LOG114 Datasheet, PDF (1/30 Pages) Burr-Brown (TI) – Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER

LOG114
SBOS301A − MAY 2004 − REVISED MARCH 2007
Single-Supply, High-Speed, Precision
LOGARITHMIC AMPLIFIER
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 V_LOGOUT 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
R₆
V_LOGOUT
9(2)
+IN₄ −IN₄
10
11
Q₁
LOG114
I₁
4
V_{CM IN}
A₁
5
200Ω
R₁⁽¹⁾
1250Ω
R₂
A₄
V_O4⁽³⁾
12
I₁ and I₂ are current inputs
from a photodiode
or other current source
I₂
3
I_REF
R_REF
16
V_REF
2.5V
Q₂
A
(
3
4)
200Ω
1250Ω
R₃⁽¹⁾
R₄
A₂
A₅
REF
1 _{V REF GND}
8
6
V+ V−
7 _Com
14
− IN₅
13 ^+IN5
15 ^VO5
NOTES: (1) Thermally dependent R₁ and R₃
provide temperature compensation.
(2) V_LOGOUT = 0.375 × log(I₁/I₂).
(3) V_O4 = 0.375 × K × log(I₁/I₂)
K = 1 + R₆/R₅.
(4) Differential Amplifier (A₃) Gain = 6.25
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PRODUCTION DATA information is current as of publication date. Products
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Production processing does not necessarily include testing of all parameters.
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