AD AD8307ARZ Low cost dc-500 mhz, 92 db logarithmic amplifier Datasheet

Low Cost DC-500 MHz, 92 dB
Logarithmic Amplifier
AD8307
FUNCTIONAL BLOCK DIAGRAM
FEATURES
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
AD8307
VPS 7
INP 8
INM 1
7.5mA
BAND GAP REFERENCE
AND BIASING
SIX 14.3dB 900MHz
AMPLIFIER STAGES
+INP
ENB
5
INT
4
OUT
3
OFS
MIRROR
1.1kΩ
–INP
6
3
NINE DETECTOR CELLS
SPACED 14.3dB
COM 2
2
2µA
/dB
12.5kΩ
COM
INPUT-OFFSET
COMPENSATION LOOP
01082-001
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 minimum at 7.5 mA typ
DC to 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 μA sleep current
Figure 1.
GENERAL DESCRIPTION
The AD8307 is the first logarithmic amplifier made available in an
8-lead (SOIC-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 components. 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 CMOScompatible control pin can disable the AD8307 to a standby
current of less than 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 degrades
only slightly at 500 MHz. There is no minimum frequency limit.
The AD8307 can be used at audio frequencies of 20 Hz or 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 can be lowered, for example to 15 mV/dB, to
permit utilization of the full dynamic range.
The AD8307 exhibits excellent supply insensitivity and temperature
stability of the scaling parameters. The unique combination 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 makes this product
useful in numerous applications requiring the reduction of a
signal to its decibel equivalent.
The AD8307 operates over the industrial temperature range of
−40°C to +85°C, and is available in 8-lead SOIC and PDIP
packages.
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 that may result from its use. Specifications subject to change without notice. No
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One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
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Fax: 781.461.3113
©2006 Analog Devices, Inc. All rights reserved.
AD8307
TABLE OF CONTENTS
Features .............................................................................................. 1
Input Interface ............................................................................ 14
Applications....................................................................................... 1
Offset Interface ........................................................................... 15
Functional Block Diagram .............................................................. 1
Output Interface ......................................................................... 15
General Description ......................................................................... 1
Theory of Operation ...................................................................... 17
Revision History ............................................................................... 2
Basic Connections...................................................................... 17
Specifications..................................................................................... 3
Input Matching ........................................................................... 17
Absolute Maximum Ratings............................................................ 4
Narrow-Band Matching ............................................................ 18
ESD Caution.................................................................................. 4
Slope and Intercept Adjustments ............................................. 19
Pin Configuration and Function Descriptions............................. 5
Applications Information .............................................................. 20
Typical Performance Characteristics ............................................. 6
Buffered Output.......................................................................... 20
Log Amp Theory .............................................................................. 9
Four Pole Filter ........................................................................... 20
Progressive Compression .......................................................... 10
1 μW to 1 kW 50 Ω Power Meter............................................. 21
Demodulating Log Amps .......................................................... 11
Measurement System with 120 dB Dynamic Range.............. 21
Intercept Calibration.................................................................. 12
Operation at Low Frequencies.................................................. 22
Offset Control ............................................................................. 12
DC-Coupled Applications......................................................... 22
Extension of Range..................................................................... 13
Operation Above 500 MHz....................................................... 23
Interfaces.......................................................................................... 14
Outline Dimensions ....................................................................... 24
Enable Interface .......................................................................... 14
Ordering Guide .......................................................................... 24
REVISION HISTORY
10/06—Rev. B to Rev. C
Updated Format..................................................................Universal
Changes to Table 1............................................................................ 3
Changes to Table 3............................................................................ 5
Changes to Offset Interface........................................................... 15
Changes to Output Interface......................................................... 15
Updated captions to Outline Dimensions................................... 24
Changes to Ordering Guide .......................................................... 24
6/03—Rev. A to Rev. B
Renumbered TPCs and Figures........................................Universal
Changes to Ordering Guide ............................................................ 3
Changes to Figure 24...................................................................... 17
Deleted Evaluation Board Information ....................................... 18
Updated Outline Dimensions ....................................................... 19
Rev. C | Page 2 of 24
AD8307
SPECIFICATIONS
VS = 5 V, TA = 25°C, RL ≥ 1 MΩ, unless otherwise noted.
Table 1.
Parameter
GENERAL CHARACTERISTICS
Input Range (±3 dB Error)
Input Range (±1 dB Error)
Logarithmic Conformance
Logarithmic Slope
vs. Temperature
Logarithmic Intercept
vs. Temperature
Input Noise Spectral Density
Operating Noise Floor
Output Resistance
Internal Load Capacitance
Response Time
Upper Usable Frequency 3
Lower Usable Frequency
AMPLIFIER CELL CHARACTERISTICS
Cell Bandwidth
Cell Gain
INPUT CHARACTERISTICS
DC Common-Mode Voltage
Common-Mode Range
DC Input Offset Voltage 4
Incremental Input Resistance
Input Capacitance
Bias Current
POWER INTERFACES
Supply Voltage
Supply Current
Disabled
Conditions
From noise floor to maximum input
From noise floor to maximum input
f ≤ 100 MHz, central 80 dB
f = 500 MHz, central 75 dB
Unadjusted 1
Sine amplitude, unadjusted 2
Equivalent sine power in 50 Ω
Inputs shorted
RSOURCE = 50 Ω/2
Pin 4 to ground
Min
23
23
−87
−88
10
Small signal, 10% to 90%,
0 mV to100 mV, CL = 2 pF
Large signal, 10% to 90%,
0 V to 2.4 V, CL = 2 pF
AC-coupled input
−3 dB
AC-coupled input
Either input (small signal)
RSOURCE ≤ 50 Ω
Drift
Differential
Either pin to ground
Either input
−0.3
Typ
92
88
±0.3
±0.5
25
20
−84
1.5
−78
12.5
3.5
400
27
27
−77
−76
15
500
10
MHz
Hz
900
14.3
MHz
dB
3.2
1.6
50
0.8
1.1
1.4
10
25
V
V
μV
μV/°C
kΩ
pF
μA
5.5
10
750
V
mA
μA
VS − 1
500
This can be adjusted downward by adding a shunt resistor from the output to ground. A 50 kΩ resistor reduces the nominal slope to 20 mV/dB.
This can be adjusted in either direction by a voltage applied to Pin 5, with a scale factor of 8 dB/V.
See the Operation Above 500 MHz section.
4
Normally nulled automatically by internal offset correction loop. May be manually nulled by a voltage applied between Pin 3 and ground; see the
Applications Information section.
2
3
Rev. C | Page 3 of 24
dB
dB
dB
dB
mV/dB
mV/dB
μV
dBm
dBm
nV/√Hz
dBm
kΩ
pF
ns
ns
8
150
1
±1
Unit
500
2.7
VENB ≥ 2 V
VENB ≤ 1 V
Max
AD8307
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
Supply
Input Voltage (Pin 1 and Pin 8)
Storage Temperature Range, N, R
Ambient Temperature Range, Rated
Performance Industrial, AD8307AN,
AD8307AR
Lead Temperature Range
(Soldering 10 sec)
Ratings
7.5 V
VSUPPLY
−65°C to +125°C
−40°C to +85°C
Stresses above those listed under Absolute Maximum Ratings
can cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods can affect
device reliability.
ESD CAUTION
300°C
Rev. C | Page 4 of 24
AD8307
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
INM 1
COM 2
AD8307
8
INP
7
VPS
6 ENB
TOP VIEW
OUT 4 (Not to Scale) 5 INT
01082-002
OFS 3
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
Mnemonic
INM
COM
OFS
OUT
INT
ENB
VPS
INP
Description
Signal Input Minus Polarity. Normally at VPOS/2.
Common Pin (Usually Grounded).
Offset Adjustment. External capacitor connection.
Logarithmic (RSSI) Output Voltage. ROUT = 12.5 kΩ.
Intercept Adjustment, ±3 dB (see the Slope and Intercept Adjustments section).
CMOS-Compatible Chip Enable. Active when high.
Positive Supply: 2.7 V to 5.5 V.
Signal Input Plus Polarity. Normally at VPOS/2. Due to the symmetrical nature of the response, there is no special
significance to the sign of the two input pins. DC resistance from INP to INM = 1.1 kΩ.
Rev. C | Page 5 of 24
AD8307
TYPICAL PERFORMANCE CHARACTERISTICS
8
3
7
1
5
ERROR (dB)
4
3
TEMPERATURE ERROR @ +85°C
0
TEMPERATURE ERROR @ +25°C
–1
TEMPERATURE ERROR @ –40°C
2
–2
0
1.0
01082-003
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
–3
–80
2.0
01082-006
SUPPLY CURRENT (mA)
2
6
–60
VENB (V)
–40
–20
0
20
INPUT LEVEL (dBm)
Figure 3. Supply Current vs. VENB Voltage (5 V)
Figure 6. Log Conformance vs. Input Level (dBm) at +25°C, +85°C, and −40°C
3
8
INPUT FREQUENCY 10MHz
6
2
5
INPUT FREQUENCY 100MHz
VOUT (V)
4
3
1
INPUT FREQUENCY 300MHz
2
INPUT FREQUENCY 500MHz
0
1.0
01082-004
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
0
–80
2.0
01082-007
SUPPLY CURRENT (mA)
7
–60
VENB (V)
3
1.5
2
1.0
0
20
0.5
ERROR (dB)
CFO VALUE = 0.01µF
0
–0.5
–1
FREQUENCY INPUT = 100MHz
CFO VALUE = 1µF
CFO VALUE = 0.1µF
–1.0
01082-005
–2
–60
–40
–20
0
–1.5
–80
20
01082-008
ERROR (dB)
FREQUENCY INPUT = 300MHz
0
–3
–80
–20
Figure 7. VOUT vs. Input Level (dBm) at Various Frequencies
Figure 4. Supply Current vs. VENB Voltage (3 V)
1
–40
INPUT LEVEL (dBm)
–60
–40
–20
0
20
INPUT LEVEL (dBm)
INPUT LEVEL (dBm)
Figure 5. Log Conformance vs. Input Level (dBm) 100 MHz, and 300 MHz
Rev. C | Page 6 of 24
Figure 8. Log Conformance vs. CFO Values at 1 kHz Input Frequency
AD8307
3.0
3
100MHz
INT PIN = 3.0V
10MHz, INT = –96.52dBm
2.5
2
1
ERROR (dB)
INT PIN = 4.0V
10MHz, INT = –87.71dBm
1.5
NO CONNECT ON INT
10MHz, INT = –82.90dBm
1.0
+INPUT
0
–1
0.5
–INPUT
01082-009
–2
0
–80
–70
–60
–50
–40
–30
–20
–10
0
10
01082-012
VOUT (V)
2.0
–3
–80
20
–60
INPUT LEVEL (dBm)
Figure 9. VOUT vs. Input Level at 5 V Supply; Showing Intercept Adjustment
0
20
3
2
INT VOLTAGE
INT = 1.0V, INT = –86dBm
500MHz
1
ERROR (dB)
INT VOLTAGE
INT NO CONNECT, INT = –71dBm
1.5
1.0
0
–1
0
–80
–70
–60
–50
–40
–30
–20
–10
0
–2
01082-010
INT VOLTAGE
INT = 2.0V, INT = –78dBm
0.5
100MHz
01082-013
2.5
VOUT (V)
–20
Figure 12. Log Conformance vs. Input Level at 100 MHz Showing
Response to Alternative Inputs
3.0
2.0
–40
INPUT LEVEL (dBm)
–3
–90
10
–70
INPUT LEVEL (dBm)
–50
–30
–10
10
INPUT LEVEL (dBm)
Figure 13. Log Conformance vs. Input at 100 MHz, 500 MHz;
Input Driven Differentially Using Transformer
Figure 10. VOUT vs. Input Level at 3 V Supply Using AD820 as Buffer,
Gain = +2; Showing Intercept Adjustment
3
2.5
2
2.0
500MHz
1.0
ERROR (dB)
VOUT (V)
1
1.5
100MHz @ –40°C
100MHz @ +25°C
0
100MHz
–1
0.5
–60
–40
–20
01082-011
0
–3
–70
20
01082-014
–2
100MHz @ +85°C
0
–80
10MHz
–60
–50
–40
–30
–20
–10
0
10
INPUT LEVEL (dBm)
INPUT LEVEL (dBm)
Figure 11. VOUT vs. Input Level at Three Temperatures (−40°C, +25°C, +85°C)
Rev. C | Page 7 of 24
Figure 14. Log Conformance vs. Input Level at 3 V Supply
Using AD820 as Buffer, Gain = +2
20
AD8307
2V
VOUT
CH 1
CH1 200mV
VOUT
CH1
CH1 500mV
CH1 GND
500ns
CH2 2.00V
CH2
GND
01082-015
GND
INPUT
SIGNAL
CH2
VOUT
CH 1
200ns
CH2 1.00V
Figure 15. Power-Up Response Time
01082-018
VENB
CH 2
Figure 18. VOUT Rise Time
CH1 200mV
CH1 500mV
2.5V
INPUT
SIGNAL
CH2
VENB
CH 2
CH2
GND
VOUT
CH1
01082-016
GND
500ns
Figure 16. Power-Down Response Time
Figure 19. Large Signal Response Time
HP8648B
10MHz REF CLK
SIGNAL
GENERATOR PULSE MODE IN
PULSE
VPS = 5.0V
MODULATION
MODE
0.1µF
1nF
RF OUT
VPS = 5.0V
HP8648B
SIGNAL
GENERATOR
HP8112A
PULSE
GENERATOR
OUT
0.1µF
1nF
RF OUT
SYNCH OUT
NC
8
7
6
8
5
52.3Ω
AD8307
3
1nF
NC = NO CONNECT
TEK P6139A
10x PROBE
TRIG
HP8112A
OUT
PULSE
GENERATOR
NC
5
AD8307
1
4
NC
TEK744A
SCOPE
2
3
4
NC
TRIG
1nF
01082-017
2
6
OUT
INM COM OFS OUT
INM COM OFS OUT
1
7
EXT TRIG
INP VPS ENB INT
INP VPS ENB INT
52.3Ω
200ns
CH2 1.00V
TEK P6204
FET PROBE
TEK744A
SCOPE
NC = NO CONNECT
Figure 20. Test Setup for VOUT Pulse Response
Figure 17. Test Setup for Power-Up/Power-Down Response Time
Rev. C | Page 8 of 24
TRIG
01082-020
CH2 2.00V
01082-019
CH1 GND
AD8307
LOG AMP THEORY
Logarithmic amplifiers perform a more complex operation than
that of classical linear amplifiers, and their circuitry is significantly
different. A good grasp of what log amps do and how they work
can prevent many pitfalls in their application. The essential
purpose of a log amp is not to amplify, though amplification is
utilized to achieve the function. Rather, it is to compress a
signal of wide dynamic range to its decibel equivalent. It is thus
a measurement device. A better term might be logarithmic
converter, since its basic function is the conversion of a signal
from one domain of representation to another, via a precise
nonlinear transformation.
Logarithmic compression leads to situations that can be
confusing or paradoxical. For example, a voltage offset added to
the output of a log amp is equivalent to a gain increase ahead of
its input. In the usual case where all the variables are voltages,
and regardless of the particular structure, the relationship
between the variables can be expressed as:
VOUT = VY log (VIN /VX )
(1)
where:
VOUT is the output voltage.
VY is the slope voltage; the logarithm is usually taken to base 10
(in which case VY is also the volts per decade).
VIN is the input voltage.
VX is the intercept voltage.
All log amps implicitly require two references, here, VX and VY,
which determine the scaling of the circuit. The absolute
accuracy of a log amp cannot be any better than the accuracy of
its scaling references. Equation 1 is mathematically incomplete
in representing the behavior of a demodulating log amp such as
the AD8307, where VIN has an alternating sign. However, the
basic principles are unaffected, and this can be safely used as the
starting point in the analyses of log amp scaling.
VOUT
5VY
4VY
3VY
(the log intercept) at the unique value VIN = VX and ideally
becomes negative for inputs below the intercept. In the ideal
case, the straight line describing VOUT for all values of VIN
continues indefinitely in both directions. The dotted line shows
that the effect of adding an offset voltage VSHIFT to the output is
to lower the effective intercept voltage VX. Exactly the same
alteration could be achieved by raising the gain (or signal level)
ahead of the log amp by the factor VSHIFT/VY. For example, if VY
is 500 mV per decade (25 mV/dB), an offset of +150 mV added
to the output appears to lower the intercept by two tenths of a
decade, or 6 dB. Adding an offset to the output is thus
indistinguishable from applying an input level that is 6 dB higher.
The log amp function described by Equation 1 differs from that
of a linear amplifier in that the incremental gain δVOUT/δVIN is a
very strong function of the instantaneous value of VIN, as is
apparent by calculating the derivative. For the case where the
logarithmic base is δ,
δVOUT
V
= Y
VIN
δVIN
That is, the incremental gain is inversely proportional to the
instantaneous value of the input voltage. This remains true for
any logarithmic base, which is chosen as 10 for all decibel
related purposes. It follows that a perfect log amp is required to
have infinite gain under classical small signal (zero amplitude)
conditions. Less ideally, this result indicates that, whatever
means are used to implement a log amp, accurate response
under small signal conditions (that is, at the lower end of the
dynamic range) demands the provision of a very high gain
bandwidth product. A further consequence of this high gain is
that, in the absence of an input signal, even very small amounts
of thermal noise at the input of a log amp cause a finite output
for zero input. This results in the response line curving away
from the ideal shown in Figure 21 toward a finite baseline,
which can be either above or below the intercept. Note that the
value given for this intercept can be an extrapolated value, in
which case the output can not cross zero, or even reach it, as is
the case for the AD8307.
While Equation 1 is fundamentally correct, a simpler formula is
appropriate for specifying the calibration attributes of a log amp
like the AD8307, which demodulates a sine wave input:
VSHIFT
LOWER INTERCEPT
2VY
VOUT = VSLOPE (PIN – P0)
VY
(3)
where:
VIN = VX
0dBc
VIN = 102VX
+40dBc
VIN = 104VX
+80dBc
01082-021
LOG VIN
VOUT = 0
VIN = 10–2VX
–40dBc
(2)
VOUT is the demodulated and filtered baseband (video or
RSSI) output.
VSLOPE is the logarithmic slope, now expressed in V/dB (typically
between 15 mV/dB and 30 mV/dB).
–2V Y
Figure 21. Ideal Log Amp Function
Figure 21 shows the input/output relationship of an ideal log
amp, conforming to Equation 1. The horizontal scale is
logarithmic and spans a wide dynamic range, shown here as
over 120 dB, or six decades. The output passes through zero
PIN is the input power, expressed in decibels relative to some
reference power level.
P0 is the logarithmic intercept, expressed in decibels relative to
the same reference level.
Rev. C | Page 9 of 24
AD8307
in the case of the AD8307, VY is traceable to an on-chip band
gap reference, while VX is derived from the thermal voltage
kT/q and is later temperature corrected.
OUTPUT
A/1
0
A
A
INPUT
A
VW
Let the input of an N-cell cascade be VIN, and the final output
VOUT. For small signals, the overall gain is simply AN. A six stage
system in which A = 5 (14 dB) has an overall gain of 15,625
(84 dB). The importance of a very high small signal gain in
implementing the logarithmic function has been noted;
however, this parameter is only of incidental interest in the
design of log amps.
From here onward, rather than considering gain, analyze the
overall nonlinear behavior of the cascade in response to a
simple dc input, corresponding to the VIN of Equation 1. For
very small inputs, the output from the first cell is V1 = AVIN.
The output from the second cell is V2 = A2 VIN, and so on, up to
VN = AN VIN. At a certain value of VIN, the input to the Nth cell,
VN–1, is exactly equal to the knee voltage EK. Thus, VOUT = AEK
and since there are N–1 cells of gain A ahead of this node,
calculate VIN = EK /AN–1. This unique situation corresponds to
the lin-log transition, (labeled 1 in Figure 24). Below this input,
the cascade of gain cells acts as a simple linear amplifier, while
for higher values of VIN, it enters into a series of segments that
lie on a logarithmic approximation (dotted line).
VOUT
Figure 22. Cascade of Nonlinear Gain Cells
To develop the theory, first consider a scheme slightly different
from that employed in the AD8307, but simpler to explain and
mathematically more straightforward to analyze. This approach
is based on a nonlinear amplifier unit, called an A/1 cell, with
the transfer characteristic shown in Figure 23.
(4A–3) E K
The local small signal gain δVOUT/δVIN is A, maintained for all
inputs up to the knee voltage EK, above which the incremental
gain drops to unity. The function is symmetrical: the same drop
in gain occurs for instantaneous values of VIN less than –EK. The
large signal gain has a value of A for inputs in the range −EK ≤
VIN ≤ +EK, but falls asymptotically toward unity for very large
inputs. In logarithmic amplifiers based on this amplifier
function, both the slope voltage and the intercept voltage must
be traceable to the one reference voltage, EK. Therefore, in this
fundamental analysis, the calibration accuracy of the log amp is
dependent solely on this voltage. In practice, it is possible to
separate the basic references used to determine VY and VX and
AEK
2
3
(3A–2) E K
3
(A–1) EK
(2A–1) E K
2
1
RATIO
OF A
LOG VIN
0
EK/AN–1
EK/AN–2
EK/AN–3
EK/AN–4
01082-024
A
STAGE N
01082-022
VX
STAGE N–1
EK
Figure 23. A/1 Amplifier Function
Most high speed, high dynamic range log amps use a cascade of
nonlinear amplifier cells (Figure 22) to generate the logarithmic
function from a series of contiguous segments, a type of
piecewise linear technique. This basic topology immediately
opens up the possibility of enormous gain bandwidth products.
For example, the AD8307 employs six cells in its main signal
path, each having a small signal gain of 14.3 dB (×5.2) and a
−3 dB bandwidth of about 900 MHz. The overall gain is about
20,000 (86 dB) and the overall bandwidth of the chain is some
500 MHz, resulting in the incredible gain bandwidth product
(GBW) of 10,000 GHz, about a million times that of a typical op
amp. This very high GBW is an essential prerequisite for
accurate operation under small signal conditions and at high
frequencies. In Equation 2, however, the incremental gain
decreases rapidly as VIN increases. The AD8307 continues to
exhibit an essentially logarithmic response down to inputs as
small as 50 μV at 500 MHz.
STAGE 2
SLOPE = 1
SLOPE = A
PROGRESSIVE COMPRESSION
STAGE 1
AEK
01082-023
The most widely used reference in RF systems is decibels above
1 mW in 50 Ω, written dBm. Note that the quantity (PIN – P0) is
just dB. The logarithmic function disappears from the formula
because the conversion has already been implicitly performed
in stating the input in decibels. This is strictly a concession to
popular convention; log amps manifestly do not respond to
power (tacitly, power absorbed at the input), but rather to input
voltage. The use of dBV (decibels with respect to 1 V rms) is
more precise, though still incomplete, since waveform is involved,
too. Since most users think about and specify RF signals in terms of
power, more specifically, in dBm re: 50 Ω, this convention is used in
specifying the performance of the AD8307.
Figure 24. First Three Transitions
Continuing this analysis, the next transition occurs when the
input to the (N–1) stage just reaches EK; that is, when VIN =
EK /AN–2. The output of this stage is then exactly AEK, and it is
easily demonstrated (from the function shown in Figure 23)
that the output of the final stage is (2A–1) EK (labeled 2 in
Figure 24). Thus, the output has changed by an amount (A–1)EK
for a change in VIN from EK /AN–1 to EK/AN–2, that is, a ratio change
Rev. C | Page 10 of 24
AD8307
of A. At the next critical point (labeled 3 in Figure 24), the input
is again A times larger and VOUT has increased to (3A–2)EK, that
is, by another linear increment of (A–1)EK.
VY =
Linear Change in VOUT
Decades Change in VIN
=
( A − 1)EK
log10 ( A)
(4)
Note that only two design parameters are involved in
determining VY, namely, the cell gain A and the knee voltage EK,
while N, the number of stages, is unimportant in setting the
slope of the overall function. For A = 5 and EK = 100 mV, the
slope would be a rather awkward 572.3 mV per decade
(28.6 mV/dB). A well designed log amp has rational scaling
parameters.
The intercept voltage can be determined by using two pairs of
transition points on the output function (consider Figure 24).
The result is
VX =
EK
( N +1 / ( A −1))
A
(5)
For the case under consideration, using N = 6, calculate
VZ = 4.28 μV. However, be careful about the interpretation of
this parameter, since it was earlier defined as the input voltage
at which the output passes through zero (see Figure 21). Clearly,
in the absence of noise and offsets, the output of the amplifier
chain shown in Figure 23 can be zero when, and only when,
VIN = 0. This anomaly is due to the finite gain of the cascaded
amplifier, which results in a failure to maintain the logarithmic
approximation below the lin-log transition (point 1 in Figure 24).
Closer analysis shows that the voltage given by Equation 5
represents the extrapolated, rather than actual, intercept.
DEMODULATING LOG AMPS
Log amps based on a cascade of A/1 cells are useful in baseband
applications because they do not demodulate their input signal.
However, baseband and demodulating log amps alike can be
made using a different type of amplifier stage, called an A/0 cell.
Its function differs from that of the A/1 cell in that the gain
above the knee voltage EK falls to zero, as shown by the solid
line in Figure 25. This is also known as the limiter function, and
a chain of N such cells are often used to generate hard limited
output in recovering the signal in FM and PM modes.
tanh
SLOPE = A
0
EK
INPUT
01082-025
A/0
OUTPUT
Further analysis shows that right up to the point where the
input to the first cell is above the knee voltage, VOUT changes by
(A–1)EK for a ratio change of A in VIN. This can be expressed as
a certain fraction of a decade, which is simply log10(A). For
example when A = 5, a transition in the piecewise linear output
function occurs at regular intervals of 0.7 decade (log10(A), or
14 dB divided by 20 dB). This insight allows us to immediately
write the volts per decade scaling parameter, which is also the
scaling voltage, VY, when using base 10 logarithms, as
SLOPE = 0
AEK
Figure 25. A/0 Amplifier Functions (Ideal and Tanh)
The AD640, AD606, AD608, AD8307, and various other
Analog Devices, Inc. communications products incorporating a
logarithmic IF amplifier all use this technique. It becomes
apparent that the output of the last stage can no longer provide
the logarithmic output, since this remains unchanged for all inputs
above the limiting threshold, which occurs at VIN = EK/AN−1.
Instead, the logarithmic output is now generated by summing
the outputs of all the stages. The full analysis for this type of log
amp is only slightly more complicated than that of the previous
case. It is readily shown that, for practical purposes, the intercept
voltage VX is identical to that given in Equation 5, while the
slope voltage is
VY =
AEK
log10 ( A )
(6)
Preference for the A/0 style of log amp, over one using A/1 cells,
stems from several considerations. The first is that an A/0 cell
can be very simple. In the AD8307 it is based on a bipolar
transistor differential pair, having resistive loads, RL, and an
emitter current source, IE. This exhibits an equivalent knee
voltage of EK = 2 kT/q and a small signal gain of A = IERL/EK.
The large signal transfer function is the hyperbolic tangent
(see dotted line in Figure 25). This function is very precise, and
the deviation from an ideal A/0 form is not detrimental. In fact,
the rounded shoulders of the tanh function result in a lower
ripple in the logarithmic conformance than that obtained using
an ideal A/0 function.
An amplifier built of these cells is entirely differential in
structure and can thus be rendered very insensitive to
disturbances on the supply lines and, with careful design, to
temperature variations. The output of each gain cell has an
associated transconductance (gm) cell, which converts the
differential output voltage of the cell to a pair of differential
currents, which are summed simply by connecting the outputs
of all the gm (detector) stages in parallel. The total current is
then converted back to a voltage by a transresistance stage to
generate the logarithmic output. This scheme is depicted, in
single sided form, in Figure 26.
Rev. C | Page 11 of 24
AD8307
A/0
gm
A/0
gm
A3VIN
A/0
gm
A4VIN
gm
motion of VX resulting from the temperature variation of EK. Do
this by adding an offset with the required temperature behavior.
VLIM
A/0
gm
IOUT
01082-026
A2VIN
AVIN
VIN
Figure 26. Log Amp Using A/0 Stages and Auxiliary Summing Cells
The chief advantage of this approach is that the slope voltage
can now be decoupled from the knee voltage EK = 2 kT/q, which
is inherently PTAT. By contrast, the simple summation of the
cell outputs would result in a very high temperature coefficient
of the slope voltage given in Equation 6. To do this, the detector
stages are biased with currents (not shown) which are rendered
stable with temperature. These are derived either from the
supply voltage (as in the AD606 and AD608) or from an
internal band gap reference (as in the AD640 and AD8307).
This topology affords complete control over the magnitude and
temperature behavior of the logarithmic slope, decoupling it
completely from EK.
A further step is needed to achieve the demodulation response,
required when the log amp is to convert an alternating input
into a quasi-dc baseband output. This is achieved by altering the
gm cells used for summation purposes to also implement the
rectification function. Early discrete log amps based on the
progressive compression technique used half-wave rectifiers.
This made post-detection filtering difficult. The AD640 was the
first commercial monolithic log amp to use a full wave rectifier,
a practice followed in all subsequent Analog Devices types.
These detectors can be modeled as being essentially linear gm
cells, but producing an output current independent of the sign
of the voltage applied to the input of each cell. That is, they
implement the absolute value function. Since the output from
the later A/0 stages closely approximates an amplitude
symmetric square wave for even moderate input levels (most
stages of the amplifier chain operate in a limiting mode), the
current output from each detector is almost constant over each
period of the input. Somewhat earlier detector stages produce a
waveform having only very brief dropouts, while the detectors
nearest the input produce a low level, almost sinusoidal
waveform at twice the input frequency. These aspects of the
detector system result in a signal that is easily filtered, resulting
in low residual ripple on the output.
INTERCEPT CALIBRATION
All monolithic log amps from Analog Devices include accurate
means to position the intercept voltage VX (or equivalent power for
a demodulating log amp). Using the scheme shown in Figure 26,
the basic value of the intercept level departs considerably from that
predicted by the simpler analyses given earlier. However, the
intrinsic intercept voltage is still proportional to EK, which is PTAT
(Equation 5). Recalling that the addition of an offset to the output
produces an effect that is indistinguishable from a change in the
position of the intercept, it is possible to cancel the left-right
The precise temperature shaping of the intercept positioning offset
results in a log amp having stable scaling parameters, making it a
true measurement device, for example, as a calibrated received
signal strength indicator (RSSI). In this application, one is more
interested in the value of the output for an input waveform that
is invariably sinusoidal. Although the input level can
alternatively be stated as an equivalent power, in dBm, be sure
to work carefully. It is essential to know the load impedance in
which this power is presumed to be measured.
In RF practice, it is generally safe to assume a reference impedance
of 50 Ω in which 0 dBm (1 mW) corresponds to a sinusoidal
amplitude of 316.2 mV (223.6 mV rms). The intercept can likewise
be specified in dBm. For the AD8307, it is positioned at −84 dBm,
corresponding to a sine amplitude of 20 μV. It is important to bear
in mind that log amps do not respond to power, but to the voltage
applied to their input.
The AD8307 presents a nominal input impedance much higher
than 50 Ω (typically 1.1 kΩ low frequencies). A simple input
matching network can considerably improve the sensitivity of
this type of log amp. This increases the voltage applied to the
input and thus alters the intercept. For a 50 Ω match, the
voltage gain is 4.8 and the entire dynamic range moves down by
13.6 dB (see Figure 35). Note that the effective intercept is a
function of waveform. For example, a square wave input reads
6 dB higher than a sine wave of the same amplitude and a
Gaussian noise input 0.5 dB higher than a sine wave of the same
rms value.
OFFSET CONTROL
In a monolithic log amp, direct coupling between the stages is
used for several reasons. First, this avoids the use of coupling
capacitors, which typically have a chip area equal to that of a
basic gain cell, thus considerably increasing die size. Second, the
capacitor values predetermine the lowest frequency at which the
log amp can operate; for moderate values, this can be as high as
30 MHz, limiting the application range. Third, the parasitic
(backplate) capacitance lowers the bandwidth of the cell, further
limiting the applications.
However, the very high dc gain of a direct-coupled amplifier
raises a practical issue. An offset voltage in the early stages of
the chain is indistinguishable from a real signal. For example, if
it were as high as 400 μV, it would be 18 dB larger than the
smallest ac signal (50 μV), potentially reducing the dynamic
range by this amount. This problem is averted by using a global
feedback path from the last stage to the first, which corrects this
offset in a similar fashion to the dc negative feedback applied
around an op amp. The high frequency components of the
signal must be removed to prevent a reduction of the HF gain in
the forward path.
In the AD8307, this is achieved by an on-chip filter, providing
sufficient suppression of HF feedback to allow operation above
Rev. C | Page 12 of 24
AD8307
1 MHz. To extend the range below this frequency, an external
capacitor can be added. This permits the high-pass corner to be
lowered to audio frequencies using a capacitor of modest value.
Note that this capacitor has no effect on the minimum signal
frequency for input levels above the offset voltage: this extends
down to dc (for a signal applied directly to the input pins). The
offset voltage varies from part to part; some exhibit essentially
stable offsets of under 100 μV without the benefit of an offset
adjustment.
EXTENSION OF RANGE
The theoretical dynamic range for the basic log amp shown in
Figure 26 is AN. For A = 5.2 (14.3 dB) and N = 6, it is 20,000 or
86 dB. The actual lower end of the dynamic range is largely
determined by the thermal noise floor, measured at the input of
the chain of amplifiers. The upper end of the range is extended
upward by the addition of top end detectors. The input signal is
applied to a tapped attenuator, and progressively smaller signals
are applied to three passive rectifying gm cells whose outputs are
summed with those of the main detectors. With care in design,
the extension to the dynamic range can be seamless over the full
frequency range. For the AD8307, it amounts to a further 27 dB.
Therefore, the total dynamic range is theoretically 113 dB. The
specified range of 90 dB (−74 dBm to +16 dBm) is for high
accuracy and calibrated operation, and includes the low end
degradation due to thermal noise and the top end reduction due
to voltage limitations. The additional stages are not, however,
redundant, but are needed to maintain accurate logarithmic
conformance over the central region of the dynamic range, and
in extending the usable range considerably beyond the specified
range. In applications where log conformance is less demanding,
the AD8307 can provide over 95 dB of range.
Rev. C | Page 13 of 24
AD8307
INTERFACES
ENABLE INTERFACE
The chip enable interface is shown in Figure 28. The currents in
the diode-connected transistors control the turn on and turn off
states of the band gap reference and the bias generator, and are a
maximum of 100 μA when Pin 6 is taken to 5 V, under worstcase conditions. Left unconnected, or at a voltage below 1 V, the
AD8307 is disabled and consume a sleep current of under 50 μA;
tied to the supply, or a voltage above 2 V, it is fully enabled. The
internal bias circuitry is very fast, typically <100 ns for either off
or on. In practice, the latency period before the log amp exhibits
its full dynamic range is more likely to be limited by factors
relating to the use of ac coupling at the input or the settling of
the offset control loop.
AD8307
40kΩ
ENB 6
TO BIAS
STAGES
COM
2
Figure 28. Enable Interface
7
AD8307
INP 8
INM 1
7.5mA
+INP
S
BAND GAP REFERENCE
AND BIASING
1.1kΩ
MIRROR
–INP 3
2µA
/dB
COM 2
ENB
5
INT
2
CP
INP
4
OUT
INM
3
OFS
125Ω
2kΩ
6kΩ
Q1
4kΩ
TOP-END
DETECTORS
~3kΩ
Q2
1
TYP 2.2V FOR
3V SUPPLY,
3.2V AT 5V
CM
COM
INPUT-OFFSET
COMPENSATION LOOP
2kΩ
8
CD
12.5kΩ
125Ω
6kΩ
COM
SIX 14.3dB 900MHz
AMPLIFIER STAGES
NINE DETECTOR CELLS
SPACED 14.3dB
6
01082-027
VPS 7
VPS
COM
S
2
Figure 27. Main Features of the AD8307
IE
2.4mA
01082-029
The differential current-mode outputs of the nine detectors are
summed and then converted to single sided form in the output
stage, nominally scaled 2 μA/dB. The logarithmic output
voltage is developed by applying this current to an on-chip
12.5 kΩ resistor, resulting in a logarithmic slope of 25 mV/dB
(that is, 500 mV/decade) at Pin OUT. This voltage is not
buffered, allowing the use of a variety of special output
interfaces, including the addition of post-demodulation
filtering. The last detector stage includes a modification to
temperature stabilize the log intercept, which is accurately
positioned to make optimal use of the full output voltage range
available. The intercept can be adjusted using the INT pin,
which adds or subtracts a small current to the signal current.
tolerance is typically within ±20%. Similarly, the capacitors have
a typical tolerance of ±15% and essentially zero temperature or
voltage sensitivity. Most interfaces have additional small junction
capacitances associated with them, due to active devices or ESD
protection; these can be neither accurate nor stable. Component
numbering in each of these interface diagrams is local.
01082-028
The AD8307 comprises six main amplifier/limiter stages, each
having a gain of 14.3 dB and small signal bandwidth of
900 MHz; the overall gain is 86 dB with a −3 dB bandwidth of
500 MHz. These six cells, and their associated gm styled full
wave detectors, handle the lower two-thirds of the dynamic
range. Three top end detectors, placed at 14.3 dB taps on a
passive attenuator, handle the upper third of the 90 dB range.
Biasing for these cells is provided by two references: one
determines their gain; the other is a band gap circuit that
determines the logarithmic slope and stabilizes it against supply
and temperature variations. The AD8307 can be enabled/
disabled by a CMOS-compatible level at ENB (Pin 6). The first
amplifier stage provides a low voltage noise spectral density
(1.5 nV/√Hz).
COM
The last gain stage also includes an offset sensing cell. This
generates a bipolarity output current when the main signal path
has an imbalance due to accumulated dc offsets. This current is
integrated by an on-chip capacitor (which can be increased in
value by an off-chip component at OFS). The resulting voltage
is used to null the offset at the output of the first stage. Since it
does not involve the signal input connections, whose ac-coupling
capacitors otherwise introduce a second pole in the feedback
path, the stability of the offset correction loop is assured.
The AD8307 is built on an advanced, dielectrically isolated,
complementary bipolar process. Most resistors are thin film
types having a low temperature coefficient of resistance (TCR)
and high linearity under large signal conditions. Their absolute
Figure 29. Signal Input Interface
INPUT INTERFACE
Figure 29 shows the essentials of the signal input interface. CP
and CM are the parasitic capacitances to ground; CD is the
differential input capacitance, mostly due to Q1 and Q2. In
most applications, both input pins are ac-coupled. The switches
close when Enable is asserted. When disabled, the inputs float,
bias current IE is shut off, and the coupling capacitors remain
charged. If the log amp is disabled for long periods, small
leakage currents discharge these capacitors. If they are poorly
matched, charging currents at power-up can generate a
Rev. C | Page 14 of 24
AD8307
In most applications, the signal is single sided and can be
applied to either Pin 1 or Pin 8, with the other pin ac-coupled to
ground. Under these conditions, the largest input signal that
can be handled by the AD8307 is 10 dBm (sine amplitude of
±1 V) when operating from a 3 V supply; 16 dBm can be
handled using a 5 V supply. The full 16 dBm can be achieved for
supplies down to 2.7 V, using a fully balanced drive. For
frequencies above about 10 MHz, this is most easily achieved
using a matching network. Using such a network, having an
inductor at the input, the input transient is eliminated.
Occasionally, it is desirable to use the dc-coupled potential of
the AD8307. The main challenge here is to present signals to
the log amp at the elevated common-mode input level,
requiring the use of low noise, low offset buffer amplifiers.
Using dual supplies of ±3 V, the input pins can operate at
ground potential.
OFFSET INTERFACE
The input referred dc offsets in the signal path are nulled via the
interface associated with Pin 3, shown in Figure 30. Q1 and Q2
are the first stage input transistors, with their corresponding
load resistors (125 Ω). Q3 and Q4 generate small currents,
which can introduce a dc offset into the signal path. When the
voltage on OFS is at about 1.5 V, these currents are equal, and
nominally 64 μA. When OFS is taken to ground, Q4 is off and
the effect of the current in Q3 is to generate an offset voltage of
64 μV × 125 Ω = 8 mV. Since the first stage gain is ×5, this is
equivalent to a input offset (INP to INM) of 1.6 mV. When OFS
is taken to its most positive value, the input referred offset is
reversed to −1.6 mV. If true dc coupling is needed, down to very
small inputs, this automatic loop must be disabled, and the
residual offset eliminated using a manual adjustment.
In normal operation, however, using an ac-coupled input signal,
the OFS pin should be left open. Any residual input offset
voltage is then automatically nulled by the action of the
feedback loop. The gm cell, which is gated off when the chip is
disabled, converts any output offset (sensed at a point near the
end of the cascade of amplifiers) to a current. This is integrated
by the on-chip capacitor CHP, and any added external
capacitance COFS, so as to generate an error voltage, which is
applied back to the input stage in the polarity needed to null the
output offset. From a small signal perspective, this feedback
alters the response of the amplifier, which, rather than behaving
as a fully dc-coupled system, now exhibits a zero in its ac
transfer function, resulting in a closed loop high-pass corner at
about 1.5 MHz.
7
125Ω
INPUT
STAGE
MAIN GAIN
STAGES
Q1
Q2
BIAS, ~1.2V
VPS
125Ω
64µA AT
BALANCE
S
OFS
Q3
36kΩ
Q4
3
48kΩ
COFS
TO LAST
DETECTOR
gm
AVERAGE
ERROR
CURRENT
CHP
2
COM
01082-030
transient input voltage that can block the lower reaches of the
dynamic range until it has become much less than the signal.
Figure 30. Offset Interface and Offset Nulling Path
The offset feedback is limited to a range of ±1.6 mV; signals larger
than this override the offset control loop, which only affects
performance for very small inputs. An external capacitor reduces
the high-pass corner to arbitrarily low frequencies; using 1 μF this
corner is below 10 Hz. All ADI log amps use an offset nulling loop;
the AD8307 differs in using this single sided form.
OUTPUT INTERFACE
The outputs from the nine detectors are differential currents,
having an average value that is dependent on the signal input
level, plus a fluctuation at twice the input frequency. The
currents are summed at nodes LGP and LGM in Figure 31.
Further currents are added at these nodes, to position the
intercept, by slightly raising the output for zero input, and to
provide temperature compensation. Since the AD8307 is not
laser trimmed, there is a small uncertainty in both the log slope
and the log intercept. These scaling parameters can be adjusted.
For zero signal conditions, all the detector output currents are
equal. For a finite input of either polarity, their difference is
converted by the output interface to a single sided unipolar
current nominally scaled 2 μA/dB (40 μA/decade), at Pin OUT.
An on-chip 12.5 kΩ resistor, R1, converts this current to a
voltage of 25 mV/dB. C1 and C2 are effectively in shunt with R1
and form a low-pass filter pole with a corner frequency of about
5 MHz. The pulse response settles to within 1% of the final
value within 300 ns. This integral low-pass filter provides
adequate smoothing in many IF applications. At 10.7 MHz, the
2f ripple is 12.5 mV in amplitude, equivalent to ±0.5 dB, and
only 0.5 mV (±0.02 dB) at f = 50 MHz. A filter capacitor CFLT
added from Pin OUT to ground lowers this corner frequency.
Using 1 μF, the ripple is maintained to less than ±0.5 dB down
to input frequencies of 100 Hz. Note that COFS should also be
increased in low frequency applications, and is typically made
equal to CFLT.
Rev. C | Page 15 of 24
AD8307
3pF
LGP
2µA/dB
0–220µA
25mV/dB
OUT 4
Rev. C | Page 16 of 24
1.25kΩ
CFLT
60kΩ
1.25kΩ
C2
1pF
C1
2.5pF
7
VPS
5
INT
2
COM
8.25kΩ
~400mV
FROM ALL
DETECTORS
LGM
Note that while the AD8307 can operate down to supply
voltages of 2.7 V, the output voltage limit is reduced when the
supply drops below 4 V. This characteristic is the result of
necessary headroom requirements, approximately two VBE
drops, in the design of the output stage.
1.25kΩ
R1
12.5kΩ
1.25kΩ
BIAS
60µA
Figure 31. Simplified Output Interface
01082-031
It can be desirable to increase the speed of the output response,
with the penalty of increased ripple. One way to do this is
simply by connecting a shunt load resistor from Pin OUT to
ground, which raises the low-pass corner frequency. This also
alters the logarithmic slope, for example to 7.5 mV/dB using a
5.36 kΩ resistor, while reducing the 10% to 90% rise time to
25 ns. The ripple amplitude for 50 MHz input remains 0.5 mV,
but this is now equivalent to ±0.07 dB. If a negative supply is
available, the output pin can be connected directly to the
summing node of an external op amp connected as an inverting
mode transresistance stage.
AD8307
THEORY OF OPERATION
Careful shielding is essential. A ground plane should be used to
provide a low impedance connection to the common pin,
COM, for the decoupling capacitor(s) used at VPS, and as the
output ground. It is inadvisable to assume that the ground plane
is an equipotential. Neither of the inputs should be ac-coupled
directly to the ground plane, but should be kept separate from
it, being returned instead to the low associated with the source.
This can mean isolating the low side of an input connector with
a small resistance to the ground plane.
the log amp side of the coupling capacitors; in the former case,
smaller capacitors can be used for a given frequency range; in
the latter case, the effective RIN is lowered directly at the log
amp inputs.
Figure 33 shows the output versus the input level, in dBm, when
driven from a terminated 50 Ω generator, for sine inputs at
10 MHz, 100 MHz, and 500 MHz; Figure 34 shows the typical
logarithmic conformance under the same conditions. Note that
+10 dBm corresponds to a sine amplitude of 1 V, equivalent to
an rms power of 10 mW in a 50 Ω termination. However, if the
termination resistor is omitted, the input power is negligible.
The use of dBm to define input level therefore needs to be
considered carefully in connection with the AD8307.
3.0
2.5
10MHz
OUTPUT VOLTAGE (V)
The AD8307 has very high gain and a bandwidth from dc to
over 1 GHz, at which frequency the gain of the main path is still
over 60 dB. Consequently, it is susceptible to all signals within
this very broad frequency range that find their way to the input
terminals. It is important to remember that these are
indistinguishable from the wanted signal, and has the effect of
raising the apparent noise floor (that is, lowering the useful
dynamic range). For example, while the signal of interest can be
an IF of 50 MHz, any of the following could easily be larger
than the IF signal at the lower extremities of its dynamic range:
60 Hz hum (picked up due to poor grounding techniques);
spurious coupling (from a digital clock source on the same PC
board); local radio stations; and so on.
BASIC CONNECTIONS
500MHz
4.7Ω
01082-033
–50
–40
–30
–20
–10
0
10
20
5
4
3
500MHz
2
VP, 2.7V TO 5.5V
AT ~8mA
1
0
–1
10MHz
100MHz
–2
7
6
–4
5
–5
–80
AD8307
–60
–50
–40
–30
–20
–10
0
10
20
Figure 34. Logarithmic Law Conformance at 10 MHz, 100 MHz, and 500 MHz
4
NC
C2 = CC
NC = NO CONNECT
OUTPUT
25mV/dB
INPUT MATCHING
01082-032
3
–70
INPUT LEVEL (dBm)
INM COM OFS OUT
2
01082-034
–3
INP VPS ENB INT
1
–60
Figure 33. Log Response at 10 MHz, 100 MHz, and 500 MHz
NC
8
–70
INPUT LEVEL (dBm)
C1 = CC
1.1kΩ
100MHz
1.0
0
–80
ERROR (dB)
0.1µF
RIN ≈
1.5
0.5
Figure 32 shows the simple connections suitable for many
applications. The inputs are ac coupled by C1 and C2, which
should have the same value, say, CC. The coupling time constant
is RIN CC/2, thus forming a high-pass corner with a 3 dB
attenuation at fHP = 1/(pRINCC ). In high frequency applications,
fHP should be as large as possible in order to minimize the
coupling of unwanted low frequency signals. Conversely, in low
frequency applications, a simple RC network forming a lowpass filter should be added at the input for the same reason. For
the case where the generator is not terminated, the signal range
should be expressed in terms of the voltage response, and
extends from −85 dBV to +6 dBV.
INPUT
–75dBm TO
+16dBm
RT
2.0
Figure 32. Basic Connections
Where it is necessary to terminate the source at a low impedance,
the resistor RT should be added, with allowance for the shunting
effect of the basic 1.1 kΩ input resistance (RIN) of the AD8307.
For example, to terminate a 50 Ω source, a 52.3 Ω 1% tolerance
resistor should be used. This can be placed on the input side or
Where higher sensitivity is required, an input matching network
is valuable. Using a transformer to achieve the impedance
transformation also eliminates the need for coupling capacitors,
which lowers the offset voltage generated directly at the input,
and balances the drives to Pin INP and Pin INM. The choice of
turns ratio depends somewhat on the frequency. At frequencies
below 50 MHz, the reactance of the input capacitance is much
higher than the real part of the input impedance. In this
Rev. C | Page 17 of 24
AD8307
0.1µF
frequency range, a turns ratio of about 1:4.8 lowers the input
impedance to 50 Ω while raising the input voltage, thus
lowering the effect of the short-circuit noise voltage by the same
factor. There is a small contribution from the input noise
current, so the total noise is reduced by a lesser factor. The
intercept is also lowered by the turns ratio; for a 50 Ω match, it
is reduced by 20 log10 (4.8) or 13.6 dB.
4.7Ω
VP, 2.7V TO 5.5V
AT ~8mA
C1
NC
50Ω INPUT
–88dBm TO
+3dBm
8
7
6
5
INP VPS ENB INT
LM
AD8307
INM COM OFS OUT
ZIN = 50Ω
1
2
3
4
NC
NARROW-BAND MATCHING
OUTPUT
25mV/dB
Transformer coupling is useful in broadband applications.
However, a magnetically-coupled transformer may not be
convenient in some situations. At high frequencies, it is often
preferable to use a narrow-band matching network, as shown in
Figure 35.
01082-035
C2
NC = NO CONNECT
Figure 35. High Frequency Input Matching Network
14
13
12
11
This has several advantages. The same voltage gain is achieved,
providing increased sensitivity, but now a measure of selectivity
is also introduced. The component count is low: two capacitors
and an inexpensive chip inductor. Further, by making these
capacitors unequal, the amplitudes at Pin INP and Pin INM can
be equalized when driving from a single sided source; that is,
the network also serves as a balun.
GAIN
10
DECIBELS
9
8
7
6
5
4
3
INPUT
2
Figure 36 shows the response for a center frequency of
100 MHz. Note the very high attenuation at low frequencies.
The high frequency attenuation is due to the input capacitance
of the log amp.
01082-036
1
0
–1
60
70
80
90
100
110
120
130
140
150
FREQUENCY (MHz)
Figure 36. Response of 100 MHz Matching Network
Table 4 provides solutions for a variety of center frequencies
(FC) and matching impedances (ZIN) of nominally 50 Ω and
100 Ω. The unequal capacitor values were chosen to provide a
well balanced differential drive, and to allow better centering of
the frequency response peak when using standard value
components; this generally results in a ZIN that is not exact. The
full AD8307 HF input impedance and the inductor losses are
included in the modeling.
Table 4. Narrow-Band Matching Values
FC (MHz)
10
20
50
100
150
200
250
500
10
20
50
100
150
200
250
500
ZIN (Ω)
45
44
46
50
57
57
50
54
103
102
99
98
101
95
92
114
C1 (pF)
160
82
30
15
10
7.5
6.2
3.9
100
51
22
11
7.5
5.6
4.3
2.2
C2 (pF)
150
75
27
13
8.2
6.8
5.6
3.3
91
43
18
9.1
6.2
4.7
3.9
2.0
Rev. C | Page 18 of 24
LM (nH)
3300
1600
680
330
220
150
100
39
5600
2700
1000
430
260
180
130
47
Voltage Gain (dB)
13.3
13.4
13.4
13.4
13.2
12.8
12.3
10.9
10.4
10.4
10.6
10.5
10.3
10.3
9.9
6.8
AD8307
SLOPE AND INTERCEPT ADJUSTMENTS
ΔdB = 20 log10
1+ M
1− M
The log intercept is adjustable over a ±3 dB range, which is
sufficient to absorb the worst-case intercept error in the
AD8307, plus some system level errors. For greater range, set RS
to zero. VR2 is adjusted while applying an accurately known
CW signal near the lower end of the dynamic range in order to
minimize the effect of any residual uncertainty in the slope. For
example, to position the intercept to −80 dBm, a test level of
−65 dBm can be applied and VR2 adjusted to produce a dc
output of 15 dB above zero at 25 mV/dB, which is +0.3 V.
(7)
0.1µF
VR2
50kΩ
C1 = CC
Rev. C | Page 19 of 24
VP, 2.7V TO 5.5V
AT ~8mA
RS
±3dB
8
INPUT
–75dBm TO
+16dBm
7
6
5
INP VPS ENB INT
AD8307
FOR VP = 3V, RS = 20kΩ
VP = 5V, RS = 51kΩ
INM COM OFS OUT
1
For example, using an rms signal level of −40 dBm with a 70%
modulation depth (M = 0.7), the decibel range is 15 dB, as the
signal varies from −47.5 dBm to −32.5 dBm.
4.7Ω
2
3
4
NC
C2 = CC
NC = NO CONNECT
20mV/dB
±10%
32.4kΩ VR1
50kΩ
Figure 37. Slope and Intercept Adjustments
01082-037
Where higher calibration accuracy is needed, the adjustments
shown in Figure 37 can be used, either singly or in combination.
The log slope is lowered to 20 mV/dB by shunting the nominally
12.5 kΩ on-chip load resistor (see Figure 31) with 50 kΩ, adjusted
by VR1. The calibration range is ±10% (18 mV/dB to 22 mV/dB),
including full allowance for the variability in the value of the
internal load. The adjustment can be made by alternately
applying two input levels, provided by an accurate signal
generator, spaced over the central portion of the log amp’s
dynamic range, for example −60 dBm and 0 dBm. An AM
modulated signal, at the center of the dynamic range, can also
be used. For a modulation depth M, expressed as a fraction, the
decibel range between the peaks and troughs over one cycle of
the modulation period is given by
AD8307
INPUT
–75dBm TO
+16dBm
VP, 2.7V TO 5.5V
VR2
50kΩ
RS
±3dB
INPUT
–75dBm TO
+16dBm
6
5
INP VPS ENB INT
FOR VP = 3V, RS = 20kΩ
VP = 5V, RS = 51kΩ
INM COM OFS OUT
2
3
NC
25mV/dB
AD8031
AD8307
4
20mV/dB
2
3
4
NC = NO CONNECT
In Figure 40, the capacitor values are chosen for operation in
the audio field, providing a corner frequency of 10 Hz, an
attenuation of 80 dB/decade above this frequency, and a 1%
settling time of 150 ms (0.1% in 175 ms). The residual ripple is
4 mV (±0.02 dB) when the input to the AD8307 is at 20 Hz.
This filter can easily be adapted to other frequencies by
proportional scaling of C5 to C7 (for example, for 100 kHz use
100 pF). Placed ahead of a digital multimeter, the convenient
slope scaling of 100 mV/dB requires only a repositioning of the
decimal point to read directly in decibels. The supply voltage for
the filter must be large enough to support the dynamic range; a
minimum of 9 V is needed for most applications; 12 V is
recommended.
4.7Ω
R1
50kΩ
VP
VR1
2kΩ
C1
10µF
R2
30.1kΩ
R1
20kΩ
COM
R1
2kΩ
COM
8
7
6
OP AMP IS AD8032 SCALE
C1 TO C8 AS NEEDED.
NOTE POLARITIES IF TANTALUM
CAPACITORS ARE USED.
422Ω
INT ±4dB
+
5
C5
1µF
C8
7.32kΩ 1µF
+
+
INP VPS ENB INT
C3
2.5nF
Figure 38. Log Amp with Buffered Output
INM COM OFS OUT
1
C1 is optional; it lowers the corner frequency of the low-pass
output filter. A value of 0.1 μF should be used for applications in
which the output is measured on a voltmeter or other low speed
device. On the other hand, when C1 is omitted, the 10% to 90%
response time is under 200 ns and is typically 300 ns to 99% of
final value. To achieve faster response times, it is necessary to
lower the load resistance at the output of the AD8307, then
restore the scale using a higher gain in the op amp. Using
8.33 kΩ, the basic slope is 10 mV/dB; this can be restored to
25 mV/dB using a buffer gain of 2.5. The overall 10% to 90%
response time is under 100 ns. Figure 39 shows how the output
current capability can be augmented to drive a 50 Ω load; RT
optionally provides reverse termination, which halves the slope
to 12.5 mV/dB.
FOUR POLE FILTER
In low frequency applications, for example, audio down to
20 Hz, it is useful to employ the buffer amplifier as a multipole
low-pass filter in order to achieve low output ripple while
maintaining a rapid response time to changes in signal level.
2
3
+
+ C2
10µF
OUTPUT
100mV/dB
100kΩ
AD8307
C4
1µF
C1
OUTPUT
50Ω
MINIMUM
Figure 39. Cable Driving Log Amp
INPUT 5mV
TO 160V rms 0.1µF
01082-038
32.4kΩ VR1
50kΩ
RT
(OPTIONAL)
R2
3.01kΩ
10mV/dB
±18%
6.34kΩ VR1
5kΩ
NC
NC = NO CONNECT
2N3904
5
NC
OUTPUT
50mV/dB
±10%
AD8031
AD8307
1
6
INP VPS ENB INT
1
4.7Ω
7
7
INM COM OFS OUT
The output can be buffered, and the slope optionally increased,
using an op amp. If the single-supply capability is to be preserved, a
suitable component is the AD8031. Like the AD8307, it is
capable of operating from a 2.7 V supply and features a rail-torail output capability; it is available in a 5-lead version and in
dual form as the 8-lead AD8032. Figure 38 shows how the slope
can be increased to 50 mV/dB (1 V per decade), requiring a
5 V supply (90 dB times 50 mV is a 4.5 V swing). VR1 provides
a ±10% slope adjustment; VR2 provides a ±3 dB intercept
range. With R2 = 4.99 kΩ, the slope is adjustable to 25 mV/dB,
allowing the use of a 2.7 V supply. Setting R2 to 80.6 kΩ, it is
raised to 100 mV/dB, providing direct reading in decibels on a
digital voltmeter. Since a 90 dB range now corresponds to a 9 V
swing, a supply of at least this amount is needed for the op amp.
8
RS
±3dB
8
BUFFERED OUTPUT
0.1µF
VP, 2.7V TO 5.5V
VR2
50kΩ
93kΩ
34kΩ
4
34kΩ
VR2
50kΩ
SLOPE C6 +
32.4kΩ 1µF
C7 +
1µF
75kΩ
80.6kΩ
COM
01082-040
The AD8307 is a highly versatile and easily applied log amp
requiring very few external components. Most applications of this
product can be accommodated using the simple connections
shown in the preceding section.
4.7Ω
01082-039
0.1µF
APPLICATIONS INFORMATION
Figure 40. Log Amp with Four Pole Low Pass Filter
Figure 40 also shows the use of an input attenuator that can
optionally be employed here, or in any other of these
applications, to produce a useful wide range ac voltmeter with
direct decibel scaling. The basic range of −73 dBm to +17 dBm
(that is, 50 μV rms to 1.6 V rms, for sine excitations) is shifted
for illustrative purposes to 5 mV to 160 V rms (at which point
the power in R1 is 512 mW). Because the basic input resistance
of the AD8307 is not precise, VR1 is used to center the signal
range at its input, doubling as a ±4 dB intercept adjustment. The
low frequency response extends to 15 Hz; a higher corner
frequency can be selected as needed by scaling C1 and C2. The
shunt capacitor C3 is used to lower the high frequency
bandwidth to about 100 kHz, and thus lower the susceptibility
to spurious signals. Other values should be chosen as needed
for the coupling and filter capacitors.
Rev. C | Page 20 of 24
AD8307
The front-end adaptation shown in Figure 41 provides the
measurement of power being delivered from a transmitter final
amplifier to an antenna. The range has been set to cover the
power range −30 dBm (7.07 mV rms, or 1 μW) to +60 dBm
(223 V rms, or 1 kW). A nominal voltage attenuation ratio of
158:1 (44 dB) is used; thus the intercept is moved from
−84 dBm to −40 dBm and the AD8307, scaled 0.25 V/decade of
power, now reads 1.5 V for a power level of 100 mW, 2.0 V at
10 W and 2.5 V at 1 kW. The general expression is
P (dBm) = 40 (VOUT − 1)
The required attenuation could be implemented using a
capacitive divider, providing a very low input capacitance, but it
is difficult to ensure accurate values of small capacitors. A better
approach is to use a resistive divider, taking the required
precautions to minimize spurious coupling into the AD8307 by
placing it in a shielded box, with the input resistor passing
through a hole in this box, as indicated in Figure 41. The
coupling capacitors shown here are suitable for f ≥ 10 MHz. A
capacitor can be added across the input pins of the AD8307 to
reduce the response to spurious HF signals, which, as already
noted, extends to over 1 GHz.
The mismatch caused by the loading of this resistor is trivial;
only 0.05% of the power delivered to the load is absorbed by the
measurement system, a maximum of 500 mW at 1 kW. The
post-demodulation filtering and slope calibration arrangements
are chosen from other applications described in this data sheet
to meet the particular system requirements. The 1 nF capacitor
lowers the risk of HF signals entering the AD8307 via the load.
TO
ANTENNA
100kΩ
1/2W
0.1µF
VP
22Ω
51pF
+5V
NC
7
6
5
INP VPS ENB INT
604Ω
INM COM OFS OUT
AD8307
1
2
3
4
NC
51pF
1nF
LEADTHROUGH
CAPACITORS,
1nF
2kΩ
VOUT
OUTPUT
NC = NO CONNECT
01082-041
50Ω INPUT
FROM P.A.
1µW TO
1kW
8
VR1
2kΩ
INT ±3dB
Figure 41. 1 μW to 1 kW 50 Ω Power Meter
MEASUREMENT SYSTEM WITH 120 dB DYNAMIC
RANGE
The dynamic range of the AD8307 can be extended further—
from 90 dB to over 120 dB—by the addition of an X-AMP® such
as the AD603. This type of variable gain amplifier exhibits a
very exact exponential gain control characteristic, which is
another way of stating that the gain varies by a constant number
of decibels for a given change in the control voltage. For the
AD603, this scaling factor is 40 dB/V, or 25 mV/dB. It is
apparent that this property of a linear-in-dB response is
characteristic of log amps; indeed, the AD8307 exhibits the
same scaling factor.
The AD603 has a very low input referred noise: 1.3 nV/√Hz at its
100 Ω input, or 0.9 nV/√Hz when matched to 50 Ω, equivalent to
0.4 μV rms, or −115 dBm, in a 200 kHz bandwidth. It is also
capable of handling inputs in excess of 1.4 V rms, or +16 dBm. It is
thus able to cope with a dynamic range of over 130 dB in this
particular bandwidth.
Now, if the gain control voltage for the X-AMP is derived from the
output of the AD8307, the effect is to raise the gain of this frontend stage when the signal is small and lower it when it is large, but
without altering the fundamental logarithmic nature of the
response. This gain range is 40 dB, which, combined with the 90 dB
range of the AD8307, again corresponds to a 130 dB range.
50Ω
INPUT
–105dBm
TO
+15dBm
L1
750nH
C1
150pF
VP, +5V
R1
187kΩ
R2
28kΩ
BANDPASS
FILTER*
4.7Ω
0.1µF
0.65V
NC
1
VPOS 8
GPOS
2
GNEG
3
VINP
4
COMM FDBK 5
R3
330Ω
VOUT 7
R4
464Ω
AD603
VNEG 6
VR1
5kΩ
INT
±8dB
VN, –5V
R5
100kΩ
8
7
6
5
INP VPS ENB INT
AD8307
INM COM OFS OUT
1
2
3
4
NC
1nF
R6
20kΩ
0.3V
TO
2.3V
R7
80.6kΩ
0.15V TO 1.15V
*FOR EXAMPLE: MURATA SFE10.7MS2G-A
NC = NO CONNECT
OUTPUT
10mV/dB
01082-042
1 μW TO 1 kW 50 Ω POWER METER
Figure 42. 120 dB Measurement System
Figure 42 shows how these two parts can work together to
provide state-of-the-art IF measurements in applications such
as spectrum/network analyzers and other high dynamic range
instrumentation. To understand the operation, note first that
the AD8307 is used to generate an output of about 0.3 V to
2.3 V. This 2 V span is divided by 2 in R5/R6/R7 to provide the
1 V span needed by the AD603 to vary its gain by 40 dB. Note
that an increase in the positive voltage applied at GNEG (Pin 2
of AD603) lowers the gain. This feedback network is tapped to
provide a convenient 10 mV/dB scaling at the output node,
which can be buffered if necessary.
The center of the voltage range fed back to the AD603 is
650 mV, and the ±20 dB gain range is centered by R1/R2. Note
that the intercept calibration of this system benefits from the
use of a well regulated 5 V supply. To absorb the insertion loss
of the filter and center the full dynamic range, the intercept is
adjusted by varying the maximum gain of the AD603, using
VR1. Figure 43 shows the AD8307 output over the range
−120 dBm to +20 dBm and the deviation from an ideal
logarithmic response. The dotted line shows the increase in the
noise floor that results when the filter is omitted; the decibel
difference is about 10 log10(50/0.2) or 24 dB, assuming a 50
MHz bandwidth from the AD603. An L-C filter can be used in
place of the ceramic filter used in this example.
Rev. C | Page 21 of 24
AD8307
2.50
See Figure 40 for a more elaborate filter. To improve the law
conformance at very low signal levels and at low frequencies, add
C4 to the offset compensation loop.
2.25
1
1.25
0
–1
ERROR
(WITH FILTER)
0.75
0.50
VIN
0.5mV TO
20V SINE
AMPLITUDE
–2
C3
750pF
–80
–60
–40
–20
INPUT LEVEL (dBm)
0
20
2
C4
1µF
C5
1µF
VOUT
25mV/dB
DC-COUPLED APPLICATIONS
It may occasionally be necessary to provide response to dc inputs.
Since the AD8307 is internally dc-coupled, there is no fundamental
reason why this is precluded. However, there is a practical
constraint since its inputs must be positioned about 2 V above the
COM potential for proper biasing of the first stage. If the source is a
differential signal at this level, it can be directly connected to the
input. For example, a microwave detector can be ac-coupled at its
RF input and its baseband load then automatically provided by the
floating RIN and CIN of the AD8307, at about VP/2.
Usually, the source is a single sided ground-referenced signal;
thus, it is necessary to provide a negative supply for the
AD8307. This can be achieved as shown in Figure 45. The
output is now referenced to this negative supply, and it is
necessary to provide an output interface that performs a
differential-to-single sided conversion. This is the purpose of
the AD830. The slope can be arranged to be 20 mV/dB, when
the output ideally runs from zero, for a dc input of 10 μV, to
2.2 μV, for an input of 4 V. The device is fundamentally
insensitive to the sign of the input signal, but with this biasing
scheme, the maximum negative input is constrained to about
−1.5 V. The transfer function after trimming and with R7 = 0 is
VOUT = (0.4 V) log10 (VIN/10 μV)
R1
4.7Ω
C1
0.1µF
R2
3.3kΩ
+5V FOR 20mV/dB
+10V FOR 50mV/dB
+15V FOR 100mV/dB
VR2
50kΩ
8
7
6
R5*
5
8
VP
INP VPS ENB INT
INM COM OFS OUT
1
2
3
7
6
INT NC
5
VN
AD830
AD8307
C1
1µF
4
VOUT
–5V
X1
X2
Y1
Y2
1
2
3
4
R7
20mV/dB
R3
1kΩ
4
Figure 44. Connections for Low Frequency Operation
A high-pass 3 dB corner frequency of nominally 3 Hz is set by the
10 μF coupling capacitors C1 and C2, which are preferably
tantalum electrolytics (note the polarity) and a low-pass 3 dB
corner frequency of 200 kHz (set by C3 and the effective resistance
at the input of 1 kΩ). The −1% amplitude error points occur at 20
Hz and 30 kHz. These are readily altered to suit other applications
by simple scaling. When C3 is zero, the low-pass corner is at 200
MHz. Note that the lower end of the dynamic range is improved by
this capacitor, which essentially provides an HF short circuit at the
input. This significantly lowers the wideband noise; the noise
reduction is about 2 dB compared to when the AD8307 is driven
from a 50 Ω source. Ensure that the output is free of postdemodulation ripple by lowering the low-pass filter time constant.
This is provided by C5; with the value shown in Figure 44, the
output time constant is 125 ms.
AD589
3
NC = NO CONNECT
The AD8307 provides excellent logarithmic conformance at
signal frequencies that can be arbitrarily low, depending only on
the values used for the input coupling capacitors. It can also be
desirable to add a low-pass input filter in order to desensitize
the log amp to HF signals. Figure 44 shows a simple arrangement,
providing coupling with an attenuation of 20 dB; the intercept is
shifted up by this attenuation, from −84 dBm to −64 dBm, and
the input range is now 0.5 mV to 20 V (sine amplitude).
TEMP
5
AD8307
1
OPERATION AT LOW FREQUENCIES
VIN
6
INP VPS ENB INT
+
R2
C2
10µF 5kΩ
Figure 43. Results for 120 dB Measurement System
+5V
NC
7
INM COM OFS OUT
01082-043
–100
4.7Ω
0.1µF
8
WITH FILTER
0.25
0
C1 R1
10µF 5kΩ
+
VR1
2kΩ
Q1
2N3904
NC = NO CONNECT
C3
0.1µF
–2V
R6
32.4kΩ
VR3
50kΩ
R8
R9
250Ω
–5V
*51kΩ FOR 20mV/dB; 5kΩ FOR 100mV/dB
Figure 45. Connections for DC-Coupled Applications
Rev. C | Page 22 of 24
01082-044
VOUT (V)
1.50
1.00
5V
2
01082-045
WITHOUT
FILTER
1.75
ERROR (dB)
2.00
AD8307
The intercept can be raised, for example, to 100 μV, with the
rationale that the dc precision does not warrant operation in
the first decade (from 10 μV to 100 μV). Likewise, the slope can
be raised to 50 mV/dB, using R7 = 3 kΩ, R8 = 2 kΩ , or to
100 mV/dB, to simplify decibel measurements on a DVM,
using R7 = 8 kΩ, R8 = 2 kΩ, which raises the maximum output
11 V, thus requiring a 15 V supply for the AD830. The output
can be made to swing in a negative direction by simply
reversing Pins 1 and 2. Low-pass filtering capacitor, C3, sets the
output rise time to about 1 ms.
6.0
5.5
5.0
4.0
1.0
3.5
0.5
3.0
0
2.5
–0.5
2.0
–1.0
ERROR (dB)
1.5
OPERATION ABOVE 500 MHZ
0.5
100µ
1m
10m
100m
1
10
VIN
Figure 46. Ideal Output and Law-Conformance Error for the DC-Coupled
AD8307 at 50 mV/dB
Figure 46 shows the output and the law-conformance error, in
the absence of noise and input offset, for the 50 mV/dB option.
Note that the error ripple for dc excitation is about twice that
for the more usual sinusoidal excitation. In practice, both the
noise and the internal offset voltage degrade the accuracy in the
first decade of the dynamic range. The latter is now manually
nulled by VR1, using a simple method that ensures very low
residual offsets.
A temporary ac signal, typically a sine wave of 100 mV in
amplitude at a frequency of about 100 Hz, is applied via the
capacitor at node TEMP; this has the effect of disturbing the
offset nulling voltage. The output voltage is then viewed on an
oscilloscope and VR1 is adjusted until the peaks of the
(frequency-doubled) waveform are exactly equal in amplitude.
This procedure can provide an input null down to about 10 μV.
The temperature drift is very low, though not specified since the
AD8307 is not principally designed to operate as a baseband log
amp; in ac modes, this offset is nulled continuously and
automatically.
The AD8307 is not intended for use above 500 MHz. However,
it does provide useful performance at higher frequencies.
Figure 47 shows a plot of the logarithmic output of the AD8307
for an input frequency of 900 MHz. The device shows good
logarithmic conformance from −50 dBm to −10 dBm. There is a
bump in the transfer function at −5 dBm, but if this is
acceptable, the device is usable over a 60 dB dynamic range
(−50 dBm to +10 dBm).
2.0
1.8
1.6
1.4
Rev. C | Page 23 of 24
1.2
1.0
0.8
0.6
0.4
01082-047
01082-046
1.0
0
10µ
Finally, the slope must be adjusted. This can be performed by
applying a low frequency square wave to the main input, having
precisely determined upper and lower voltage levels, provided
by a programmable waveform generator. A suitable choice is a
100 Hz square wave with levels of 10 mV and 1 V. The output is
a low-pass filtered square wave, and its amplitude should be
0.8 V for 20 mV/dB scaling, or 4 V for 100 mV/dB scaling.
VOUT (V)
VOUT (V)
4.5
Next, it is necessary to set the intercept. This is the purpose of
VR2, which should be adjusted after VR1. The simplest method
is to short the input and adjust VR2 for an output of 0.3 V,
corresponding to the noise floor. For more exacting
applications, a temporary sinusoidal test voltage of 1 mV in
amplitude, at about 1 MHz, should be applied, which can
require the use of a temporary on-board input attenuator. For
20 mV/dB scaling, a 10 μV dc intercept (which is 6 dB below
the ac intercept) requires adjusting the output to 0.68 V; for
100 mV/dB scaling, this becomes 3.4 V. If a 100 μV intercept is
preferred (usefully lowering the maximum output voltage),
these become 0.28 V and 1.4 V, respectively.
0.2
0
–60
–50
–40
–30
–20
PIN (dBm)
–10
0
10
Figure 47. Output vs. Input Level for a 900 MHz Input Signal
AD8307
OUTLINE DIMENSIONS
0.400 (10.16)
0.365 (9.27)
0.355 (9.02)
8
5
1
0.280 (7.11)
0.250 (6.35)
0.240 (6.10)
4
0.100 (2.54)
BSC
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.060 (1.52)
MAX
0.210 (5.33)
MAX
0.015
(0.38)
MIN
0.150 (3.81)
0.130 (3.30)
0.115 (2.92)
SEATING
PLANE
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.195 (4.95)
0.130 (3.30)
0.115 (2.92)
0.015 (0.38)
GAUGE
PLANE
0.430 (10.92)
MAX
0.005 (0.13)
MIN
0.014 (0.36)
0.010 (0.25)
0.008 (0.20)
0.070 (1.78)
0.060 (1.52)
0.045 (1.14)
070606-A
COMPLIANT TO JEDEC STANDARDS MS-001
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS.
Figure 48. 8-Lead Plastic Dual In-Line Package [PDIP]
(N-8)
Dimensions shown in inches and (millimeters)
5.00 (0.1968)
4.80 (0.1890)
8
1
5
4
1.27 (0.0500)
BSC
0.25 (0.0098)
0.10 (0.0040)
COPLANARITY
0.10
SEATING
PLANE
6.20 (0.2440)
5.80 (0.2284)
1.75 (0.0688)
1.35 (0.0532)
0.51 (0.0201)
0.31 (0.0122)
0.50 (0.0196)
0.25 (0.0099)
45°
8°
0°
0.25 (0.0098)
0.17 (0.0067)
1.27 (0.0500)
0.40 (0.0157)
COMPLIANT TO JEDEC STANDARDS MS-012-A A
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
060506-A
4.00 (0.1574)
3.80 (0.1497)
Figure 49. 8-Lead Standard Small Outline Package [SOIC_N]
Narrow Body
(R-8)
Dimensions shown in millimeters and (inches)
ORDERING GUIDE
Model
AD8307AN
AD8307ANZ1
AD8307AR
AD8307AR-REEL
AD8307AR-REEL7
AD8307ARZ1
AD8307ARZ-REEL1
AD8307ARZ-RL71
Temperature Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package Description
8-Lead PDIP
8-Lead PDIP
8-Lead SOIC_N
8-Lead SOIC_N 13" REEL
8-Lead SOIC_N 7" REEL
8-Lead SOIC_N
8-Lead SOIC_N 13" REEL
8-Lead SOIC_N 7" REEL
1
Z = Pb-free part.
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D01082-0-10/06(C)
Rev. C | Page 24 of 24
Package Option
N-8
N-8
R-8
R-8
R-8
R-8
R-8
R-8