LOG114
SBOS301A − MAY 2004 − REVISED MARCH 2007
Single-Supply, High-Speed, Precision
LOGARITHMIC AMPLIFIER
FEATURES
DESCRIPTION
D ADVANTAGES:
The LOG114 is specifically designed for measuring
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).
D
D
D
D
D
D
D
D
− Tiny for High Density Systems
− Precision on One Supply
− Fast Over Eight Decades
− Fully-Tested Function
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.
APPLICATIONS
D ONET ERBIUM-DOPED FIBER OPTIC
AMPLIFIER (EDFA)
LASER OPTICAL DENSITY MEASUREMENT
PHOTODIODE SIGNAL COMPRESSION AMP
D
D
D LOG, LOG-RATIO FUNCTION
D ANALOG SIGNAL COMPRESSION IN FRONT
D
OF ANALOG-TO-DIGITAL (ADC) CONVERTER
ABSORBANCE MEASUREMENT
The output signal at VLOGOUT has a scale factor of 0.375V
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
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.
R5
V LOGOUT
9 (2)
R6
10
+IN 4
11
−IN 4
LOG114
Q1
200Ω
R (1)
I1
4
V CM IN
1250Ω
R2
1
A1
A4
5
I1 and I 2 are current inputs
from a photodiode
or other current source
VO4(3)
A 3(4)
Q2
13
200Ω
R (1)
I2
3
IREF
12
1250Ω
R4
3
A5
A2
15
+IN 5
VO5
R REF
16
REF
VREF
2.5V
1
V REF GND
8
V+
6
7
V−
Com
14
− IN 5
NOTES: (1) Thermally dependent R 1 and R 3
provide temperature compensation.
(2) V LOGOUT = 0.375 × log(I1/I2).
(3) V O4 = 0.375 × K × log(I1/I2)
K = 1 + R 6/R 5.
(4) Differential Amplifier (A3 ) Gain = 6.25
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.
Copyright 2004−2007, Texas Instruments Incorporated
! !
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"#$$%
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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
handled with appropriate precautions. Failure to observe
proper handling and installation procedures can cause damage.
ABSOLUTE MAXIMUM RATINGS(1)
Supply Voltage, V+ to V− . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12V
Signal Input Terminals, Voltage(2) . . . . . (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
(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.
(2) Input terminals are diode-clamped to the power-supply rails.
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
Input signals that can swing more than 0.5V beyond the supply
rails should be current-limited to 10mA or less.
(3) Short-circuit to ground.
FEATURES
PRODUCT
Logarithmic Transimpedance Amplifier, 5V, Eight Decades
LOG114
Logarithmic Transimpedance, 36V, 7.5 Decades
LOG112
Resistor-Feedback Transimpedance, 5V, 5.5 Decades
OPA380,
OPA381
Switched Integrator Transimpedance, Six Decades
IVC102
Direct Digital Converter, Six Decades
DDC112
ORDERING INFORMATION(1)
PRODUCT
PACKAGE-LEAD
PACKAGE DESIGNATOR
LOG114
QFN-16
RGV
PACKAGE MARKING
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
VO5
−IN5
+IN5
16
15
14
13
1
3
I1
4
10 +IN4
9
5
6
7
8
V+
I2
11 −IN4
Com
2
V−
NC
12 VO4
Exposed
thermal
die pad on
underside
(Must be
connected to V−)
VCM IN
VREF GND
VREF
Top View
QFN−16 (4mm x 4mm)
NC = No Connection
2
VLOGOUT
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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 TA = +25°C, RVLOGOUT = 10kΩ, VCM = GND, unless otherwise noted.
LOG114
PARAMETER
CONDITIONS
CORE LOG FUNCTION
MIN
IIN/VOUT Equation
TYP
MAX
VO = (0.375V) Log (I1/I2)
UNITS
V
LOG CONFORMITY ERROR(1)
Initial
1nA to 100µA (5 decades)
100pA to 3.5mA (7.5 decades)
0.1
0.2
%
0.009
0.017
dB
0.9
%
0.08
dB
1mA to 10mA
See Typical Characteristics
1nA to 100µA (5 decades)
0.1
100pA to 3.5mA (7.5 decades)
0.5
%
1mA to 10mA
See Typical Characteristics
%
Initial Scaling Factor
100pA to 10mA
0.375
Scaling Factor Error
1nA to 100µA
0.4
±2.5
%
0.035
0.21
dB
Over Temperature
0.4
%
TRANSFER FUNCTION (GAIN)(2)
Over Temperature
V/decade
TMIN to TMAX
1.5
±3.5
%
+15°C to +50°C
0.7
±3
%
±1
±4
INPUT, A1 and A2
Offset Voltage
VOS
vs Temperature
dV/dT
TMIN to TMAX
+15
vs Power Supply
PSRR
VS = ±2.25V to ±5.5V
75
Input Bias Current
IB
vs Temperature
Input Common-Mode Voltage Range
Voltage Noise
Current Noise
in
µV/V
pA
Doubles every 10°C
VCM
en
400
±5
TMIN to TMAX
mV
µV/°C
(V−)+1.5 to
(V+)−1.5
V
f = 0.1Hz to 10kHz
3
µVrms
f = 1kHz
30
nV/√Hz
f = 1kHz
4
fA/√Hz
±11
±50
mV
TMIN to TMAX
±15
±65
mV
OUTPUT, A3 (VLOGOUT)
Output Offset, VOSO, Initial
VOSO
Over Temperature
Full-Scale Output (FSO)(3)
Gain Bandwidth Product
Short-Circuit Current
(V−) + 0.6
GBW
IIN = 1µA
ISC
Capacitive Load
(V+) − 0.6
V
50
MHz
±18
mA
100
pF
OP AMP, A4 and A5
Input Offset Voltage
±250
VOS
±1000
µV
dV/dT
TMIN to TMAX
±2
vs Supply
PSRR
VS = ±4.5V to ±5.5V
30
vs Common-Mode Voltage
CMRR
74
dB
IB
−1
µA
vs Temperature
Input Bias Current
Input Offset Current
250
±0.05
IOS
Input Voltage Range
(V−)
Input Noise f = 0.1Hz to 10Hz
f = 1kHz
Current Noise
µV/°C
in
µV/V
µA
(V+) − 2
V
2
µVPP
13
nV/√Hz
2
pA/√Hz
Open-Loop Voltage Gain
AOL
100
dB
Gain Bandwidth Product
GBW
15
MHz
5
V/µs
Slew Rate
Settling Time 0.01%
SR
tS
Rated Output
Short-Circuit Current
G = −1, 3V Step, CL = 100pF
(V−) + 0.5
ISC
µs
1.5
(V+) − 0.5
+4/−10
V
mA
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 TA = +25°C, RVLOGOUT = 10kΩ, VCM = GND, unless otherwise noted.
LOG114
PARAMETER
CONDITIONS
MIN
TOTAL ERROR(4, 5)
TYP
MAX
UNITS
See Typical Characteristics
FREQUENCY RESPONSE, Core Log(6)
BW, 3dB I1 or I2 =
IAC = 10% of IDC value, IREF = 1µA
1nA
5
kHz
10nA
12
kHz
100nA
120
kHz
1µA
2.3
MHz
10µA to 1mA (ratio 1:100)
>5
MHz
1mA to 3.5mA (ratio 1:3.5)
>5
MHz
3.5mA to 10mA (ratio 1:2.9)
>5
MHz
8nA to 240nA (ratio 1:30)
0.7
µs
10nA to 100nA (ratio 1:10)
1.5
µs
10nA to 1µA (ratio 1:100)
0.15
µs
10nA to 10µA (ratio 1:1k)
0.07
µs
10nA to 1mA (ratio 1:100k)
0.06
µs
1
µs
8nA to 240nA (ratio 1:30)
1
µs
10nA to 100nA (ratio 1:10)
2
µs
10nA to 1µA (ratio 1:100)
0.25
µs
10nA to 10µA (ratio 1:1k)
0.05
µs
10nA to 1mA (ratio 1:100k)
0.03
µs
1
µs
Step Response
IREF = 1µA
Increasing (I1 or I2)
1mA to 10mA (ratio 1:10)
Decreasing (I1 or I2)
IREF = 1µA
1mA to 10mA (ratio 1:10)
VOLTAGE REFERENCE
Bandgap Voltage
2.5
±0.15
Error, Initial
V
±1
±25
vs Temperature
vs Supply
vs Load
%
ppm/°C
VS = ±4.5V to ±5.5V
±30
ppm/V
IO = ±2mA
±200
ppm/mA
±10
mA
Short-Circuit Current
POWER SUPPLY
Dual Supply Operating Range
Quiescent Current
±2.4
VS
IQ
±10
IO = 0
±5.5
V
±15
mA
TEMPERATURE RANGE
Specification, TMIN to TMAX
−5
+75
°C
Operating
−40
+85
°C
Storage
−55
+125
Thermal Resistance, qJA
62
°C
°C/W
(1) Log conformity error is peak deviation from the best-fit straight line of VO vs Log (I1/I2) curve expressed as a percent of peak-to-peak full-scale output. Scale factor,
K, equals 0.375V output per decade of input current.
(2) Scale factor of core log function is trimmed to 0.375V output per decade change of input current.
(3) Specified by design.
(4) Worst-case total error for any ratio of I1/I2, as the largest of the two errors, when I, and I2 are considered separately.
(5) Total error includes offset voltage, bias current, gain, and log conformity.
(6) 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 TA = +25°C, RVLOGOUT = 10kΩ, VCM = +2.5V, unless otherwise noted.
LOG114
PARAMETER
CONDITIONS
CORE LOG FUNCTION
IIN/VOUT Equation
MIN
TYP
MAX
VO = (0.375V) Log (I1/I2) + VCM
UNITS
V
LOG CONFORMITY ERROR(1)
Initial
1nA to 100µA (5 decades)
100pA to 3.5mA (7.5 decades)
0.1
0.25
%
0.009
0.022
dB
0.9
%
0.08
dB
1mA to 10mA
See Typical Characteristics
1nA to 100µA (5 decades)
0.1
100pA to 3.5mA (7.5 decades)
0.5
1mA to 10mA
See Typical Characteristics
Initial Scaling Factor
10nA to 100µA
0.375
Scaling Factor Error
1nA to 100µA
0.4
±2.5
%
0.0.35
0.21
dB
Over Temperature
0.4
%
%
TRANSFER FUNCTION (GAIN)(2)
Over Temperature
V/decade
TMIN to TMAX
0.035
±3.5
%
+15°C to +50°C
0.7
±3
%
±1
±7
INPUT, A1 and A2
Offset Voltage
VOS
mV
vs Temperature
dV/dT
TMIN to TMAX
+30
µV/°C
vs Power Supply
PSRR
VS = +4.5V to +5.5V
300
µV/V
Input Bias Current
IB
±5
pA
vs Temperature
Input Common-Mode Voltage Range
Voltage Noise
Current Noise
TMIN to TMAX
Doubles every 10°C
VCM
en
in
(V−)+1.5 to
(V+)−1.5
V
f = 0.1Hz to 10kHz
3
µVrms
f = 1kHz
30
nV/√Hz
f = 1kHz
4
fA/√Hz
OUTPUT, A3 (VLOGOUT)
Output Offset, VOSO, Initial
VOSO
Over Temperature
TMIN to TMAX
Full Scale Output (FSO)(3)
Gain Bandwidth Product
Short-Circuit Current
VS = +5V
GBW
±14
±65
mV
±18
±80
mV
(V−) + 0.6
IIN = 1µA
ISC
Capacitive Load
(V+) − 0.6
V
50
MHz
±18
mA
100
pF
OP AMP, A4 and A5
Input Offset Voltage
±250
VOS
±4000
µV
dV/dT
TMIN to TMAX
±2
µV/°C
vs Supply
PSRR
VS = +4.8V to +5.5V
30
µV/V
vs Common-Mode Voltage
CMRR
70
dB
IB
−1
µA
vs Temperature
Input Bias Current
Input Offset Current
±0.05
IOS
Input Voltage Range
(V−)
Input Noise f = 0.1Hz to 10Hz
in
V
28
µVPP
nV/√Hz
2
pA/√Hz
1
f = 1kHz
Current Noise
µA
(V+) − 1.5
Open-Loop Voltage Gain
AOL
100
dB
Gain Bandwidth Product
GBW
15
MHz
5
V/µs
Slew Rate
Settling Time 0.01%
SR
tS
Rated Output
Short-Circuit Current
G = −1, 3V Step, CL = 100pF
(V−) + 0.5
ISC
µs
1.5
(V+) − 0.5
+4/−10
V
mA
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 TA = +25°C, RVLOGOUT = 10kΩ, VCM = +2.5V, unless otherwise noted.
LOG114
PARAMETER
CONDITIONS
MIN
TOTAL ERROR(4, 5)
TYP
MAX
UNITS
See Typical Characteristics
FREQUENCY RESPONSE, Core Log(6)
BW, 3dB I1 or I2 =
IAC = 10% of IDC value, IREF = 1µA
1nA
5
kHz
10nA
12
kHz
100nA
120
kHz
1µA
2.3
MHz
10µA to 1mA (ratio 1:100)
>5
MHz
1mA to 3.5mA (ratio 1:3.5)
>5
MHz
3.5mA to 10mA (ratio 1:2.9)
>5
MHz
8nA to 240nA (ratio 1:30)
0.7
µs
10nA to 100nA (ratio 1:10)
1.5
µs
10nA to 1µA (ratio 1:100)
0.15
µs
10nA to 10µA (ratio 1:1k)
0.07
µs
10nA to 1mA (ratio 1:100k)
0.06
µs
1
µs
8nA to 240nA (ratio 1:30)
1
µs
10nA to 100nA (ratio 1:10)
2
µs
10nA to 1µA (ratio 1:100)
0.25
µs
10nA to 10µA (ratio 1:1k)
0.05
µs
10nA to 1mA (ratio 1:100k)
0.03
µs
1
µs
Step Response
IREF = 1µA
Increasing (I1 or I2)
1mA to 10mA (ratio 1:10)
Decreasing (I1 or I2)
IREF = 1µA
1mA to 10mA (ratio 1:10)
VOLTAGE REFERENCE
Bandgap Voltage
2.5
±0.15
Error, Initial
V
±1
±25
vs Temperature
vs Supply
vs Load
%
ppm/°C
VS = +4.8V to +11V
±30
ppm/V
IO = ±2mA
±200
ppm/mA
±10
mA
Short-Circuit Current
POWER SUPPLY
Single Supply Operating Range
Quiescent Current
VS
IQ
4.8
±10
IO = 0
11
V
±15
mA
TEMPERATURE RANGE
Specification, TMIN to TMAX
−5
+75
°C
Operating
−40
+85
°C
Storage
−55
+125
Thermal Resistance, qJA
62
°C
°C/W
(1) Log conformity error is peak deviation from the best-fit straight line of VO vs Log (I1/I2) curve expressed as a percent of peak-to-peak full-scale output. Scale factor,
K, equals 0.375V output per decade of input current.
(2) Scale factor of core log function is trimmed to 0.375V output per decade change of input current.
(3) Specified by design.
(4) Worst-case total error for any ratio of I1/I2, as the largest of the two errors, when I, and I2 are considered separately.
(5) Total error includes offset voltage, bias current, gain, and log conformity.
(6) 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
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
TYPICAL CHARACTERISTICS: VS = +5V
All specifications at TA = +25°C, RVLOGOUT = 10kΩ, VCM = GND, unless otherwise noted.
ONE CYCLE OF NORMALIZED TRANSFER FUNCTION
0.40
1.5
0.35
Normalized Output Voltage (V)
Normalized Output Voltage (V)
NORMALIZED TRANSFER FUNCTION
2.0
1.0
0.5
0
−0.5
−1.0
−1.5
−2.0
10−4
10−3
10−2
10−1
0.30
0.25
0.20
0.15
0.10
0.05
0
1
101
102
103
104
1
10
Current Ratio (I1/I2)
40
Current Ratio (I 1/ I2)
SCALING FACTOR ERROR (I2 = reference 100pA to 10mA)
VLOGOUT vs I 1 INPUT (I2 = 1µA)
2.5
2.0
1.5
1.0
20
10
+70_ C
VLOGOUT (V)
Gain Error (%)
30
+25_ C
0_ C
0
−10
−20
0.5
0
−0.5
−1.0
−10_ C
−1.5
+80_ C
+90_ C
100pA 1nA
−2.0
−2.5
10nA 100nA 1µA
10µA 100µA 1mA
10mA
100pA 1nA
10nA 100nA 1µA
Input Current (I1)
2.0
VLOGOUT vs I2 INPUT (I1 = 1µA)
10mA
VLOGOUT vs I REF
4
100pA
1.5
3
1.0
1nA
10nA
2
1µA
0.5
VLOGOUT (V)
VLOGOUT (V)
10µA 100µA 1mA
Input Current (I1)
0
−0.5
−1.0
100nA
1
0
−1
10µA
−2
−1.5
−2.0
−3
−2.5
100pA 1nA
−4
100pA 1nA
10nA 100nA 1µA
10µA 100µA 1mA
Input Current (I1)
10mA
1mA 100µA
10mA
10nA 100nA 1µA
10µA 100µA 1mA
10mA
IREF (I2)
7
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
TYPICAL CHARACTERISTICS: VS = +5V (continued)
All specifications at TA = +25°C, RVLOGOUT = 10kΩ, VCM = GND, unless otherwise noted.
AVERAGE TOTAL ERROR AT +80_C
100
80
80
60
60
I1 = 1mA
40
20
I1 = 10µA I = 100µA
1
0
−20
−40
20
−100
I1 = 100µA
0
−40
−60
I1 = 10nA
I1 = 1nA
100µA
200µA
I 1 = 100nA
400µA
600µA
I 1 = 1µA
−80
I1 = 1µA
1mA
100µA
400µA
200µA
AVERAGE TOTAL ERROR AT −10_C
LOG CONFORMITY vs TEMPERATURE
7.5 Decade
I1 = 1mA
40
I1 = 1nA
20
0
I1 = 10nA
−40
I1 = 100µA
1.0
Linearity (%)
Total Error (mV)
1mA
1.2
I1 = 1µA
−80
0.8
7 Decade
0.6
5 Decade
4 Decade
6 Decade
0.4
I1 = 10µA I = 100nA
1
−60
0.2
−100
0
100µA
200µA
400µA
600µA
800µA
−10
1mA
0
10
I2
0.40
0.35
0.08
+90_ C
Linearity (%)
0_C
−10_ C
+80_ C
40
50
60
70
80
90
5 DECADE LOG CONFORMITY vs I REF
+90_ C
0.25
0.20
+80_ C
0.15
+70_ C
0.10
0.05
+25_C
+70_ C
0.04
100pA 1nA
30
0.30
0.07
0.06
20
Temperature (_ C)
4 DECADE LOG CONFORMITY vs IREF
0.09
Linearity (%)
800µA
1.4
60
10nA 100nA 1µA
IREF (I1)
8
600µA
I2
80
−20
I1 = 1nA, 10nA,
100nA
−100
800µA
I2
100
I 1 = 10µA
−20
−60
−80
I1 = 1mA
40
Total Error (mV)
Total Error (mV)
AVERAGE TOTAL ERROR AT +25_C
100
10µA 100µA 1mA
0.05
10mA
−10_C, 0_ C, +25_C
0
100pA 1nA 10nA 100nA 1µA
I REF (I1)
10µA 100µA 1mA
10mA
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
TYPICAL CHARACTERISTICS: VS = +5V (continued)
All specifications at TA = +25°C, RVLOGOUT = 10kΩ, VCM = GND, unless otherwise noted. For ac measurements, small signal means up to approximately 10% of dc
level.
6 DECADE LOG CONFORMITY vs IREF
0.45
8 DECADE LOG CONFORMITY (100pA to 3.5mA)
1.6
1.5
0.40
+90_ C
1.4
Linearity (%)
0.35
+80_C
0.30
+70_C
1.3
1.2
0_ C
0.25
−10_C, 0_ C, +25_ C
+70_ C
0.9
100pA 1nA
10nA 100nA 1µA
20
10µA 100µA 1mA
100pA 1nA
10mA
10nA 100nA 1µA
10µA 100µA 1mA
IREF (I1)
Input Current (I1 or I 2)
SMALL−SIGNAL VLOGOUT
SMALL−SIGNAL AC RESPONSE I1
(10% sine modulation)
−5
Normalized LOG Output (dB)
10
1µA
1mA
0
100nA
−10
10mA
0
10mA
Normalized VLOGOUT (%)
−10_ C
1.0
0.20
10nA
−20
100µA
−30
10µA
10µA
−10
100µA
−15
−20
1µA
1nA
−25
−30
1mA
100nA
10nA
−35
−40
−45
−40
−50
10
100
1k
10k
100k
1M
10M
100M
100
1k
10k
Frequency (Hz)
160
−5
140
10µA
10M
100M
225
120
100µA
−15
180
1µA
1nA
−25
Gain (dB)
100
−20
1mA
10nA
−30
100nA
−35
80
135
60
Phase
Gain
40
90
20
−40
0
−45
−20
45
−40
−50
100
1M
A3 GAIN AND PHASE vs FREQUENCY
0
−10
100k
Frequency (Hz)
SMALL−SIGNAL AC RESPONSE I2
(10% sine modulation)
Normalized LOG Output (dB)
+25_C
+80_ C
1.1
Phase (_ )
Linearity (%)
+90_C
1k
10k
100k
1M
Frequency (Hz)
10M
100M
100
1k
10k
100k
1M
10M
0
40M
Frequency (Hz)
9
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
TYPICAL CHARACTERISTICS: VS = +5V (continued)
All specifications at TA = +25°C, RVLOGOUT = 10kΩ, VCM = GND, unless otherwise noted.
A4 and A5 GAIN AND PHASE vs FREQUENCY
180
140
3
120
Phase
Gain
90
60
40
Phase (_)
80
45
20
Normalized Output (dB)
0
135
100
Gain (dB)
A4 and A5 NONINVERTING CLOSED−LOOP RESPONSE
0
−20
1
10
100
1k
10k
100k
1M
10M
G=1
−3
G = 10
−6
−9
−12
−15
0
18M
1k
10k
A4 and A5 INVERTING CLOSED−LOOP RESPONSE
30
1M
10M
100M
A4 and A5 CAPACITIVE LOAD RESPONSE
10
G = +1
20
0
10
0
−10
−10
G = −10
−20
Gain (dB)
Gain (dB)
100k
Frequency (Hz)
Frequency (Hz)
G = −1
−30
−40
C = 100pF
−20
C < 10pF
−30
−50
−60
−40
−70
−80
−50
1k
10k
100k
1M
Frequency (Hz)
10
10M
60M
1k
10k
100k
1M
Frequency (Hz)
10M
50M
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
Either I1 or I2 can be held constant to serve as the reference current, with the other input being used for the input signal. The value of the reference current is selected
such that the output at VLOGOUT (pin 9) is zero when the
reference current and input current are equal. An onchip 2.5V reference is provided for use in generating the
reference current.
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 according to equation (1).
V LOGOUT + 0.375
log 10
Two additional amplifiers, A4 and A5, are included in the
LOG114 for use in scaling, offsetting, filtering, threshold
detection, or other functions.
ǒII Ǔ
1
BASIC CONNECTIONS
2
(1)
The output at VLOGOUT can be digitized directly, or scaled
for an ADC input using an uncommitted or external op
amp.
Figure 1 and Figure 2 show the LOG114 in typical dual
and single-supply configurations, respectively. To reduce 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.
An offsetting voltage (VCom) can be connected to the
Com pin to raise the voltage at VLOGOUT. When an
offsetting voltage is used, the transfer function
becomes:
V LOGOUT + 0.375
log 10
ǒII Ǔ ) V
1
Com
(2)
2
R8
56.2kΩ
R7
100kΩ
R5
100kΩ
9
VLOGOUT(1)
Q1
IREF
1µF
4 I1
5 VCM IN
Input Signal
RREF 100pA to 10mA
2.5MΩ
R1
10
R6
66.5kΩ
11
+IN4 −IN4
R2
A1
A4
Q2
A3
3 I2
R3
R4
A5
A2
16 VREF
LOG114
VO4(2) 12
+IN5
13
VO5
15
2.5VREF
VREF GND
1
V+
8
V−
1000pF
10µF
6
1000pF
10µF
+
+5V
+
Com
7
−IN5
14
NOTE: (1) VLOGOUT = 0.375 × log(I1/I2)
(2) VO4 = −0.249 × log(I1/I2) + 1.5V
−5V
Figure 1. Dual Supply Configuration Example for Best Accuracy Over Eight Decades.
11
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
R6
66.5kΩ
R5
100kΩ
R7
100kΩ
R8
316kΩ
REF3040
or
REF3240
4.096V
Reference
Input current
from photodiode
or current source
Q1
I1
IµA
RREF
1.62MΩ
9
VLOGOUT(2)
4
I1
5
VCM IN(1)
R1
3
11
−IN4
R2
A4
A3
I2
R4
R3
A5
A2
Photodiode(4)
+2.5V
16
VREF
2.5VREF
VREF GND
1
V
−IN5
V− Com
8
+
1000pF
6
7
1
10µF
VCom = +2.5V
+5V
NOTE: (1)
(2)
(3)
(4)
In single−supply configuration, VCM IN must be connected to ≥ 1V.
VLOGOUT = 0.375 × log(I1/I2) + 2.5V.
VO4 = −0.249 × log(I 1/I2) + 1.5V.
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
LOG114
A1
Q2
I2
10
+IN4
VO4(3) 12
+IN5
13
VO5
15
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
4. The A4 amplifier scales and offsets the VLOGOUT
signal for use by the ADC using the equation:
DESIGN EXAMPLE FOR DUAL-SUPPLY
CONFIGURATION
Given these conditions:
D V+ = 5V and V− = −5V
D 100pA ≤ Input signal
D The stage following the LOG114 is an analog-todigital converter (ADC) with +5V supply and
+2.5V reference voltage, so VO4 swings from
+0.5V to +2.5V.
1. Due to LOG114 symmetry, you can choose either
I1 or I2 as the signal input pin. Choosing I1 as the
reference makes the resistor network around A4
simpler. (Note: Current must flow into pins 3 (I1) and
pin 4 (I2).)
2. Select the magnitude of the reference current.
Since the signal (I2) spans eight decades, set I1 to
1µA − four decades above the minimum I2 value.
(Note that it does not have to be placed in the
middle. If I2 spanned seven decades, I1 could be set
three decades above the minimum and four
decades below the maximum I2 value.) This
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(∆VO4 per ∆I2) when the current signal decreases.
VLOGOUT + 0.375
Therefore, choose the final A4 output:
0V ≤ VO4 ≤ +2.5V
This output results in a 2.5V range for the 3V VLOGOUT
range, or 2.5V/3V scaling factor.
5. When I2 = 10mA, VLOGOUT = −1.5V. Using the
equation in step 5:
V O4 + *SFACTOR
+ 0.375
The A4 amplifier configuration for VO4 = −2.5/3(VLOGOUT)
+ 0V is seen in Figure 3.
The overall transer function is:
V O4 + *0.249
+ 0.375
ǒ
log
Ǔ
1mA
100pA
(7)
Internal A4 Output Amplifier
R6
82.5kΩ
+5V
VO4 = −2/3 (VLOGOUT)
A4
I1
I2
VREF
+2.5V
+ ) 1.5V
(3)
Therefore, the expected voltage range at the output
of amplifier A3 is:
* 1.5V v V LOGOUT v ) 1.5V
1
VLOGOUT
I1
I2
ǒǓ
log
ǒII Ǔ ) 1.5V
log
2
1mA
ǒ10mA
Ǔ + * 1.5V
VLOGOUT + 0.375
(6)
Therefore, VOFFSET = 0V
log
For I2 + 100pA :
ǒVLOGOUTǓ ) VOFFSET
0V + *2.5Vń3V(*1.5V) ) VOFFSET
R5
100kΩ
ǒǓ
log
(5)
The A4 amplifier is specified with a rated output swing
capability from (V−) +0.5V to (V+) − 0.5V.
3. Using Equation (1) calculate the expected range of
log outputs at VLOGOUT:
For I2 + 10mA :
ǒVLOGOUTǓ ) VOFFSET
V O4 + *SFACTOR
(4)
−5V
R7
100kΩ
R8
37.4kΩ
10mA
I2
100pA
VO4
0V
+2.5V
A4 amplifier used to scale and offset VLOGOUT for 0V to 2.5V output.
Figure 3. Operational Amplifier Configuration for
Scaling the Output Going to ADC Stage.
13
"#$$%
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
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.
DESIGN EXAMPLE FOR SINGLE-SUPPLY
CONFIGURATION
Given these conditions:
D
D
D
D
V+ = 5V
V− = GND
100pA ≤ Input signal ≤ 10mA
The stage following the LOG114 is an analog to
digital converter (ADC) with +5V supply and
+2.5V reference voltage
1. Choose either I1 or I2 as the signal input pin. For this
example, I2 is used. Choosing I1 as the reference
current makes the resistor network around A4
simpler. (Note: Current only flows into the I1 and I2
pins.)
2. Select the magnitude of the reference current.
Since the signal (I2) spans eight decades, set I1 to
1µA − four decades above the minimum I2 value,
and four decades below the maximum I2 value.
(Note that it does not have to be placed in the
middle. If I2 spanned seven decades, I1 could be set
three decades above the minimum and four
decades below the maximum I2 value.) This
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(∆VO4 per ∆I2) when the current signal decreases.
3. Using Equation (1) calculate the expected range of
log outputs at VLOGOUT:
+ 0.375
ǒǓ
log
1mA
ǒ10mA
Ǔ + * 1.5V
For I2 + 100pA :
+ 0.375
ǒ
log
5. The A4 amplifier scales and offsets the VLOGOUT
signal for use by the ADC using the equation:
ǒVLOGOUTǓ ) VOFFSET
V O4 + *SFACTOR
(11)
The A4 amplifier is specified with a rated output swing
capability from (V−) +0.5V to (V+) − 0.5V.
Therefore, choose the final A4 output:
+0.5V ≤ VO4 ≤ +2.5V
This output results in a 2V range for the 3V VLOGOUT
range, or 2V/3V scaling factor.
6. When I2 = 10mA, VLOGOUT = +1V, and VO4 = 2.5V.
Using the equation in step 5:
V O4 + *SFACTOR
ǒVLOGOUTǓ ) VOFFSET
2.5V + *2Vń3V(1V) ) VOFFSET
(12)
The overall transer function is:
V O4 + *0.249
ǒǓ
I1
log
I2
Ǔ
1mA
100pA
+ ) 1.5V
* 1.5V v V LOGOUT v ) 1.5V
ǒII Ǔ ) 1.5V
log
1
(13)
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, RP, 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
and the corresponding VOFFSET is 3.3V. This circuit
configuration is shown in Figure 4b.
2
(8)
Therefore, the expected voltage range at the output
of amplifier A3 is:
14
(10)
The A4 amplifier configuration for VO4 = −2/3(VLOGOUT) +
3.16 is seen in Figure 4a.
I1
I2
log
VLOGOUT + 0.375
) 1V v V LOGOUT v ) 4V
Therefore, VOFFSET = 3.16V
For I2 + 10mA :
VLOGOUT + 0.375
4. Select an offset voltage, VCom to use for centering
the output between (V−) + 0.6V and (V+) − 0.6V,
which is the full-scale output capability of the A3
amplifier. Choosing VCom = 2.5V, and recalculating
the expected voltage output range for VLOGOUT using
Equation (2), results in:
(9)
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
Internal A4 Output Amplifier
R5
100kΩ
External Output Amplifier
R5
100kΩ
R6
66.5kΩ
R6
82.5kΩ
VLOGOUT
VLOGOUT
+5V
A4
VO4 = −2/3 (VLOGOUT) + 3.16
10mA
I2
VREF
+2.5V
100pA
R7
100kΩ
R8
316kΩ
VOUT = −2.5/3 (VLOGOUT) + 3.3
OPA335
VREF
+2.5V
2.5V
VO4
RP(1)
10mA
I2
100pA
−5V
R7
100kΩ
R8
267kΩ
VOUT
0.5V
2.5V
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 RP connected to −5V to achieve 0V output.
Figure 4. Operational Amplifier Configuration for Scaling and Offsetting the Output Going to ADC Stage.
ADVANTAGES OF DUAL−SUPPLY OPERATION
VCM IN (Pin 5)
The LOG114 performs very well on a single +5V supply
by level-shifting pin 7 (Com) to half-supply and raising
the common-mode voltage (pin 5, VCM IN) of the input
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:
The VCM IN pin is used to bias the A1 and A2 amplifier into
its common-mode input voltage range, (V−) + 1.5V to
(V+) − 1.5V.
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
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.
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 differential amplifier, A3, within its linear range. This voltage can
provide an asymmetrical offset of the VLOGOUT voltage.
15
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
SETTING THE REFERENCE CURRENT
When the LOG114 is used to compute logarithms, either I1 or I2 can be held constant to become the reference current to which the other is compared.
ply system, and a maximum value of 7mV in a +5V supply system. Resistor temperature stability and noise
contributions should also be considered.
If IREF is set to the lowest current in the span of the signal
current (as shown in the front page figure), VLOGOUT will
range from:
V LOGOUT + 0.375
log 10
ǒI maxI minsignalǓ ^ 0V
VREF = 100mV
1
1
R1
(14)
R3
1
+5V
to some maximum value:
V LOGOUT + 0.375
log 10
ǒ
R2
Ǔ
I 1 min
I 1 max signal
(15)
A better way to achieve higher accuracy is to choose
IREF to be in the center of the full signal range. For
example, for a signal range of 1nA to 1mA, it is better
to use this approach:
Ǹ1mAń1nA + 1mA dc
A1
IREF
R3 >> R2
While convenient, this approach does not usually result
in best performance, because I1 min accuracy is difficult
to achieve, particularly if it is < 20nA.
I REF + I SIGNAL min
VOS
−
+
Figure 5. T-Network for Reference Current.
VREF may be an external precision voltage reference, or
the on-chip 2.5V voltage reference of the LOG114.
IREF can be derived from an external current source,
such as that shown in Figure 6.
(16)
than it is to set IREF = 1nA. It is much easier and more
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 resistor is used, the value may be large depending on IREF.
If IREF is 10nA and +2.5V is used:
RREF = 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
amplifier A1 has a maximum value of 4mV in a ±5V sup-
16
RREF
3.6kΩ
2N2905
+15V
6V
IN834
−15V
IREF =
6V
RREF
Figure 6. Temperature-Compensated Current Source.
"#$$%
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SBOS301A − MAY 2004 − REVISED MARCH 2007
NEGATIVE INPUT CURRENTS
The LOG114 functions only with positive input currents
(conventional current flows into input current pins). In
QA
I IN
situations where negative input currents are needed,
the example circuits in Figure 7, Figure 8, and Figure 9
may be used.
QB
National
LM394
D1
D2
OPA703
IOUT
Figure 7. Current Inverter/Current Source.
+5V
+3.3V
1/2
OPA2335
1.5kΩ
1kΩ
10nA to 1mA
(+3.3V
Back Bias)
+5V
BSH203
1/2
OPA2335
10nA to 1mA
Pin 3 or Pin 4
LOG114
Photodiode
Figure 8. Precision Current Inverter/Current Source.
1kΩ
100kΩ
100kΩ
+5V
10nA to 1mA
+3.3V
Back Bias
1/2
OPA2335
+5V
1.5kΩ
+3.3V
1/2
OPA2335
Photodiode
1.5kΩ
100kΩ
100kΩ
LOG114
10nA to 1mA
Pin 3 or Pin 4
Figure 9. Precision Current Inverter/Current Source.
17
"#$$%
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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 current inputs. Voltage inputs may be handled directly by
using a low-impedance voltage source with series resistors, but the dynamic input range is limited to approximately 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 application. Absorbance of the sample is A = log λ1′/λ1. If
D1 and D2 are matched, A ∝ (0.375V) log(I1/I2).
R5
9
V LOGOUT(1)
Q1
I1
Sample
λ1
I1
5
V CM IN
R1
+IN 4
A3
I2
R4
R3
A5
A2
D2
16
LO G114
R2
Q2
3
10
− IN 4
A4
λ 1′
λ1
10
A1
D1
I2
Light
Source
4
R6
V REF
V O4(2) 12
+IN 5
13
V O5
15
2.5V REF
V REF GND
1
V+
8
V−
6
Com
− IN 5
7
14
+5V
NO TES: (1) V LOGOUT = 0.375 × log(I1/I 2).
(2) V O4 = 0.375 × K × log(I1/I 2)
K = 1 + R 6/R 5.
Figure 10. Using the LOG114 to Measure Absorbance.
18
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SBOS301A − MAY 2004 − REVISED MARCH 2007
DATA COMPRESSION
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. (Suggested products: ADS7818, ADS7834).
I1
LOG114
VLOGOUT
I2
V+
+3.3V OPERATION
V−
TPS60241
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 generate a +5V supply for the LOG114, as shown in
Figure 11.
+3.3V
C1
1µF
C1
1µF
VIN
VOUT
C1+
C2+
C1−
C2−
GND
EN
+5V
C2
1µF
C0
1µF
Likewise, the TPS6040 negative charge pump may be
connected to the +5V output of the TPS60241 to generate 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+
+5V
V−
−5V
CFLY
1µF
TPS60241
C1
1µF
CFLY−
CFLY+
+5V
+3.3V
C1
1µF
VIN
VOUT
C1+
C2+
C1−
C2−
GND
EN
IN
C2
1µF
CO
1µF
CI
1µF
TPS60400
GND
OUT
−5V
CO
1µF
Figure 12. Creating a ±5V Supply from a +3.3V Supply.
19
"#$$%
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
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
connecting one of the photodiodes to the LOG114 I1
input, and the other to the I2 input. The differential
amplifier would then be eliminated.
ERBIUM-DOPED FIBER OPTIC AMPLIFIER
(EDFA)
The LOG114 was designed for optical networking systems. 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-controlled current source (VCCS), which then drives the power 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.
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.
Tap
Tap
1%
1%
Fiber
Pump Laser
OPA569
IL
Power
Op Amp
VCCS
PID
VERROR
Diff
I1
LOG114
IREF1
VOUT1
VOUT2
I2
LOG114
REF
IREF2
DAC
RREF1
RREF2
Figure 13. Erbium-Doped Fiber Optic Amplifier (EDFA) block diagram.
20
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www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
INSIDE THE LOG114
The LOG114 uses two matched logarithmic amplifiers
(A1 and A2 with logging diodes in the feedback loops) to
generate the outputs log (I1) and log (I2), respectively.
The gain of 6.25 differential amplifier (A3) subtracts the
output of A2 from the output of A1, resulting in [log (I1)
− log (I2)], or log (I1/I2). The symmetrical design of the
A1 and A2 logarithmic amps allows I1 and I2 to be used
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 VLOGOUT versus log (I1/I2)
curve. Log conformity is then expressed as a percent of
ideal full−scale output. Thus, the nonlinearity error expressed in volts over m decades is:
DEFINITION OF TERMS
Transfer Function
The ideal transfer function of the LOG114 is:
V LOGOUT + 0.375
VLOGOUT (NONLIN) = 0.375V/decade • 2Nm
ǒǓ
I
log 1
12
where N is the log conformity error, in percent.
(17)
This transfer function can be seen graphically in the typical characteristic curve, VLOGOUT vs IREF.
When a pedestal, or offset, voltage (VCom) is connected
to the Com pin, an additional offset term is introduced
into the equation:
V LOGOUT + 0.375
ǒ1I Ǔ ) V
log
1
2
Log Conformity
For the LOG114, log conformity is calculated in the
same way as linearity and is plotted as I1/I2 on a semilog scale. In many applications, log conformity is the
most important specification. This condition is true because 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.
Com
(18)
Accuracy
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
over
a
wide
dynamic range. The accuracy for any combination of
inputs is determined from the total error specification.
Total Error
The total error is the deviation of the actual output from
the ideal output. Thus,
VLOGOUT(ACTUAL) = VLOGOUT(IDEAL) ± Total Error
It represents the sum of all the individual components
of error normally associated with the log amp when operating in the current input mode. The worst-case error
for any given ratio of I1/I2 is the largest of the two errors
when I1 and I2 are considered separately. Temperature
can also affect total error.
Errors RTO and RTI
As with any transfer function, errors generated by the
function may be Referred-to-Output (RTO) or Referredto-Input (RTI). In this respect, log amps have a unique
property: given some error voltage at the log amp output, that error corresponds to a constant percent of the
input, regardless of the actual input level.
INDIVIDUAL ERROR COMPONENTS
The ideal transfer function with current input is:
V LOGOUT
IDEAL
ǒ1I Ǔ
+ 0.375
log
1
(19)
The actual transfer function with the major components
of error is:
2
0.375(1 " DK)
ǒII Ǔ " 2Nm " V
log
1
OSO
2
(20)
where:
∆K = gain error (0.4%, typ, as specified in the Electrical Characteristics table)
IB1 = bias current of A1 (5pA, typ)
IB2 = bias current of A2 (5pA, typ)
m = number of decades over which the log
conformity error is specified
N = log conformity error (0.1%, typ for m = 5 decades;
0.9% typ for m = 7.5 decades)
VOSO = output offset voltage (11mV, typ for ±5V supplies; 14mV, typ for +5V supplies)
To determine the typical error resulting from these error
components, first compute the ideal output. Then calculate the output again, this time including the individual
error components. Then determine the error in percent
using Equation (21):
%error +
ŤV LOGOUT IDEAL*V LOGOUT TYPŤ
V LOGOUTIDEAL
100%
(21)
21
"#$$%
www.ti.com
SBOS301A − MAY 2004 − REVISED MARCH 2007
For example, in a system configured for measurement
of five decades, with I1 = 1mA, and I2 = 10µA:
V LOGOUT
VLOGOUT
IDEAL
+ 0.375
ǒ
+ 0.375(1 " 0.004)
TYP
Ǔ
*3
log 10*5 + 0.75V
10
ǒ1010
log
−3*5
−5*5
(22)
Ǔ
10−12
10−12
The exposed leadframe die pad on the bottom of
the package should be connected to V−.
" 2(0.001)(5) " 0.011
(23)
Using the positive error components (+∆K, +2Nm, and
+VOSO) to calculate the maximum typical output:
V LOGOUT
TYP
+ 0.774V
(24)
Therefore, the error in percent is:
%error +
|0.75*0.774|
0.75
100% + 3.2%
(25)
QFN PACKAGE
The LOG114 comes in a QFN-16 package. This leadless 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 characteristics.
QFN packages are physically small, have a smaller
routing area, improved thermal performance, and improved electrical parasitics. Additionally, the absence of
external leads eliminates bent-lead issues.
22
The QFN package can be easily mounted using standard 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.
QFN LAYOUT GUIDELINES
The exposed leadframe die pad on the QFN package
should be soldered to a thermal pad on the PCB. A mechanical drawing showing an example layout is attached 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.
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 provide structural integrity and long-term reliability.
PACKAGE OPTION ADDENDUM
www.ti.com
14-Oct-2022
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
Samples
(4/5)
(6)
LOG114AIRGVR
ACTIVE
VQFN
RGV
16
2500
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
0 to 70
LOG
114
Samples
LOG114AIRGVT
ACTIVE
VQFN
RGV
16
250
RoHS & Green
NIPDAU
Level-2-260C-1 YEAR
0 to 70
LOG
114
Samples
(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)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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