LOG114AIRGVRG4 [BB]
Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER; 单电源，高速，高精度对数放大器
| 型号： | LOG114AIRGVRG4 |
| 厂家： | 
BURR-BROWN CORPORATION |
| 描述： | Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER |
| 文件： | 总30页 (文件大小：606K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LOG114
SBOS301A − MAY 2004 − REVISED MARCH 2007
Single-Supply, High-Speed, Precision
LOGARITHMIC AMPLIFIER
FD EATURES
DESCRIPTION
The LOG114 is specifically designed for measuring
ADVANTAGES:
− Tiny for High Density Systems
low-level and wide dynamic range currents in
communications, lasers, medical, and industrial
systems. The device computes the logarithm or log-ratio
of an input current or voltage relative to a reference
current or voltage (logarithmic transimpedance
amplifier).
− Precision on One Supply
− Fast Over Eight Decades
− Fully-Tested Function
D
D
D
D
D
D
D
D
TWO SCALING AMPLIFIERS
WIDE INPUT DYNAMIC RANGE:
Eight Decades, 100pA to 10mA
2.5V REFERENCE
STABLE OVER TEMPERATURE
LOW QUIESCENT CURRENT: 10mA
DUAL OR SINGLE SUPPLY: + 5V, +5V
PACKAGE: Small QFN-16 (4mm x 4mm)
SPECIFIED TEMPERATURE RANGE:
−5°C to +75°C
High precision is ensured over a wide dynamic range of
input signals on either bipolar ( 5V) or single (+5V)
supply. Special temperature drift compensation circuitry
is included on-chip. In log-ratio applications, the signal
current may be from a high impedance source such as
a photodiode or resistor in series with a low impedance
voltage source. The reference current is provided by a
resistor in series with a precision internal voltage
reference, photo diode, or active current source.
The output signal at V
has a scale factor of 0.375V
LOGOUT
AD PPLICATIONS
out per decade of input current, which limits the output
so that it fits within a 5V or 10V range. The output can be
scaled and offset with one of the available additional
amplifiers, so it matches a wide variety of ADC input
ranges. Stable dc performance allows accurate
ONET ERBIUM-DOPED FIBER OPTIC
AMPLIFIER (EDFA)
D
D
D
D
LASER OPTICAL DENSITY MEASUREMENT
PHOTODIODE SIGNAL COMPRESSION AMP
LOG, LOG-RATIO FUNCTION
measurement of low-level signals over
a wide
temperature range. The LOG114 is specified over a
−5°C to +75°C temperature range and can operate from
−40°C to +85°C.
ANALOG SIGNAL COMPRESSION IN FRONT
OF ANALOG-TO-DIGITAL (ADC) CONVERTER
R
R
5
6
D
ABSORBANCE MEASUREMENT
−
V
IN
4
+IN
LOGOUT
4
(2)
10
11
9
LOG114
Q
1
Ω
Ω
Ω
200
1250
I
1
(1)
R
R
2
1
4
A
1
V
CM IN
(3)
5
A
A
V
4
O4
5
12
(4)
A
3
Q
I
and I are current inputs
2
2
1
+IN
from a photodiode
or other current source
13
15
Ω
(1)
200
1250
I
2
R
R
4
3
V
3
5
O5
A
2
R
V
REF
16
I
REF
REF
1
NOTES: (1) Thermally dependent R and R
1
3
REF
provide temperature compensation.
2.5V
×
= 0.375 log(I /I ).
1 2
(2) V
LOGOUT
8
6
7
14
V
Com
×
×
K log(I /I )
(3) V = 0.375
REF GND
O4
1 2
K = 1 + R /R .
(4) Differential Amplifier (A ) Gain = 6.25
−
−
IN
V+
V
6
5
5
3
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments
semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
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ꢀꢎ ꢍ ꢙꢔꢓ ꢑ ꢊꢍ ꢋ ꢖꢎ ꢍ ꢓ ꢕ ꢒ ꢒ ꢊꢋ ꢟ ꢙꢍ ꢕ ꢒ ꢋꢍꢑ ꢋꢕ ꢓꢕ ꢒꢒ ꢐꢎ ꢊꢘ ꢞ ꢊꢋꢓ ꢘꢔꢙ ꢕ ꢑꢕ ꢒꢑꢊ ꢋꢟ ꢍꢌ ꢐꢘ ꢘ ꢖꢐ ꢎ ꢐꢏ ꢕꢑꢕ ꢎ ꢒꢚ
Copyright 2004−2007, Texas Instruments Incorporated
www.ti.com
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SBOS301A − MAY 2004 − REVISED MARCH 2007
This integrated circuit can be damaged by ESD. Texas
Instruments recommends that all integrated circuits be
(1)
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V+ to V− . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12V
handledwith appropriate precautions. Failure to observe
(2)
proper handling and installation procedures can cause damage.
Signal Input Terminals, Voltage . . . . . (V−) −0.5V to (V+) + 0.5V
Current(2) . . . . . . . . . . . . . . . . . . . . 10mA
Output Short-Circuit(3) . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous
Operating Temperature . . . . . . . . . . . . . . . . . . . . . . −40°C to +85°C
Storage Temperature . . . . . . . . . . . . . . . . . . . . . . . −55°C to +125°C
Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +150°C
ESD Rating (Human Body Model) . . . . . . . . . . . . . . . . . . . . 2000V
ESD damage can range from subtle performance degradation to
complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes could
cause the device not to meet its published specifications.
PRECISION CURRENT MEASUREMENT
PRODUCTS
(1)
Stresses above these ratings may cause permanent damage.
Exposure to absolute maximum conditions for extended periods
may degrade device reliability. These are stress ratings only, and
functional operation of the device at these or any other conditions
beyond those specified is not implied.
FEATURES
PRODUCT
Logarithmic Transimpedance Amplifier, 5V, Eight Decades
Logarithmic Transimpedance, 36V, 7.5 Decades
LOG114
LOG112
(2)
(3)
Input terminals are diode-clamped to the power-supply rails.
Input signals that can swing more than 0.5V beyond the supply
rails should be current-limited to 10mA or less.
OPA380,
OPA381
Resistor-Feedback Transimpedance, 5V, 5.5 Decades
IVC102
Short-circuit to ground.
Switched Integrator Transimpedance, Six Decades
Direct Digital Converter, Six Decades
DDC112
(1)
ORDERING INFORMATION
PRODUCT
PACKAGE-LEAD
PACKAGE DESIGNATOR
PACKAGE MARKING
LOG114
QFN-16
RGV
LOG114
(1)
For the most current package and ordering information, see the Package Option Addendum at the end of this document, or see the TI web site
at www.ti.com.
PIN CONFIGURATION
QFN-16
Top View
16
15
14
13
1
2
3
4
12
11
10
9
VREF GND
VO4
Exposed
thermal
die pad on
underside
(Must be
−
NC
I2
IN4
+IN4
−
connected to V )
I1
VLOGOUT
5
6
7
8
QFN−16 (4mm x 4mm)
NC = No Connection
2
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SBOS301A − MAY 2004 − REVISED MARCH 2007
ELECTRICAL CHARACTERISTICS: VS = + 5V
Boldface limits apply over the specified temperature range, TA = −5°C to +75°C.
All specifications at T = +25°C, R
= 10kΩ, V = GND, unless otherwise noted.
A
VLOGOUT
CM
LOG114
PARAMETER
CONDITIONS
MIN
TYP
= (0.375V) Log (I /I )
MAX
UNITS
CORE LOG FUNCTION
I
/V
Equation
V
O
V
IN OUT
1 2
(1)
LOG CONFORMITY ERROR
Initial
1nA to 100µA (5 decades)
0.1
0.009
0.9
0.2
%
dB
%
0.017
100pA to 3.5mA (7.5 decades)
0.08
dB
1mA to 10mA
1nA to 100µA (5 decades)
100pA to 3.5mA (7.5 decades)
1mA to 10mA
See Typical Characteristics
Over Temperature
0.1
0.5
0.4
%
%
%
See Typical Characteristics
(2)
TRANSFER FUNCTION (GAIN)
Initial Scaling Factor
100pA to 10mA
0.375
0.4
V/decade
Scaling Factor Error
1nA to 100µA
2.5
0.21
3.5
3
%
dB
%
0.035
1.5
Over Temperature
T
to T
MIN MAX
+15°C to +50°C
0.7
%
INPUT, A and A
1
2
Offset Voltage
V
1
4
mV
µV/°C
µV/V
pA
OS
vs Temperature
dV/dT
PSRR
T
to T
+ 15
MIN
MAX
vs Power Supply
V
S
=
2.25V to 5.5V
75
400
Input Bias Current
I
B
5
vs Temperature
T
to T
Doubles every 10°C
MIN
MAX
Input Common-Mode Voltage Range
V
CM
(V−)+1.5 to
(V+)−1.5
V
Voltage Noise
e
n
f = 0.1Hz to 10kHz
f = 1kHz
3
30
4
µVrms
nV/√Hz
fA/√Hz
Current Noise
i
n
f = 1kHz
OUTPUT, A (V
3
)
LOGOUT
Output Offset, V
, Initial
V
11
50
65
mV
mV
V
OSO
OSO
Over Temperature
Full-Scale Output (FSO)
Gain Bandwidth Product
Short-Circuit Current
Capacitive Load
T
to T
15
MIN
MAX
(3)
(V−) + 0.6
(V+) − 0.6
GBW
I
IN
= 1µA
50
18
MHz
mA
pF
I
SC
100
OP AMP, A and A
4
5
Input Offset Voltage
vs Temperature
vs Supply
V
250
2
1000
250
µV
µV/°C
µV/V
dB
OS
dV/dT
T
to T
MIN
MAX
PSRR
CMRR
V
S
=
4.5V to 5.5V
30
vs Common-Mode Voltage
Input Bias Current
Input Offset Current
Input Voltage Range
Input Noise f = 0.1Hz to 10Hz
f = 1kHz
74
I
B
−1
µA
µA
I
0.05
OS
(V−)
(V+) − 2
V
2
13
2
µV
PP
nV/√Hz
pA/√Hz
dB
Current Noise
i
n
Open-Loop Voltage Gain
Gain Bandwidth Product
Slew Rate
A
100
15
5
OL
GBW
SR
MHz
V/µs
µs
Settling Time 0.01%
Rated Output
t
S
G = −1, 3V Step, C = 100pF
1.5
L
(V−) + 0.5
(V+) − 0.5
V
Short-Circuit Current
I
+4/−10
mA
SC
3
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SBOS301A − MAY 2004 − REVISED MARCH 2007
ELECTRICAL CHARACTERISTICS: VS = + 5V (continued)
Boldface limits apply over the specified temperature range, TA = −5°C to +75°C.
All specifications at T = +25°C, R
= 10kΩ, V
= GND, unless otherwise noted.
A
VLOGOUT
CM
LOG114
TYP
PARAMETER
CONDITIONS
MIN
MAX
UNITS
(4, 5)
TOTAL ERROR
See Typical Characteristics
(6)
FREQUENCY RESPONSE, Core Log
BW, 3dB I or I
1
=
I
= 10% of I
value, I
= 1µA
= 1µA
= 1µA
2
AC
DC
REF
1nA
5
kHz
kHz
10nA
100nA
1µA
12
120
2.3
> 5
> 5
> 5
kHz
MHz
MHz
MHz
MHz
10µA to 1mA (ratio 1:100)
1mA to 3.5mA (ratio 1:3.5)
3.5mA to 10mA (ratio 1:2.9)
Step Response
Increasing (I or I )
I
REF
1
2
8nA to 240nA (ratio 1:30)
10nA to 100nA (ratio 1:10)
10nA to 1µA (ratio 1:100)
10nA to 10µA (ratio 1:1k)
10nA to 1mA (ratio 1:100k)
1mA to 10mA (ratio 1:10)
Decreasing (I or I )
0.7
1.5
µs
µs
µs
µs
µs
µs
0.15
0.07
0.06
1
I
REF
1
2
8nA to 240nA (ratio 1:30)
10nA to 100nA (ratio 1:10)
10nA to 1µA (ratio 1:100)
10nA to 10µA (ratio 1:1k)
10nA to 1mA (ratio 1:100k)
1mA to 10mA (ratio 1:10)
1
2
µs
µs
µs
µs
µs
µs
0.25
0.05
0.03
1
VOLTAGE REFERENCE
Bandgap Voltage
Error, Initial
2.5
0.15
25
V
1
%
vs Temperature
vs Supply
ppm/°C
ppm/V
ppm/mA
mA
V
S
=
4.5V to 5.5V
2mA
30
vs Load
I
O
=
200
10
Short-Circuit Current
POWER SUPPLY
Dual Supply Operating Range
Quiescent Current
V
2.4
5.5
15
V
S
I
Q
I
O
= 0
10
mA
TEMPERATURE RANGE
Specification, T
Operating
to T
−5
+75
+85
°C
°C
°C
MIN
MAX
−40
−55
Storage
+125
Thermal Resistance, q
62
°C/W
JA
(1)
Log conformity error is peak deviation from the best-fit straight line of V vs Log (I /I ) curve expressed as a percent of peak-to-peak full-scale output. Scale factor,
O
1 2
K, equals 0.375V output per decade of input current.
Scale factor of core log function is trimmed to 0.375V output per decade change of input current.
Specified by design.
(2)
(3)
(4)
(5)
(6)
Worst-case total error for any ratio of I /I , as the largest of the two errors, when I, and I are considered separately.
1
2
2
Total error includes offset voltage, bias current, gain, and log conformity.
Small signal bandwidth (3dB) and transient response are a function of the level of input current. Smaller input current amplitude results in lower bandwidth.
4
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SBOS301A − MAY 2004 − REVISED MARCH 2007
ELECTRICAL CHARACTERISTICS: VS = +5V
Boldface limits apply over the specified temperature range, TA = −5°C to +75°C.
All specifications at T = +25°C, R
= 10kΩ, V = +2.5V, unless otherwise noted.
A
VLOGOUT
CM
LOG114
PARAMETER
CONDITIONS
MIN
TYP
= (0.375V) Log (I /I ) + V
CM
MAX
UNITS
CORE LOG FUNCTION
I
/V
Equation
V
V
IN OUT
O
1
2
(1)
LOG CONFORMITY ERROR
Initial
1nA to 100µA (5 decades)
0.1
0.009
0.9
0.25
%
dB
%
0.022
100pA to 3.5mA (7.5 decades)
0.08
dB
1mA to 10mA
1nA to 100µA (5 decades)
100pA to 3.5mA (7.5 decades)
1mA to 10mA
See Typical Characteristics
Over Temperature
0.1
0.5
0.4
%
%
See Typical Characteristics
(2)
TRANSFER FUNCTION (GAIN)
Initial Scaling Factor
10nA to 100µA
1nA to 100µA
0.375
0.4
V/decade
Scaling Factor Error
2.5
0.21
3.5
3
%
dB
%
0.0.35
0.035
0.7
Over Temperature
T
to T
MIN MAX
+15°C to +50°C
%
INPUT, A and A
1
2
Offset Voltage
V
1
7
mV
µV/°C
µV/V
pA
OS
vs Temperature
dV/dT
PSRR
T
to T
+ 30
MIN
MAX
vs Power Supply
V
S
= +4.5V to +5.5V
300
Input Bias Current
I
B
5
vs Temperature
T
to T
Doubles every 10°C
MIN
MAX
Input Common-Mode Voltage Range
V
CM
(V−)+1.5 to
(V+)−1.5
V
Voltage Noise
e
n
f = 0.1Hz to 10kHz
f = 1kHz
3
30
4
µVrms
nV/√Hz
fA/√Hz
Current Noise
i
n
f = 1kHz
OUTPUT, A (V
3
)
LOGOUT
Output Offset, V
, Initial
V
14
65
80
mV
mV
V
OSO
OSO
Over Temperature
T
to T
18
MIN
MAX
(3)
Full Scale Output (FSO)
V
I
= +5V
(V−) + 0.6
(V+) − 0.6
S
Gain Bandwidth Product
Short-Circuit Current
Capacitive Load
GBW
= 1µA
50
18
MHz
mA
pF
IN
I
SC
100
OP AMP, A and A
4
5
Input Offset Voltage
vs Temperature
V
250
2
4000
µV
µV/°C
µV/V
dB
OS
dV/dT
T
to T
MIN
MAX
vs Supply
PSRR
CMRR
V
S
= +4.8V to +5.5V
30
vs Common-Mode Voltage
Input Bias Current
Input Offset Current
Input Voltage Range
Input Noise f = 0.1Hz to 10Hz
f = 1kHz
70
I
B
−1
µA
µA
I
0.05
OS
(V−)
(V+) − 1.5
V
1
28
2
µV
PP
nV/√Hz
pA/√Hz
dB
Current Noise
i
n
Open-Loop Voltage Gain
Gain Bandwidth Product
Slew Rate
A
100
15
5
OL
GBW
SR
MHz
V/µs
µs
Settling Time 0.01%
Rated Output
t
S
G = −1, 3V Step, C = 100pF
1.5
L
(V−) + 0.5
(V+) − 0.5
V
Short-Circuit Current
I
+4/−10
mA
SC
5
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SBOS301A − MAY 2004 − REVISED MARCH 2007
ELECTRICAL CHARACTERISTICS: VS = +5V (continued)
Boldface limits apply over the specified temperature range, TA = −5°C to +75°C.
All specifications at T = +25°C, R
= 10kΩ, V
= +2.5V, unless otherwise noted.
A
VLOGOUT
CM
LOG114
TYP
PARAMETER
CONDITIONS
MIN
MAX
UNITS
(4, 5)
TOTAL ERROR
See Typical Characteristics
(6)
FREQUENCY RESPONSE, Core Log
BW, 3dB I or I
1
=
I
= 10% of I
value, I
= 1µA
= 1µA
= 1µA
2
AC
DC
REF
1nA
5
kHz
kHz
10nA
100nA
1µA
12
120
2.3
> 5
> 5
> 5
kHz
MHz
MHz
MHz
MHz
10µA to 1mA (ratio 1:100)
1mA to 3.5mA (ratio 1:3.5)
3.5mA to 10mA (ratio 1:2.9)
Step Response
Increasing (I or I )
I
REF
1
2
8nA to 240nA (ratio 1:30)
10nA to 100nA (ratio 1:10)
10nA to 1µA (ratio 1:100)
10nA to 10µA (ratio 1:1k)
10nA to 1mA (ratio 1:100k)
1mA to 10mA (ratio 1:10)
Decreasing (I or I )
0.7
1.5
µs
µs
µs
µs
µs
µs
0.15
0.07
0.06
1
I
REF
1
2
8nA to 240nA (ratio 1:30)
10nA to 100nA (ratio 1:10)
10nA to 1µA (ratio 1:100)
10nA to 10µA (ratio 1:1k)
10nA to 1mA (ratio 1:100k)
1mA to 10mA (ratio 1:10)
1
2
µs
µs
µs
µs
µs
µs
0.25
0.05
0.03
1
VOLTAGE REFERENCE
Bandgap Voltage
Error, Initial
2.5
0.15
25
V
1
%
vs Temperature
vs Supply
ppm/°C
ppm/V
ppm/mA
mA
V
S
= +4.8V to +11V
30
vs Load
I
O
=
2mA
200
10
Short-Circuit Current
POWER SUPPLY
Single Supply Operating Range
Quiescent Current
V
4.8
11
15
V
S
I
Q
I
O
= 0
10
mA
TEMPERATURE RANGE
Specification, T
Operating
to T
−5
+75
+85
°C
°C
°C
MIN
MAX
−40
−55
Storage
+125
Thermal Resistance, q
62
°C/W
JA
(1)
Log conformity error is peak deviation from the best-fit straight line of V vs Log (I /I ) curve expressed as a percent of peak-to-peak full-scale output. Scale factor,
O
1 2
K, equals 0.375V output per decade of input current.
Scale factor of core log function is trimmed to 0.375V output per decade change of input current.
Specified by design.
(2)
(3)
(4)
(5)
(6)
Worst-case total error for any ratio of I /I , as the largest of the two errors, when I, and I are considered separately.
1
2
2
Total error includes offset voltage, bias current, gain, and log conformity.
Small signal bandwidth (3dB) and transient response are a function of the level of input current. Smaller input current amplitude results in lower bandwidth.
6
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SBOS301A − MAY 2004 − REVISED MARCH 2007
TYPICAL CHARACTERISTICS: VS = + 5V
All specifications at T = +25°C, R
= 10kΩ, V
= GND, unless otherwise noted.
A
VLOGOUT
CM
ONE CYCLE OF NORMALIZED TRANSFER FUNCTION
NORMALIZED TRANSFER FUNCTION
2.0
1.5
1.0
0.5
0
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
−
−
−
−
0.5
1.0
1.5
2.0
10−4
10−3 10−2 10−1
1
101
102
103
104
1
10
Current Ratio (I1/I2)
Current Ratio (I1/ I2)
SCALING FACTOR ERROR (I2 = reference 100pA to 10mA)
µ
VLOGOUT vs I1 INPUT (I2 = 1 A)
40
2.5
2.0
1.5
1.0
0.5
0
30
20
10
0
_
_
+70 C
+25 C
_
0 C
−
−
−
−
−
0.5
1.0
1.5
2.0
2.5
−
_
10 C
−
10
_
+80 C
_
+90 C
−
20
µ
µ
µ
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
µ
µ
µ
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
Input Current (I1)
Input Current (I1)
µ
VLOGOUT vs I2 INPUT (I1 = 1 A)
VLOGOUT vs IREF
4
2.0
100pA
1.5
1.0
0.5
0
3
2
1
0
1
2
3
4
1nA
10nA
100nA
µ
1 A
−
−
−
−
−
0.5
1.0
1.5
2.0
2.5
−
µ
10 A
−
−
−
µ
100 A
1mA
10mA
µ
µ
µ
µ
µ
µ
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
10mA
Input Current (I1)
IREF (I2)
7
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TYPICAL CHARACTERISTICS: VS = + 5V (continued)
All specifications at T = +25°C, R
= 10kΩ, V = GND, unless otherwise noted.
A
VLOGOUT
CM
_
AVERAGE TOTAL ERROR AT +80 C
_
AVERAGE TOTAL ERROR AT +25 C
100
80
60
40
20
0
100
80
60
40
20
0
I1 = 1mA
I1 = 1mA
µ
I1 = 100 A
µ
I1 = 10 A
µ
I1 = 10 A
µ
I1 = 100 A
−
−
−
−
20
40
60
80
−
−
−
−
20
40
60
80
µ
I1 = 1 A
I1 = 1nA, 10nA,
100nA
I1 = 10nA
I1 = 1nA
µ
I1 = 1 A
I1 = 100nA
−
100
−
100
100 A
µ
µ
µ
µ
µ
100 A
200 A
400 A
600 A
800 A
1mA
µ
µ
µ
µ
µ
200 A
400 A
600 A
800 A
1mA
I2
I2
−
_
AVERAGE TOTAL ERROR AT 10 C
LOG CONFORMITY vs TEMPERATURE
7.5 Decade
100
1.4
80
60
40
20
0
1.2
1.0
0.8
0.6
0.4
0.2
0
I1 = 1mA
I1 = 1nA
7 Decade
5 Decade
−
−
−
−
20
40
60
80
I1 = 10nA
µ
I1 = 100 A
6 Decade
4 Decade
µ
I1 = 10 A
I1 = 100nA
µ
I1 = 1 A
−
100
100 A
µ
µ
µ
µ
µ
200 A
400 A
600 A
800 A
1mA
−
10
0
10
20
30
40
50
60
70
80
90
I2
_
Temperature ( C)
4 DECADE LOG CONFORMITY vs IREF
5 DECADE LOG CONFORMITY vs IREF
0.09
0.08
0.07
0.06
0.05
0.04
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
_
+90 C
_
+90 C
_
0 C
−
_
10 C
_
_
+80 C
+80 C
_
+70 C
_
+25 C
_
+70 C
−
_
_
_
10 C, 0 C, +25 C
µ
µ
µ
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
µ
µ
µ
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
IREF (I1)
IREF (I1)
8
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TYPICAL CHARACTERISTICS: VS = + 5V (continued)
All specifications at T = +25°C, R
= 10kΩ, V
= GND, unless otherwise noted. For ac measurements, small signal means up to approximately 10% of dc
A
VLOGOUT
CM
level.
6 DECADE LOG CONFORMITY vs IREF
8 DECADE LOG CONFORMITY (100pA to 3.5mA)
0.45
0.40
0.35
0.30
0.25
0.20
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
_
+90 C
_
+90 C
_
+80 C
_
+70 C
_
+25 C
_
+80 C
_
0 C
−
_
10 C
_
+70 C
−
_
_
_
10 C, 0 C, +25 C
µ
µ
µ
µ
µ
µ
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
100pA 1nA 10nA 100nA 1 A 10 A 100 A 1mA 10mA
IREF (I1)
Input Current (I1 or I2)
SMALL−SIGNALAC RESPONSE I1
SMALL−SIGNAL VLOGOUT
(10% sine modulation)
0
20
10mA
−
5
10
15
20
25
30
35
40
45
50
10
0
µ
10 A
−
−
−
−
−
−
−
−
−
µ
1 A
µ
100 A
1mA
µ
1 A
100nA
1nA
1mA
−
−
−
−
10
20
30
40
10nA
100nA
10nA
µ
100 A
µ
10 A
10
100
1k
10k
100k
1M
10M
100M
100
1k
10k
100k
1M
10M
100M
Frequency (Hz)
Frequency (Hz)
SMALL−SIGNALAC RESPONSE I2
(10% sine modulation)
A3 GAIN AND PHASE vs FREQUENCY
160
140
120
100
80
225
180
135
90
0
−
5
µ
10 A
−
−
−
−
−
−
−
−
−
10
15
20
25
30
35
40
45
50
µ
100 A
µ
1 A
1nA
10nA
1mA
60
Phase
Gain
40
100nA
20
0
45
−
−
20
40
0
100
1k
10k
100k
1M
10M 40M
100
1k
10k
100k
1M
10M
100M
Frequency (Hz)
Frequency (Hz)
9
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SBOS301A − MAY 2004 − REVISED MARCH 2007
TYPICAL CHARACTERISTICS: VS = + 5V (continued)
All specifications at T = +25°C, R
= 10kΩ, V
= GND, unless otherwise noted.
A
VLOGOUT
CM
A4 and A5 GAIN AND PHASE vs FREQUENCY
A
and A5 NONINVERTING CLOSED−LOOP RESPONSE
4
140
120
100
80
180
135
90
45
0
3
0
3
6
9
G = 1
−
−
−
Phase
Gain
G = 10
60
40
20
−
12
15
0
−
−
20
1
10
100
1k
10k
100k
1M
10M 18M
1k
10k
100k
1M
10M
100M
Frequency (Hz)
Frequency (Hz)
A4 and A5 CAPACITIVE LOAD RESPONSE
A4 and A5 INVERTING CLOSED−LOOP RESPONSE
10
0
30
20
10
0
G = +1
−
−
−
−
−
10
20
30
40
50
−
10
20
30
40
50
60
70
80
−
G = 10
C = 100pF
C < 10pF
−
−
G =
1
−
−
−
−
−
−
1k
10k
100k
1M
10M
50M
1k
10k
100k
1M
10M
60M
Frequency (Hz)
Frequency (Hz)
10
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SBOS301A − MAY 2004 − REVISED MARCH 2007
Either I or I can be held constant to serve as the refer-
ence current, with the other input being used for the in-
put signal. The value of the reference current is selected
1
2
APPLICATIONS INFORMATION
OVERVIEW
The LOG114 is a precision logarithmic amplifier that is
capable of measuring currents over a dynamic range of
eight decades. It computes the logarithm, or log ratio,
of an input current relative to a reference current ac-
cording to equation (1).
such that the output at V
(pin 9) is zero when the
LOGOUT
reference current and input current are equal. An on-
chip 2.5V reference is provided for use in generating the
reference current.
Two additional amplifiers, A and A , are included in the
4
5
LOG114 for use in scaling, offsetting, filtering, threshold
detection, or other functions.
I1
I2
ǒ Ǔ
VLOGOUT + 0.375 log10
(1)
BASIC CONNECTIONS
The output at V
can be digitized directly, or scaled
LOGOUT
Figure 1 and Figure 2 show the LOG114 in typical dual
and single-supply configurations, respectively. To re-
duce the influence of lead inductance of power-supply
lines, it is recommended that each supply be bypassed
with a 10µF tantalum capacitor in parallel with a 1000pF
ceramic capacitor as shown in Figure 1 and Figure 2.
Connecting these capacitors as close to the LOG114
V+ supply pin to ground as possible improves supply−
related noise rejection.
for an ADC input using an uncommitted or external op
amp.
An offsetting voltage (V
) can be connected to the
Com
Com pin to raise the voltage at V
. When an
LOGOUT
offsetting voltage is used, the transfer function
becomes:
I1
I2
ǒ Ǔ
VLOGOUT + 0.375 log10
) VCom
(2)
R8
56.2k
R7
100k
Ω
Ω
R5
100k
R6
66.5k
Ω
Ω
10
11
9
(1)
VLOGOUT
+IN4 IN
−
LOG114
4
Q1
IREF
1 F
µ
I1
4
5
R1
R2
A1
(2)
VCM IN
VO4
12
A4
+IN5
VO5
Q2
13
15
A3
Input Signal
100pAto 10mA
RREF
2.5M
Ω
I2
3
R3
R4
A5
A2
VREF
16
2.5VREF
VREF GND
IN
−
V+
V−
Com
7
5
8
6
1
14
1000pF
1000pF
10 F
10 F
µ
µ
NOTE: (1) VLOGOUT = 0.375 log(I /I )
×
1
2
+
+
(2) VO4
= 0.249 log(I /I ) + 1.5V
− ×
1 2
+5V
5V
−
Figure 1. Dual Supply Configuration Example for Best Accuracy Over Eight Decades.
11
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SBOS301A − MAY 2004 − REVISED MARCH 2007
R5
R6
Ω
Ω
66.5k
100k
R7
Ω
100k
R8
Ω
316k
10
11
9
(2)
VLOGOUT
−
+IN4
IN4
LOG114
Q1
I1
µ
I A
REF3040
or
I1
4
5
R1
R2
REF3240
4.096V
Reference
A1
(1)
(3)
RREF
VCM IN
VO4
12
A4
Ω
1.62M
+IN5
VO5
Q2
13
15
A3
Input current
from photodiode
or current source
I2
I2
3
R3
R4
A5
A2
Photodiode(4)
VREF
16
+2.5V
2.5VREF
−
IN5
1
VREF GND
V
−
V
6
Com
7
8
1
+
µ
10 F
1000pF
VCom = +2.5V
+5V
≥
NOTE: (1) In single−supply configuration, VCM IN must be connected to 1V.
×
(2) VLOGOUT = 0.375 log(I1/I2) + 2.5V.
− ×
= 0.249 log(I1/I2) + 1.5V.
(3) VO4
(4) The cathode of the photodiode is returned to VREF resulting in zero bias across it. The cathode
could be returned to a voltage more positive than VCM IN to create a reverse bias for reducing
photodiode capacitance, which increases speed.
Figure 2. Single-Supply Configuration Example for Measurement Over Eight Decades.
12
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SBOS301A − MAY 2004 − REVISED MARCH 2007
DESIGN EXAMPLE FOR DUAL-SUPPLY
CONFIGURATION
4. The A amplifier scales and offsets the V
4 LOGOUT
signal for use by the ADC using the equation:
Given these conditions:
ǒ
Ǔ
VO4 + *SFACTOR VLOGOUT ) VOFFSET
(5)
D
D
V+ = 5V and V− = −5V
100pA ≤ Input signal
The A amplifier is specified with a rated output swing
capability from (V−) +0.5V to (V+) − 0.5V.
4
D
The stage following the LOG114 is an analog-to-
digital converter (ADC) with +5V supply and
+2.5V reference voltage, so V swings from
Therefore, choose the final A output:
4
O4
0V ≤ V ≤ +2.5V
O4
+0.5V to +2.5V.
This output results in a 2.5V range for the 3V V
range, or 2.5V/3V scaling factor.
LOGOUT
1. Due to LOG114 symmetry, you can choose either
I or I as the signal input pin. Choosing I as the
1
2
1
5. When I = 10mA, V
= −1.5V. Using the
2
LOGOUT
reference makes the resistor network around A4
simpler. (Note: Current must flow into pins 3 (I ) and
equation in step 5:
1
pin 4 (I ).)
2
ǒ
Ǔ
VO4 + *SFACTOR VLOGOUT ) VOFFSET
2. Select the magnitude of the reference current.
0V + *2.5Vń3V(*1.5V) ) VOFFSET
(6)
Since the signal (I ) spans eight decades, set I to
2
1
Therefore, V
= 0V
OFFSET
1µA − four decades above the minimum I value.
2
(Note that it does not have to be placed in the
middle. If I spanned seven decades, I could be set
The A amplifier configuration for V = −2.5/3(V
+ 0V is seen in Figure 3.
)
4
O4
LOGOUT
2
1
three decades above the minimum and four
The overall transer function is:
decades below the maximum I value.) This
2
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(∆V per ∆I ) when the current signal decreases.
I1
I2
+ *0.249 logǒ Ǔ) 1.5V
VO4
(7)
O4
2
3. Using Equation (1) calculate the expected range of
log outputs at V
Internal A Output Amplifier
4
:
LOGOUT
R5
R6
For I2 + 10mA :
Ω
Ω
100k
82.5k
VLOGOUT
I1
+ 0.375 logǒ Ǔ
VLOGOUT
+5V
I2
1mA
−
VO4
=
2/3 (VLOGOUT
)
+ 0.375 logǒ Ǔ + * 1.5V
A4
10mA
For I2 + 100pA :
10mA
I2
I1
−
5V
R8
100pA
VREF
+2.5V
+ 0.375 logǒ Ǔ
VLOGOUT
I2
R7
100k
0V
+2.5V
1mA
+ 0.375 logǒ Ǔ+ ) 1.5V
100pA
Ω
Ω
37.4k
VO4
(3)
Therefore, the expected voltage range at the output
A4 amplifier used to scale and offset VLOGOUT for 0V to 2.5V output.
of amplifier A is:
3
Figure 3. Operational Amplifier Configuration for
Scaling the Output Going to ADC Stage.
* 1.5V v VLOGOUT v ) 1.5V
(4)
13
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SBOS301A − MAY 2004 − REVISED MARCH 2007
DESIGN EXAMPLE FOR SINGLE-SUPPLY
CONFIGURATION
This result would be fine in a dual−supply system
(V+ = +5V, V− = −5V) where the output can swing
below ground, but does not work in a single supply
+5V system. Therefore, an offset voltage must be
added to the system.
Given these conditions:
D
D
D
V+ = 5V
V− = GND
100pA ≤ Input signal ≤ 10mA
4. Select an offset voltage, V
to use for centering
Com
the output between (V−) + 0.6V and (V+) − 0.6V,
which is the full-scale output capability of the A
3
D
The stage following the LOG114 is an analog to
digital converter (ADC) with +5V supply and
+2.5V reference voltage
amplifier. Choosing V
= 2.5V, and recalculating
Com
the expected voltage output range for V
Equation (2), results in:
using
LOGOUT
1. Choose either I or I as the signal input pin. For this
1
2
example, I is used. Choosing I as the reference
current makes the resistor network around A4
2
1
) 1V v VLOGOUT v ) 4V
(10)
simpler. (Note: Current only flows into the I and I
pins.)
1
2
5. The A amplifier scales and offsets the V
4
LOGOUT
signal for use by the ADC using the equation:
2. Select the magnitude of the reference current.
Since the signal (I ) spans eight decades, set I to
ǒ
Ǔ
VO4 + *SFACTOR VLOGOUT ) VOFFSET
2
1
(11)
1µA − four decades above the minimum I value,
2
The A amplifier is specified with a rated output swing
capability from (V−) +0.5V to (V+) − 0.5V.
4
and four decades below the maximum I value.
2
(Note that it does not have to be placed in the
middle. If I spanned seven decades, I could be set
Therefore, choose the final A4 output:
2
1
three decades above the minimum and four
+0.5V ≤ V ≤ +2.5V
O4
decades below the maximum I value.) This
2
This output results in a 2V range for the 3V V
range, or 2V/3V scaling factor.
LOGOUT
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(∆V per ∆I ) when the current signal decreases.
6. When I = 10mA, V
= +1V, and V = 2.5V.
O4
O4
2
2
LOGOUT
Using the equation in step 5:
ǒ
Ǔ
VO4 + *SFACTOR VLOGOUT ) VOFFSET
3. Using Equation (1) calculate the expected range of
log outputs at V
:
2.5V + *2Vń3V(1V) ) VOFFSET
LOGOUT
(12)
) +
Therefore, V
= 3.16V
OFFSET
For I2 + 10mA :
The A amplifier configuration for V = −2/3(V
4
O4
LOGOUT
I1
+ 0.375 logǒ Ǔ
VLOGOUT
3.16 is seen in Figure 4a.
I2
The overall transer function is:
1mA
+ 0.375 logǒ Ǔ + * 1.5V
10mA
For I2 + 100pA :
I1
I2
+ *0.249 logǒ Ǔ) 1.5V
VO4
I1
(13)
+ 0.375 logǒ Ǔ
VLOGOUT
I2
A similar process can be used for configuring an
external rail-to-rail output op amp, such as the OPA335.
Because the OPA335 op amp can swing down to 0V
using a pulldown resistor, R , connected to −5V (for
details, refer to the OPA335 data sheet, available for
download at www.ti.com), the scaling factor is 2.5V/3V
1mA
+ 0.375 logǒ Ǔ+ ) 1.5V
100pA
(8)
P
Therefore, the expected voltage range at the output
of amplifier A is:
3
and the corresponding V
configuration is shown in Figure 4b.
is 3.3V. This circuit
OFFSET
* 1.5V v VLOGOUT v ) 1.5V
(9)
14
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Internal A Output Amplifier
4
External Output Amplifier
R5
100k
R6
66.5k
R5
100k
R6
82.5k
Ω
Ω
Ω
Ω
VLOGOUT
VLOGOUT
+5V
VO4
=
2/3 (VLOGOUT) + 3.16
10mA
VOUT
=
2.5/3 (VLOGOUT) + 3.3
−
−
A4
OPA335
(1)
10mA
100pA
RP
I2
I2
100pA
VREF
VREF
+2.5V
5V
−
+2.5V
R7
100k
R8
316k
R7
100k
R8
267k
Ω
2.5V
0.5V
2.5V
VO4
VOUT
Ω
Ω
Ω
0.5V
a) A4 amplifier used to scale and offset VLOGOUT for 0.5V to 2.5V output.
b) OPA335 amplifier used to scale and offset VLOGOUT for 0V to 2.5V output.
NOTE: (1) See OPA335 data sheet for use of R connected to 5V to achieve 0V output.
−
P
Figure 4. Operational Amplifier Configuration for Scaling and Offsetting the Output Going to ADC Stage.
ADVANTAGES OF DUAL−SUPPLY OPERATION
V
(Pin 5)
CM IN
The V
pin is used to bias the A and A amplifier into
1 2
The LOG114 performs very well on a single +5V supply
by level-shifting pin 7 (Com) to half-supply and raising
CMIN
its common-mode input voltage range, (V−) + 1.5V to
(V+) − 1.5V.
the common-mode voltage (pin 5, V
) of the input
CM IN
amplifiers. This level−shift places the input amplifiers in
the linear operating range. However, there are also
some advantages to operating the LOG114 on dual 5V
supplies. These advantages include:
INPUT CURRENT RANGE
To maintain specified accuracy, the input current range
of the LOG114 should be limited from 100pA to 3.5mA.
Input currents outside of this range may compromise
the LOG114 performance. Input currents larger than
3.5mA result in increased nonlinearity. An absolute
maximum input current rating of 10mA is included to
prevent excessive power dissipation that may damage
the input transistor.
1) eliminating the need for the +4.096V precision
reference;
2) eliminating a small additional source of error arising
from the noise and temperature drift of the level−shifting
voltage; and
3) allowing increased magnitude of a reverse bias
voltage on the photodiode.
COM (PIN 7) VOLTAGE RANGE
The voltage on the Com pin is used to bias the differen-
tial amplifier, A , within its linear range. This voltage can
3
provide an asymmetrical offset of the V
voltage.
LOGOUT
15
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SBOS301A − MAY 2004 − REVISED MARCH 2007
SETTING THE REFERENCE CURRENT
ply system, and a maximum value of 7mV in a +5V sup-
ply system. Resistor temperature stability and noise
contributions should also be considered.
When the LOG114 is used to compute logarithms, ei-
ther I or I can be held constant to become the refer-
1
2
ence current to which the other is compared.
If I is set to the lowest current in the span of the signal
REF
current (as shown in the front page figure), V
range from:
will
LOGOUT
VREF = 100mV
R1 R3
I1 min
I1 max signal
VOS
ǒ
Ǔ^ 0V
VLOGOUT + 0.375 log10
−
+
1
(14)
+5V
A1
IREF
to some maximum value:
R2
R3 >> R2
I1 min
I1 max signal
ǒ
Ǔ
VLOGOUT + 0.375 log10
(15)
While convenient, this approach does not usually result
in best performance, because I min accuracy is difficult
1
Figure 5. T-Network for Reference Current.
may be an external precision voltage reference, or
to achieve, particularly if it is < 20nA.
A better way to achieve higher accuracy is to choose
I
to be in the center of the full signal range. For
V
REF
REF
example, for a signal range of 1nA to 1mA, it is better
to use this approach:
the on-chip 2.5V voltage reference of the LOG114.
I
can be derived from an external current source,
REF
such as that shown in Figure 6.
Ǹ
IREF + ISIGNAL min 1mAń1nA + 1mA dc
(16)
than it is to set I
= 1nA. It is much easier and more
REF
precise (that is, dc accuracy, temperature stability, and
lower noise) to establish a 1mA dc current level than a
1nA level for the reference current.
IREF
2N2905
The reference current may be derived from a voltage
source with one or more resistors. When a single resis-
RREF
Ω
3.6k
2N2905
+15V
tor is used, the value may be large depending on I
.
REF
−
15V
If I
is 10nA and +2.5V is used:
REF
6V
IN834
6V
RREF
IREF
=
R
REF
= 2.5V/10nA = 250MΩ
A voltage divider may be used to reduce the value of the
resistor, as shown in Figure 5. When using this method,
one must consider the possible errors caused by the
amplifier input offset voltage. The input offset voltage of
Figure 6. Temperature-Compensated Current Source.
amplifier A has a maximum value of 4mV in a 5V sup-
1
16
ꢠꢂ ꢡ ꢢꢢꢣ
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
NEGATIVE INPUT CURRENTS
situations where negative input currents are needed,
the example circuits in Figure 7, Figure 8, and Figure 9
may be used.
The LOG114 functions only with positive input currents
(conventional current flows into input current pins). In
QA
QB
IIN
National
LM394
D1
D2
OPA703
IOUT
Figure 7. Current Inverter/Current Source.
+5V
+3.3V
1/2
OPA2335
Ω
1.5k
Ω
1k
+5V
1/2
OPA2335
BSH203
(+3.3V
Back Bias)
10nA to 1mA
LOG114
10nA to 1mA
Pin 3 or Pin 4
Photodiode
Figure 8. Precision Current Inverter/Current Source.
Ω
1k
Ω
100k
Ω
100k
+5V
10nA to 1mA
+3.3V
1/2
OPA2335
Back Bias
+5V
Ω
Ω
1.5k
1.5k
+3.3V
1/2
OPA2335
Photodiode
Ω
Ω
100k
100k
LOG114
10nA to 1mA
Pin 3 or Pin 4
Figure 9. Precision Current Inverter/Current Source.
17
ꢠ ꢂꢡ ꢢꢢ ꢣ
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
VOLTAGE INPUTS
noise from these sources must be considered and can
limit the usefulness of this technique.
The LOG114 provides the best performance with cur-
rent inputs. Voltage inputs may be handled directly by
using a low-impedance voltage source with series resis-
tors, but the dynamic input range is limited to approxi-
mately three decades of input voltage. This limitation
exists because of the magnitude of the required input
voltage and size of the corresponding series resistor.
For 10nA of input current, a 10V voltage source and a
1GΩ resistor would be required. Voltage and current
APPLICATION CIRCUITS
LOG RATIO
One of the more common uses of log ratio amplifiers is
to measure absorbance. See Figure 10 for a typical ap-
plication. Absorbance of the sample is A = log λ ′/λ . If
1
1
D and D are matched, A ∝ (0.375V) log(I /I ).
1
2
1 2
R
R
6
5
10
+IN
10
IN
9
(1)
V
−
LOG114
LOGOUT
4
4
Q
1
I
4
5
1
R
R
2
1
A
(2)
1
V
V
12
CM IN
O4
I
1
2
A
A
4
D
D
Sample
1
+IN
V
Q
5
13
15
2
A
3
λ
′
λ
1
1
I
I
3
2
O5
R
R
λ
3
4
1
5
Light
A
2
Source
2
V
16
REF
2.5V
REF
−
−
V
IN
5
V+
V
Com
7
REF GND
8
6
1
14
+5V
×
= 0.375 log(I /I ).
1 2
NOTES: (1) V
LOGOUT
×
×
log(I /I )
(2) V = 0.375
K
O4
1 2
K = 1 + R /R .
6
5
Figure 10. Using the LOG114 to Measure Absorbance.
18
ꢠꢂ ꢡ ꢢꢢꢣ
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
DATA COMPRESSION
I
1
In many applications, the compressive effects of the
logarithmic transfer function are useful. For example, a
LOG114 preceding a 12-bit ADC can produce the
dynamic range equivalent to a 20-bit converter. (Sug-
gested products: ADS7818, ADS7834).
V
LOGOUT
LOG114
I
2
−
V
V+
+3.3V OPERATION
TPS60241
+5V
C
For systems with only a +3.3V power supply, the
TPS60241 zero-ripple switched cap buck-boost 2.7V to
5.5V input to 5V output converter may be used to gener-
ate a +5V supply for the LOG114, as shown in
Figure 11.
V
V
OUT
IN
+3.3V
C
C
C
1+
2+
C
µ
1 F
1
2
C
µ
1 F
C
1µF
0
1
µ
1 F
C
1
−
2
−
GND EN
Likewise, the TPS6040 negative charge pump may be
connected to the +5V output of the TPS60241 to gener-
ate a −5V supply to create a ±5V supply for the
LOG114, as Figure 12 illustrates.
Figure 11. Creating a +5V Supply from a +3.3V Supply.
I1
VLOGOUT
LOG114
I2
−
V
V+
+5V
−
5V
CFLY
µ
1 F
TPS60241
CFLY− CFLY+
TPS60400 OUT
GND
+5V
−
5V
+3.3V
VIN
VOUT
C2+
IN
C1+
C1
C1
C2
CO
CO
CI
µ
1 F
µ
1 F
µ
1 F
µ
1 F
µ
1 F
µ
1 F
C1
C2
−
−
GND EN
Figure 12. Creating a 5V Supply from a +3.3V Supply.
19
ꢠ ꢂꢡ ꢢꢢ ꢣ
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
ERBIUM-DOPED FIBER OPTIC AMPLIFIER
(EDFA)
An alternate design of the system shown in Figure 13
is possible because the LOG114 inherently takes the
log ratio. Therefore, one log amp can be eliminated by
The LOG114 was designed for optical networking sys-
tems. Figure 13 shows a block diagram of the LOG114
in a typical EDFA application. This application uses two
log amps to measure the optical input and output power
of the amplifier. A difference amplifier subtracts the log
output signals of both log amps and applies an error
voltage to the proportional-integral-derivative (PID)
controller. The controller output adjusts a voltage-con-
connecting one of the photodiodes to the LOG114 I
1
input, and the other to the I input. The differential
2
amplifier would then be eliminated.
The LOG114 is uniquely suited for most EDFA
applications because of its fast rise and fall times
(typically less than 1µs for a 100:1 current input step).
It also measures a very wide dynamic range of up to
eight decades.
trolled current source (V ), which then drives the pow-
CCS
er op amp and pump laser. The desired optical gain is
achieved when the error voltage at the PID is zero.
The log ratio function is the optical power gain of the
EDFA. This circuitry forms an automatic power level
control loop.
Tap
1%
Tap
1%
Fiber
Pump Laser
Power
Op Amp
OPA569
IL
VCCS
PID
VERROR
Diff
VOUT1 VOUT2
I1
I2
LOG114
LOG114
REF
IREF1
IREF2
DAC
RREF1
RREF2
Figure 13. Erbium-Doped Fiber Optic Amplifier (EDFA) block diagram.
20
ꢠꢂ ꢡ ꢢꢢꢣ
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
INSIDE THE LOG114
Log Conformity
For the LOG114, log conformity is calculated in the
The LOG114 uses two matched logarithmic amplifiers
same way as linearity and is plotted as I /I on a semi-
(A and A with logging diodes in the feedback loops) to
1 2
1
2
log scale. In many applications, log conformity is the
most important specification. This condition is true be-
cause bias current errors are negligible (5pA for the
LOG114), and the scale factor and offset errors may be
trimmed to zero or removed by system calibration.
These factors leave log conformity as the major source
of error.
generate the outputs log (I ) and log (I ), respectively.
1
2
The gain of 6.25 differential amplifier (A ) subtracts the
3
output of A from the output of A , resulting in [log (I )
2
1
1
− log (I )], or log (I /I ). The symmetrical design of the
2
1 2
A and A logarithmic amps allows I and I to be used
1
2
1
2
interchangeably, and provides good bandwidth and
phase characteristics with frequency.
Log conformity is defined as the peak deviation from the
best fit straight line of the V
versus log (I /I )
LOGOUT
1 2
DEFINITION OF TERMS
curve. Log conformity is then expressed as a percent of
ideal full−scale output. Thus, the nonlinearity error ex-
pressed in volts over m decades is:
Transfer Function
The ideal transfer function of the LOG114 is:
V
= 0.375V/decade • 2Nm
LOGOUT (NONLIN)
I1
12
where N is the log conformity error, in percent.
+ 0.375 logǒ Ǔ
VLOGOUT
(17)
This transfer function can be seen graphically in the typ-
INDIVIDUAL ERROR COMPONENTS
ical characteristic curve, V
vs I
.
LOGOUT
REF
The ideal transfer function with current input is:
When a pedestal, or offset, voltage (V
)is connected
Com
to the Com pin, an additional offset term is introduced
into the equation:
I1
12
ǒ Ǔ
VLOGOUT IDEAL + 0.375 log
(19)
I1
12
The actual transfer function with the major components
of error is:
+ 0.375 logǒ Ǔ
VLOGOUT
) VCom
(18)
I1
I2
Accuracy
0.375(1 " DK) logǒ Ǔ
" 2Nm " VOSO
Accuracy considerations for a log ratio amplifier are
somewhat more complicated than for other amplifiers.
This complexity exists because the transfer function is
nonlinear and has two inputs, each of which can vary
(20)
where:
∆K = gain error (0.4%, typ, as specified in the Electri-
cal Characteristics table)
over
a
wide
dynamic range. The accuracy for any combination of
inputs is determined from the total error specification.
I
I
= bias current of A (5pA, typ)
1
B1
B2
= bias current of A (5pA, typ)
2
Total Error
m = number of decades over which the log
conformity error is specified
The total error is the deviation of the actual output from
the ideal output. Thus,
N = log conformity error (0.1%, typ for m = 5 decades;
0.9% typ for m = 7.5 decades)
V
= V
Total Error
LOGOUT(ACTUAL)
LOGOUT(IDEAL)
V
= output offset voltage (11mV, typ for 5V sup-
plies; 14mV, typ for +5V supplies)
It represents the sum of all the individual components
of error normally associated with the log amp when op-
erating in the current input mode. The worst-case error
OSO
To determine the typical error resulting from these error
components, first compute the ideal output. Then calcu-
late the output again, this time including the individual
error components. Then determine the error in percent
using Equation (21):
for any given ratio of I /I is the largest of the two errors
1 2
when I and I are considered separately. Temperature
1
2
can also affect total error.
Errors RTO and RTI
Ť
Ť
As with any transfer function, errors generated by the
function may be Referred-to-Output (RTO) or Referred-
to-Input (RTI). In this respect, log amps have a unique
property: given some error voltage at the log amp out-
put, that error corresponds to a constant percent of the
input, regardless of the actual input level.
VLOGOUT IDEAL*VLOGOUT
TYP
%error +
100%
VLOGOUTIDEAL
(21)
21
ꢠ ꢂꢡ ꢢꢢ ꢣ
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
For example, in a system configured for measurement
The QFN package can be easily mounted using stan-
dard printed circuit board (PCB) assembly techniques.
See Application Note QFN/SON PCB Attachment
(SLUA271) and Application Report Quad Flatpack No−
Lead Logic Packages (SCBA017), both available for
download at www.ti.com.
of five decades, with I = 1mA, and I = 10µA:
1
2
10*3
ǒ Ǔ+ 0.75V
VLOGOUT IDEAL + 0.375 log
10*5
(22)
(23)
10−3*5 10−12
10−5*5 10−12
+ 0.375 1 " 0.004 logǒ
" 2 0.001
Ǔ
(
)
VLOGOUT
TYP
The exposed leadframe die pad on the bottom of
the package should be connected to V−.
(
)( )
5 " 0.011
Using the positive error components (+∆K, +2Nm, and
+V ) to calculate the maximum typical output:
QFN LAYOUT GUIDELINES
OSO
The exposed leadframe die pad on the QFN package
should be soldered to a thermal pad on the PCB. A me-
chanical drawing showing an example layout is at-
tached at the end of this data sheet. Refinements to this
layout may be necessary based on assembly process
requirements. Mechanical drawings located at the end
of this data sheet list the physical dimensions for the
package and pad. The five holes in the landing pattern
are optional, and are intended for use with thermal vias
that connect the leadframe die pad to the heatsink area
on the PCB.
VLOGOUT TYP + 0.774V
(24)
Therefore, the error in percent is:
|
|
0.75*0.774
%error +
100% + 3.2%
0.75
(25)
QFN PACKAGE
The LOG114 comes in a QFN-16 package. This lead-
less package has lead contacts on all four sides of the
bottom of the package, thereby maximizing board
space. An exposed leadframe die pad on the bottom of
the package enhances thermal and electrical charac-
teristics.
Soldering the exposed pad significantly improves
board-level reliability during temperature cycling, key
push, package shear, and similar board-level tests.
Even with applications that have low-power dissipation,
the exposed pad must be soldered to the PCB to pro-
vide structural integrity and long-term reliability.
QFN packages are physically small, have a smaller
routing area, improved thermal performance, and im-
proved electrical parasitics. Additionally, the absence of
external leads eliminates bent-lead issues.
22
PACKAGE OPTION ADDENDUM
www.ti.com
7-May-2007
PACKAGING INFORMATION
Orderable Device
LOG114AIRGVR
LOG114AIRGVRG4
LOG114AIRGVT
Status (1)
ACTIVE
ACTIVE
ACTIVE
ACTIVE
Package Package
Pins Package Eco Plan (2) Lead/Ball Finish MSL Peak Temp (3)
Qty
Type
Drawing
QFN
RGV
16
16
16
16
2500 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR
no Sb/Br)
QFN
QFN
QFN
RGV
RGV
RGV
2500 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR
no Sb/Br)
250 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR
no Sb/Br)
LOG114AIRGVTG4
250 Green (RoHS & CU NIPDAU Level-2-260C-1 YEAR
no Sb/Br)
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check
http://www.ti.com/productcontent for the latest availability information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements
for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered
at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and
package, or 2) lead-based die adhesive used between the die and leadframe. The component is otherwise considered Pb-Free (RoHS
compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame
retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder
temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is
provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the
accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take
reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on
incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited
information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI
to Customer on an annual basis.
Addendum-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
17-May-2007
TAPE AND REEL INFORMATION
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
17-May-2007
Device
Package Pins
Site
MLA
MLA
Reel
Diameter Width
(mm)
Reel
A0 (mm)
4.3
B0 (mm)
4.3
K0 (mm)
1.5
P1
W
Pin1
(mm) (mm) Quadrant
(mm)
LOG114AIRGVR
LOG114AIRGVT
RGV
RGV
16
16
330
12
12
12
12 PKGORN
T2TR-MS
P
180
12
4.3
4.3
1.5
12 PKGORN
T2TR-MS
P
TAPE AND REEL BOX INFORMATION
Device
Package
Pins
Site
Length (mm) Width (mm) Height (mm)
LOG114AIRGVR
LOG114AIRGVT
RGV
RGV
16
16
MLA
MLA
346.0
190.0
346.0
212.7
29.0
31.75
Pack Materials-Page 2
PACKAGE MATERIALS INFORMATION
www.ti.com
17-May-2007
Pack Materials-Page 3
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