FEATURES
q
EASY-TO-USE COMPLETE CORE FUNCTION
q
HIGH ACCURACY: 0.01% FSO Over 5 Decades
q
WIDE INPUT DYNAMIC RANGE:
7.5 Decades, 100pA to 3.5mA
q
LOW QUIESCENT CURRENT: 1mA
q
WIDE SUPPLY RANGE:
4.5V to
18V
Precision
LOGARITHMIC AND LOG RATIO AMPLIFIER
DESCRIPTION
The LOG104
is a versatile integrated circuit that computes
the logarithm or log ratio of an input current relative to a
reference current.
The LOG104 is tested over a wide dynamic range of input
signals. In log ratio applications, a signal current can come
from a photodiode, and a reference current from a resistor in
series with a precision external reference.
The output signal at V
OUT
is trimmed to 0.5V per decade of
input current, allowing seven decades of input current, dy-
namic range.
Low DC offset voltage and temperature drift allow accurate
measurement of low-level signals over a wide environmental
temperature range. The LOG104 is specified over the tem-
perature range 5
C to +75
C, with operation over 40
C to
+85
C.
APPLICATIONS
q
LOG, LOG RATIO COMPUTATION:
Communication, Analytical, Medical, Industrial,
Test, General Instrumentation
q
PHOTODIODE SIGNAL COMPRESSION AMP
q
ANALOG SIGNAL COMPRESSION IN FRONT
OF ANALOG-TO-DIGITAL(A/D) CONVERTER
LOG104
www.ti.com
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
Copyright 2002-2004, Texas Instruments Incorporated
SBOS243B MAY 2002 REVISED JUNE 2004
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.
LOG
104
Note: Protected under US Patent #6,667,650; other patents pending.
A
2
A
1
1
5
8
4
6
3
Q
1
Q
2
C
C
I
2
I
1
R
1
R
2
V
V
OUT
V+
GND
LOG104
V
OUT
= 0.5 LOG (I
1
/I
2
)
www.ti.com
LOG104
2
SBOS243B
SPECIFIED
PACKAGE
TEMPERATURE
PACKAGE
ORDERING
TRANSPORT
PRODUCT
PACKAGE-LEAD
DESIGNATOR
RANGE
MARKING
NUMBER
MEDIA, QUANTITY
LOG104AID
SO -8
D
5
C to +75
C
LOG104
LOG104AID
Rails, 100
"
"
"
"
"
LOG104AIDR
Tape and Reel, 2500
NOTE: (1) For the most current package and ordering information, see the Package Option Addendum located at the end of this data sheet.
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Texas Instru-
ments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degrada-
tion 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.
ABSOLUTE MAXIMUM RATINGS
(1)
Supply Voltage, V+ to V .................................................................... 36V
Input Voltage ....................................................... V (0.5) to V+ (+0.5V)
Input Current ...................................................................................
10mA
Output Short-Circuit
(2)
.............................................................. Continuous
Operating Temperature .................................................... 40
C to +85
C
Storage Temperature ..................................................... 55
C to +125
C
Junction Temperature .................................................................... +150
C
Lead Temperature (soldering, 10s) ............................................... +300
C
NOTES: (1) Stresses above these ratings may cause permanent damage.
Exposure to absolute maximum conditions for extended periods may degrade
device reliability. (2) Short-circuit to ground.
PACKAGE/ORDERING INFORMATION
(1)
PIN DESCRIPTION
ELECTRICAL CHARACTERISTICS
Boldface limits apply over the specified temperature range, T
A
= 5
C to +75
C.
At T
A
= +25
C, V
S
=
5V, R
OUT
= 10k
, unless otherwise noted.
Top View
SO
I
2
NC
GND
V
NC = No Internal Connection
LOG104
I
1
NC
V
OUT
V+
8
7
6
5
1
2
3
4
LOG104AID
PARAMETER
CONDITION
MIN
TYP
MAX
UNITS
CORE LOG FUNCTION
I
IN
/ V
OUT
Equation
V
O
= (0.5V)log (I
1
/I
2
)
V
LOG CONFORMITY ERROR
(1)
Initial
1nA to 100
A (5 decades)
0.01
0.2
%
100pA to 3.5mA (7.5 decades)
0.06
%
over Temperature
1nA to 100
A (5 decades)
0.0001
%/
C
100pA to 3.5mA (7.5 decades)
(2)
0.0005
%/
C
GAIN
(3)
Initial Value
1nA to 100
A
0.5
V/decade
Gain Error
1nA to 100
A
0.15
1
%
vs Temperature
T
MIN
to T
MAX
0.003
0.01
%/
C
INPUT, A1 and A2
Offset Voltage
0.3
1.5
mV
vs Temperature
T
MIN
to T
MAX
2
V/
C
vs Power Supply (PSRR)
V
S
=
4.5V to
18V
5
50
V/V
Input Bias Current
5
pA
vs Temperature
T
MIN
to T
MAX
Doubles Every 10
C
Voltage Noise
f = 10Hz to 10kHz
3
Vrms
f = 1kHz
30
nV/
Hz
Current Noise
f = 1kHz
4
fA/
Hz
Common-Mode Voltage Range (Positive)
(V+) 2
(V+) 1.5
V
(Negative)
(V) + 2
(V) + 1.2
V
Common-Mode Rejection Ratio (CMRR)
105
dB
OUTPUT, A2 (V
OUT
)
Output Offset, V
OSO
, Initial
3
15
mV
vs Temperature
T
MIN
to T
MAX
2
V/
C
Full-Scale Output (FSO)
V
S
=
5V
(V) + 1.2
(V+) 1.5
V
Short-Circuit Current
18
mA
www.ti.com
LOG104
3
SBOS243B
TOTAL ERROR
(4)(5)
I
1
or I
2
remains fixed while other varies.
Initial
Min to Max
I
1
or I
2
= 3.5mA
75
mV
I
1
or I
2
= 1mA
20
mV
I
1
or I
2
= 100
A
20
mV
I
1
or I
2
= 10
A
20
mV
I
1
or I
2
= 1
A
20
mV
I
1
or I
2
= 100nA
20
mV
I
1
or I
2
= 10nA
20
mV
I
1
or I
2
= 1nA
20
mV
I
1
or I
2
= 350pA
20
mV
I
1
or I
2
= 100pA
20
mV
vs Temperature
I
1
or I
2
= 3.5mA
1.2
mV/
C
I
1
or I
2
= 1mA
0.4
mV/
C
I
1
or I
2
= 100
A
0.1
mV/
C
I
1
or I
2
= 10
A
0.05
mV/
C
I
1
or I
2
= 1
A
0.05
mV/
C
I
1
or I
2
= 100nA
0.09
mV/
C
I
1
or I
2
= 10nA
0.2
mV/
C
I
1
or I
2
= 1nA
0.3
mV/
C
I
1
or I
2
= 350pA
0.1
mV/
C
I
1
or I
2
= 100pA
0.3
mV/
C
vs Supply
I
1
or I
2
= 3.5mA
3.0
mV/ V
I
1
or I
2
= 1mA
0.1
mV/ V
I
1
or I
2
= 100
A
0.1
mV/ V
I
1
or I
2
= 10
A
0.1
mV/ V
I
1
or I
2
= 1
A
0.1
mV/ V
I
1
or I
2
= 100nA
0.1
mV/ V
I
1
or I
2
= 10nA
0.1
mV/ V
I
1
or I
2
= 1nA
0.25
mV/V
I
1
or I
2
= 350pA
0.1
mV/ V
I
1
or I
2
= 100pA
0.1
mV/ V
FREQUENCY RESPONSE, CORE LOG
(6)
BW, 3dB
I
2
= 10nA
C
C
= 4500pF
0.1
kHz
I
2
= 1
A
C
C
= 150pF
38
kHz
I
2
= 10
A
C
C
= 150pF
40
kHz
I
2
= 1mA
C
C
= 50pF
45
kHz
Step Response
Increasing
I
2
= 1
A to 1mA
C
C
= 150pF
11
s
I
2
= 100nA to 1
A
C
C
= 150pF
7
s
I
2
= 10nA to 100nA
C
C
= 150pF
110
s
Decreasing
I
2
= 1mA to 1
A
C
C
= 150pF
45
s
I
2
= 1
A to 100nA
C
C
= 150pF
20
s
I
2
= 100nA to 10nA
C
C
= 150pF
550
s
POWER SUPPLY
Operating Range
V
S
4.5
18
V
Quiescent Current
I
O
= 0
1
1.5
mA
TEMPERATURE RANGE
Specified Range, T
MIN
to T
MAX
5
75
C
Operating Range
40
85
C
Storage Range
55
125
C
Thermal Resistance,
JA
SO-8
150
C/W
NOTES: (1) Log Conformity Error is peak deviation from the best-fit straight line of V
OUT
versus log (I
1
/ I
2
) curve expressed as a percent of peak-to-peak full-scale.
(2) May require higher supply for full dynamic range.
(3) Output core log function is trimmed to 0.5V output per decade change of input current.
(4) Worst-case Total Error for any ratio of I
1
/I
2
is the largest of the two errors, when I
1
and I
2
are considered separately.
(5) Total I
1
+ I
2
should be kept below 4.5mA on
5V supply.
(6) Bandwidth (3dB) and transient response are a function of both the compensation capacitor and the level of input current.
ELECTRICAL CHARACTERISTICS
(Cont.)
Boldface limits apply over the specified temperature range, T
A
= 5
C to +75
C.
At T
A
= +25
C, V
S
=
5V, R
L
= 10k
, unless otherwise noted.
LOG104AID
PARAMETER
CONDITION
MIN
TYP
MAX
UNITS
www.ti.com
LOG104
4
SBOS243B
TYPICAL CHARACTERISTICS
At T
A
= +25
C, V
S
=
5V, R
L
= 10k
, unless otherwise noted.
ONE CYCLE OF NORMALIZED TRANSFER FUNCTION
Normalized Output Voltage (V)
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
2
6
1
10
3
Current Ratio, I
1
/I
2
4
8
2.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
2.0
Normalized Output V
oltage (V)
NORMALIZED TRANSFER FUNCTION
0.0001 0.001
0.01
0.1
1
10
100
1k
10k
Current Ratio, I
1
/I
2
V
OUT
= 0.5V LOG (I
1
/I
2
)
3dB FREQUENCY RESPONSE
3dB Frequency Response (Hz)
0.1
1M
100k
10k
1k
100
10
1
100
A
100
A
1mA
I
1
= 1mA
1
A
1mA
to 10
A
100nA
10nA
I
1
= 1nA
10nA
C
C
= 1000pF
C
C
= 1
F
10
A to 1
A
C
C
= 10pF
1nA
100
A
100pA
1nA
10nA
100nA
1
A
10
A
100
A
1mA
I
2
1
A
10
A
100
A
I
1
= 1nA
10nA
120
100
80
60
40
20
0
100pA 1nA
10nA 100nA
1
A
10
A 100
A 1mA 10mA
TOTAL ERROR vs INPUT CURRENT
Input Current (I
1
or I
2
)
T
otal Error (mV)
+75
C
+25
C
5
C
5.8
4.8
3.8
2.8
1.8
0.8
0.2
100pA 1nA
10nA 100nA 1
A
10
A 100
A 1mA
10mA
GAIN ERROR (I
2
= 1
A)
Input Current (I
1
or I
2
)
Gain Error (%)
5
C to 40
C
+85
C
+75
C
+25
C
MINIMUM VALUE OF COMPENSATION CAPACITOR
100M
10M
1M
100k
10k
1k
100
10
1
C
C
(pF)
100pA
1nA
10nA 100nA
1
A
10
A 100
A 1mA 10mA
I
2
I
1
= 100pA
I
1
= 1nA
I
1
= 10nA
I
1
= 100nA
1
A
I
1
= 10
A
100
A
1mA
Select C
C
for I
1
min.
and I
2
max. Values
below 2pF may be ignored.
www.ti.com
LOG104
5
SBOS243B
TYPICAL CHARACTERISTICS
(Cont.)
At T
A
= +25
C, V
S
=
5V, R
L
= 10k
, unless otherwise noted.
17
15
13
11
9
7
5
3
1
1
Input Current (I
1
or
I
2
)
Log Conformity (mV)
LOG CONFORMITY vs INPUT CURRENT
+85
C
+75
C
40
C to +25
C
100pA
1nA
10nA
100nA
1
A
10
A
100
A
1mA
LOG CONFORMITY vs TEMPERATURE
Log Conformity (m%)
350
300
250
200
150
100
50
0
40 30 20 10 0
10 20
30 40
50 60
70 80
90
Temperature (
C)
7 Decades
(100pA to 1mA)
6 Decades
(1nA to 1mA)
5 Decades
(1nA to 100
A)
8
1
6
3
4
5
V
V+
10
F
LOG104
1000pF
10
F
1000pF
I
1
I
2
V
OUT
C
C
FIGURE 1. Basic Connections of the LOG104.
APPLICATION INFORMATION
The LOG104 is a true logarithmic amplifier that uses the
base-emitter voltage relationship of bipolar transistors to
compute the logarithm, or logarithmic ratio of a current ratio.
Figure 1 shows the basic connections required for operation
of the LOG104. In order to reduce the influence of lead
inductance of power-supply lines, it is recommended that
each supply be bypassed with a 10
F tantalum capacitor in
parallel with a 1000pF ceramic capacitor, as shown in
Figure 1. Connecting the capacitors as close to the LOG104
as possible will contribute to noise reduction as well.
INPUT CURRENT RANGE
To maintain specified accuracy, the input current range of the
LOG104 should be limited from 100pA to 3.5mA. Input currents
outside of this range may compromise LOG104 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 logging transistor.
On
5V supplies, the total input current (I
1
+ I
2
) is limited to
4.5mA. Due to compliance issues internal to the LOG104, to
accommodate larger total input currents, supplies should be
increased.
Currents smaller than 100pA will result in increased errors due
the input bias currents of op amps A
1
and A
2
(typically 5pA).
The input bias currents may be compensated for, as shown in
Figure 2. The input stages of the amplifiers have FET inputs,
with input bias current doubling every 10
C, which makes the
nulling technique shown practical only where the temperature
is fairly stable.
FIGURE 2. Bias Current Nulling.
V
R
1
'
> 1M
I
2
I
1
R
2
'
10k
R
1
1M
R
2
10k
V+
8
1
4
6
3
V
OUT
5
V
V+
C
C
LOG104
GND
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