
increase versatility and simplify applications. The signal input
impedance is ~500 kΩ in shunt with ~2 pF.
The AD640 is a complete monolithic logarithmic amplifier. A single
AD640 provides up to 50 dB of dynamic range for frequencies
from dc to 120 MHz. Two AD640s in cascade can provide up to
95 dB of dynamic range at reduced bandwidth. The AD640 uses a
successive detection scheme to provide an output current propor-
tional to the logarithm of the input voltage. It is laser calibrated to
close tolerances and maintains high accuracy over the full military
temperature range using supply voltages from ±4.5 V to ±7.5 V.
The AD640 comprises five cascaded dc-coupled amplifier/limiter
stages, each having a small signal voltage gain of 10 dB and a –3 dB
bandwidth of 350 MHz. Each stage has an associated full-wave
detector, whose output current depends on the absolute value of its
input voltage. The five outputs are summed to provide the video
output (when low-pass filtered) scaled at 1 mA per decade (50 µA
per dB). On chip resistors can be used to convert this output cur-
rent to a voltage with several convenient slope options. A balanced
5. The dc-coupled signal path eliminates the need for numerous
interstage coupling capacitors and simplifies logarithmic
conversion of subsonic signals.
(continued on page 4)
FUNCTIONAL BLOCK DIAGRAM
RG1
17
RG0
16
RG2
15
LOG OUT
14
LOG COM
13
1k⍀
1k⍀
12 +V
INTERCEPT POSITIONING BIAS
S
18
19
COM
FULL-WAVE
DETECTOR
FULL-WAVE
DETECTOR
FULL-WAVE
DETECTOR
FULL-WAVE
DETECTOR
FULL-WAVE
DETECTOR
ATN OUT
20
1
SIG +IN
SIG –IN
ATN LO
11
10
SIG +OUT
SIG –OUT
10dB
10dB
10dB
10dB
10dB
AMPLIFIER/LIMITER
AMPLIFIER/LIMITER
AMPLIFIER/LIMITER
AMPLIFIER/LIMITER
AMPLIFIER/LIMITER
2
27⍀
30⍀
3
4
ATN COM
ATN COM
BL2
ITC
9
8
270⍀
5
6
7
SLOPE BIAS REGULATOR
GAIN BIAS REGULATOR
ATN IN
BL1
–V
S
*Protected under U.S. patent number 4,990,803.
REV. C
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
Fax: 781/326-8703
World Wide Web Site: http://www.analog.com
© Analog Devices, Inc., 1999
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