FEATURES
D ADVANTAGES:
Tiny for High Density Systems
Precision on One Supply
Fast Over Eight Decades
Fully-Tested Function
D TWO SCALING AMPLIFIERS
D WIDE INPUT DYNAMIC RANGE:
Eight Decades, 100pA to 10mA
D 2.5V REFERENCE
D STABLE OVER TEMPERATURE
D LOW QUIESCENT CURRENT: 10mA
D DUAL OR SINGLE SUPPLY: +5V, +5V
D PACKAGE: Small QFN-16 (4mm x 4mm)
D SPECIFIED TEMPERATURE RANGE:
5
°C to +75°C
APPLICATIONS
D ONET ERBIUM-DOPED FIBER OPTIC
AMPLIFIER (EDFA)
D LASER OPTICAL DENSITY MEASUREMENT
D PHOTODIODE SIGNAL COMPRESSION AMP
D LOG, LOG-RATIO FUNCTION
D ANALOG SIGNAL COMPRESSION IN FRONT
OF ANALOG-TO-DIGITAL (ADC) CONVERTER
D ABSORBANCE MEASUREMENT
DESCRIPTION
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).
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 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.
All trademarks are the property of their respective owners.
LOG114
SBOS301A MAY 2004 REVISED MARCH 2007
Single-Supply, High-Speed, Precision
LOGARITHMIC AMPLIFIER
www.ti.com
Copyright
20042007, Texas Instruments Incorporated
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.
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.
Q
1
200
R
1
(1)
1250
R
2
R
5
R
6
200
R3(1)
RREF
1250
R4
VO4(3)
IN
4
+IN4
V
LOGOUT
+IN5
IN
5
VO5
V
CM IN
I1
4
18
6
7
I
1 and I2 are current inputs
from a photodiode
or other current source
11
10
9(2)
14
5
3
15
13
12
16
VREF
2.5V
V
Com
V REF GND
Q
2
A
3
(4)
A
1
A2
A
4
LOG114
A
5
I
REF
I
2
V+
REF
NOTES: (1) Thermally dependent R1 and R3
provide temperature compensation.
(2) VLOGOUT = 0.375 × log(I1/I2).
(3) V
O4 =0.375 × K × log(I1/I2)
K= 1 + R6/R5.
(4) Differential Amplifier (A
3) Gain = 6.25