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零件编号 | LOG101AIDR | ||
描述 | Precision LOGARITHMIC AND LOG RATIO AMPLIFIER | ||
制造商 | Burr-Brown Corporation | ||
LOGO | |||
1 Page
LOG101
LOG101
SBOS242B – MAY 2002 – REVISED JUNE 2004
Precision
LOGARITHMIC AND LOG RATIO AMPLIFIER
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
APPLICATIONS
q LOG, LOG RATIO COMPUTATION:
Communication, Analytical, Medical, Industrial,
Test, and General Instrumentation
q PHOTODIODE SIGNAL COMPRESSION AMPS
q ANALOG SIGNAL COMPRESSION IN FRONT
OF ANALOG-TO-DIGITAL (A/D) CONVERTERS
DESCRIPTION
The LOG101 is a versatile integrated circuit that computes
the logarithm or log ratio of an input current relative to a
reference current.
The LOG101 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 VOUT is trimmed to 1V per decade of input
current allowing seven decades of input current dynamic
range.
Low DC offset voltage and temperature drift allow accurate
measurement of low-level signals over a wide environmental
temperature range. The LOG101 is specified over the tem-
perature range –5°C to +75°C, with operation over
–40°C to +85°C.
Note: Protected under US Patent #6,667,650; other patents pending.
I2
I1
1
VOUT = (1V) • LOG (I1/I2)
V+
48
Q1 Q2
CC
LOG101
A1
A2
3
VOUT
R2
R1
56
GND
V–
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.
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.
www.ti.com
Copyright © 2002-2004, Texas Instruments Incorporated
INSIDE THE LOG101
Using the base-emitter voltage relationship of matched
bipolar transistors, the LOG101 establishes a logarith-
mic function of input current ratios. Beginning with the
base-emitter voltage defined as:
VBE
=
VT
ln
IC
IS
where :
VT
=
kT
q
(1)
k = Boltzman’s constant = 1.381 • 10–23
T = Absolute temperature in degrees Kelvin
q = Electron charge = 1.602 • 10–19 Coulombs
IC = Collector current
IS = Reverse saturation current
From the circuit in Figure 11, we see that:
VL = VBE1 – VBE2
(2)
Substituting (1) into (2) yields:
VL
=
VT1
ln
I1
IS1
–
VT2
ln I 2
IS2
(3)
If the transistors are matched and isothermal and
VTI = VT2, then (3) becomes:
VL
=
VT1
ln
I1
IS
– ln I2
IS
VL
=
VT
ln I1
I2
and since
ln x = 2.3log10 x
VL
=
n
VT
log I1
I2
(4)
(5)
(6)
(7)
where n = 2.3
(8)
also
or
VOUT
= VL
R1 + R2
R1
VOUT
=
R1 + R2
R1
n
VT
log
I1
I2
VOUT
=
(1V) •
log I1
I2
(9)
(10)
(11)
I1
I1
Q1 –
– Q2
++
VBE1 VBE2
I2
A2
VOUT
A1
VOUT = (1V) • LOG
I1
I2
R2
I2 VL
R1
FIGURE 11. Simplified Model of a Log Amplifier.
It should be noted that the temperature dependance
associated with VT = kT/q is internally compensated on
the LOG101 by making R1 a temperature sensitive resis-
tor with the required positive temperature coefficient.
DEFINITION OF TERMS
TRANSFER FUNCTION
The ideal transfer function is:
VOUT = 1V • log (I1/I2)
(5)
Figure 12 shows the graphical representation of the transfer
over valid operating range for the LOG101.
ACCURACY
Accuracy considerations for a log ratio amplifier are some-
what more complicated than for other amplifiers. This is
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.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
–3.5
I2=
I2=
I2=
100pA
1nA
10nA
100pA 1nA 10nA 100nA 1µA 10µA 100µA 1mA
I
I
I
I
I
2
2
2
2
2
=
=
=
=
=
100nA
11100µ0µAµAA
1mA
VOUT = (1V) • LOG (I1/I2)
10mA
I1
FIGURE 12. Transfer Function with Varying I2 and I1.
8
www.ti.com
LOG101
SBOS242B
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页数 | 13 页 | ||
下载 | [ LOG101AIDR.PDF 数据手册 ] |
零件编号 | 描述 | 制造商 |
LOG101AID | Precision LOGARITHMIC AND LOG RATIO AMPLIFIER | Burr-Brown Corporation |
LOG101AIDR | Precision LOGARITHMIC AND LOG RATIO AMPLIFIER | Burr-Brown Corporation |
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