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
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