LOG104 [TI]
LOGARITHMIC AND LOG RATIO AMPLIFIER; 对数和对数比放大器
| 型号： | LOG104 |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | LOGARITHMIC AND LOG RATIO AMPLIFIER |
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| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LOG104
L
O
G
1
0
4
SBOS243B – MAY 2002 – REVISED JUNE 2004
Precision
LOGARITHMIC AND LOG RATIO AMPLIFIER
DESCRIPTION
FEATURES
The LOG104 is a versatile integrated circuit that computes
the logarithm or log ratio of an input current relative to a
reference current.
●
EASY-TO-USE COMPLETE CORE FUNCTION
● HIGH ACCURACY: 0.01% FSO Over 5 Decades
● WIDE INPUT DYNAMIC RANGE:
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.
7.5 Decades, 100pA to 3.5mA
● LOW QUIESCENT CURRENT: 1mA
● WIDE SUPPLY RANGE: ±4.5V to ±18V
The output signal at VOUT is trimmed to 0.5V per decade of
input current, allowing seven decades of input current, dy-
namic range.
APPLICATIONS
● LOG, LOG RATIO COMPUTATION:
Communication, Analytical, Medical, Industrial,
Test, General Instrumentation
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.
● PHOTODIODE SIGNAL COMPRESSION AMP
● ANALOG SIGNAL COMPRESSION IN FRONT
Note: Protected under US Patent #6,667,650; other patents pending.
OF ANALOG-TO-DIGITAL(A/D) CONVERTER
I2
CC
VOUT = 0.5 LOG (I1/I2)
V+
4
8
I1
LOG104
1
Q1
Q2
3
A2
A1
VOUT
R2
R1
5
6
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.
PRODUCTION DATA information is current as of publication date.
Copyright © 2002-2004, Texas Instruments Incorporated
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
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ABSOLUTE MAXIMUM RATINGS(1)
Supply Voltage, V+ to V–.................................................................... 36V
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.
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
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.
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.
PIN DESCRIPTION
Top View
SO
I1
NC
1
2
3
4
8
7
6
5
I2
NC
GND
V–
LOG104
VOUT
V+
NC = No Internal Connection
PACKAGE/ORDERING INFORMATION(1)
SPECIFIED
PACKAGE
DESIGNATOR
TEMPERATURE
RANGE
PACKAGE
MARKING
ORDERING
NUMBER
TRANSPORT
MEDIA, QUANTITY
PRODUCT
PACKAGE-LEAD
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.
ELECTRICAL CHARACTERISTICS
Boldface limits apply over the specified temperature range, TA = –5°C to +75°C.
At TA = +25°C, VS = ±5V, ROUT = 10kΩ, unless otherwise noted.
LOG104AID
PARAMETER
CONDITION
MIN
TYP
MAX
UNITS
CORE LOG FUNCTION
I
IN /VOUT Equation
VO = (0.5V)log (I1/I2)
V
LOG CONFORMITY ERROR(1)
Initial
1nA to 100µA (5 decades)
100pA to 3.5mA (7.5 decades)
1nA to 100µA (5 decades)
0.01
0.06
0.0001
0.0005
0.2
%
%
%/°C
%/°C
over Temperature
100pA to 3.5mA (7.5 decades)(2)
GAIN(3)
Initial Value
Gain Error
vs Temperature
1nA to 100µA
1nA to 100µA
TMIN to TMAX
0.5
0.15
0.003
V/decade
%
%/°C
±1
0.01
INPUT, A1 and A2
Offset Voltage
±0.3
±2
5
±5
±1.5
mV
µV/°C
µV/V
pA
vs Temperature
vs Power Supply (PSRR)
Input Bias Current
vs Temperature
Voltage Noise
TMIN to TMAX
VS = ±4.5V to ±18V
50
TMIN to TMAX
f = 10Hz to 10kHz
f = 1kHz
Doubles Every 10°C
3
30
4
µVrms
nV/√Hz
fA/√Hz
V
V
dB
Current Noise
Common-Mode Voltage Range (Positive)
(Negative)
f = 1kHz
(V+) – 2
(V–) + 2
(V+) – 1.5
(V–) + 1.2
105
Common-Mode Rejection Ratio (CMRR)
OUTPUT, A2 (VOUT
)
Output Offset, VOSO, Initial
vs Temperature
Full-Scale Output (FSO)
Short-Circuit Current
±3
±2
±15
mV
µV/°C
V
TMIN to TMAX
VS = ±5V
(V–) + 1.2
(V+) – 1.5
±18
mA
LOG104
2
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SBOS243B
ELECTRICAL CHARACTERISTICS (Cont.)
Boldface limits apply over the specified temperature range, TA = –5°C to +75°C.
At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted.
LOG104AID
PARAMETER
CONDITION
MIN
TYP
MAX
UNITS
TOTAL ERROR(4)(5)
Initial
I1 or I2 remains fixed while other varies.
Min to Max
I1 or I2 = 3.5mA
I1 or I2 = 1mA
I1 or I2 = 100µA
I1 or I2 = 10µA
I1 or I2 = 1µA
I1 or I2 = 100nA
I1 or I2 = 10nA
I1 or I2 = 1nA
I1 or I2 = 350pA
I1 or I2 = 100pA
I1 or I2 = 3.5mA
I1 or I2 = 1mA
I1 or I2 = 100µA
I1 or I2 = 10µA
I1 or I2 = 1µA
I1 or I2 = 100nA
I1 or I2 = 10nA
I1 or I2 = 1nA
I1 or I2 = 350pA
I1 or I2 = 100pA
I1 or I2 = 3.5mA
I1 or I2 = 1mA
I1 or I2 = 100µA
I1 or I2 = 10µA
I1 or I2 = 1µA
I1 or I2 = 100nA
I1 or I2 = 10nA
I1 or I2 = 1nA
±75
±20
±20
±20
±20
±20
±20
±20
±20
±20
mV
mV
mV
mV
mV
mV
mV
mV
mV
mV
vs Temperature
±1.2
±0.4
±0.1
±0.05
±0.05
±0.09
±0.2
±0.3
±0.1
±0.3
±3.0
±0.1
±0.1
±0.1
±0.1
±0.1
±0.1
±0.25
±0.1
±0.1
mV/°C
mV/°C
mV/°C
mV/°C
mV/°C
mV/°C
mV/°C
mV/°C
mV/°C
mV/°C
mV/V
mV/V
mV/V
mV/V
mV/V
mV/V
mV/V
mV/V
mV/V
mV/V
vs Supply
I1 or I2 = 350pA
I1 or I2 = 100pA
FREQUENCY RESPONSE, CORE LOG(6)
BW, 3dB
I2 = 10nA
I2 = 1µA
I2 = 10µA
I2 = 1mA
CC = 4500pF
CC = 150pF
CC = 150pF
CC = 50pF
0.1
38
40
45
kHz
kHz
kHz
kHz
Step Response
Increasing
I2 = 1µA to 1mA
I2 = 100nA to 1µA
I2 = 10nA to 100nA
Decreasing
CC = 150pF
CC = 150pF
CC = 150pF
11
7
110
µs
µs
µs
I2 = 1mA to 1µA
I2 = 1µA to 100nA
I2 = 100nA to 10nA
CC = 150pF
CC = 150pF
CC = 150pF
45
20
550
µs
µs
µs
POWER SUPPLY
Operating Range
Quiescent Current
VS
IO = 0
±4.5
±18
±1.5
V
mA
±1
TEMPERATURE RANGE
Specified Range, TMIN to TMAX
Operating Range
–5
–40
–55
75
85
125
°C
°C
°C
Storage Range
Thermal Resistance, θJA SO-8
150
°C/W
NOTES:(1) Log Conformity Error is peak deviation from the best-fit straight line of VOUT versus log (I1/I2) 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 I1/I2 is the largest of the two errors, when I1 and I2 are considered separately.
(5) Total I1 + I2 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.
LOG104
SBOS243B
3
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TYPICAL CHARACTERISTICS
At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted.
ONE CYCLE OF NORMALIZED TRANSFER FUNCTION
NORMALIZED TRANSFER FUNCTION
2.0
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
VOUT = 0.5V LOG (I1/I2)
1.5
1.0
0.5
0.0
–0.5
–1.0
–1.5
–2.0
1
2
3
4
6
8
10
0.0001 0.001 0.01
0.1
1
10
100
1k
10k
Current Ratio, I1/I2
Current Ratio, I1/I2
GAIN ERROR (I2 = 1µA)
+85°C
TOTAL ERROR vs INPUT CURRENT
120
100
80
60
40
20
0
5.8
4.8
3.8
2.8
1.8
0.8
+75°C
+75°C
+25°C
–5°C to –40°C
+25°C –5°C
–0.2
100pA 1nA 10nA 100nA 1µA 10µA 100µA 1mA 10mA
100pA 1nA 10nA 100nA 1µA 10µA 100µA 1mA 10mA
Input Current (I1 or I2)
Input Current (I1 or I2)
3dB FREQUENCY RESPONSE
MINIMUM VALUE OF COMPENSATION CAPACITOR
100M
1M
100k
10k
1k
Select CC for I1 min.
and I2 max. Values
10µA
100µA
100µA
1mA
10M
1M
100k
10k
1k
1µA
below 2pF may be ignored.
I1 = 100pA
I
= 1mA
1
100µA
I1 = 1nA
I1 = 10nA
100µA
1µA
1nA
10nA
1mA
to 10µA
10nA
I1 = 100nA
100
10
100nA
10nA
I1 = 1nA
1µA
100
10
I = 1nA
1
I1 = 10µA
100µA
1mA
1
0.1
1
100pA 1nA 10nA 100nA 1µA
10µA 100µA 1mA
100pA 1nA 10nA 100nA 1µA 10µA 100µA 1mA 10mA
I2
I2
LOG104
4
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SBOS243B
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VS = ±5V, RL = 10kΩ, unless otherwise noted.
LOG CONFORMITY vs TEMPERATURE
LOG CONFORMITY vs INPUT CURRENT
17
350
300
250
200
150
100
50
7 Decades
(100pA to 1mA)
15
13
+85°C
11
6 Decades
(1nA to 1mA)
9
7
+75°C
5 Decades
(1nA to 100µA)
5
–40°C to +25°C
3
1
0
–1
–40 –30 –20 –10
0
10 20 30 40 50 60 70 80 90
100pA
1nA 10nA 100nA 1µA
10µA 100µA 1mA
Temperature (°C)
Input Current (I1 or I2)
INPUT CURRENT RANGE
APPLICATION INFORMATION
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.
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 10µF 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.
On ±5V supplies, the total input current (I1 + I2) 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 A1 and A2 (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.
V+
10µF
1000pF
4
1
6
R2
10kΩ
3
V–
VOUT
LOG104
V+
R1
1MΩ
8
5
1
8
3
6
5
VOUT
I1
I2
I1
CC
LOG104
GND
10µF
1000pF
R1'
> 1MΩ
4
I2
V–
CC
V–
R2'
10kΩ
V+
FIGURE 1. Basic Connections of the LOG104.
FIGURE 2. Bias Current Nulling.
LOG104
SBOS243B
5
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SETTING THE REFERENCE CURRENT
V+
V+
When the LOG104 is used to compute logarithms, either I1 or
I2 can be held constant and becomes the reference current to
which the other is compared.
I1 = 2.5nA to 1mA
4
2.5V
1
3
REF3025
VOUT
1GΩ to 2.5kΩ
LOG104
100kΩ
100Ω
V
OUT is expressed as:
I
2 = 2.5nA
10MΩ
8
VOUT = (0.5V) • log (I1/I2)
(1)
+25mV
3
5
6
GND
IREF can be derived from an external current source (such as
shown in Figure 3), or it may be derived from a voltage
source with one or more resistors. When a single resistor is
used, the value may be large depending on IREF. If IREF is
10nA and +2.5V is used:
+2.5V
CC
V–
OPA335 Chopper Op Amp
–2.5V
(2)
RREF = 2.5V/10nA = 250M
FIGURE 5. Current Source with Offset Compensation.
IREF
2N2905
at different levels of input signals. Smaller input currents
require greater gain to maintain full dynamic range, and will
slow the frequency response of the LOG104.
RREF
3.6kΩ
2N2905
+15V
–15V
6V
IN834
6V
FREQUENCY COMPENSATION
IREF
=
RREF
Frequency compensation for the LOG104 is obtained by
connecting a capacitor between pins 3 and 8. The size of the
capacitor is a function of the input currents, as shown in the
Typical Characteristic Curves (Minimum Value of Compen-
sation Capacitor). For any given application, the smallest
value of the capacitor which may be used is determined by
the maximum value of I2 and the minimum value of I1. Larger
values of CC will make the LOG104 more stable, but will
reduce the frequency response.
FIGURE 3. Temperature Compensated Current Source.
A voltage divider may be used to reduce the value of the
resistor (as shown in Figure 4). When using this method, one
must consider the possible errors caused by the amplifier’s
input offset voltage. The input offset voltage of amplifier A1
has a maximum value of 1.5mV, making VREF a suggested
value of 100mV.
In an application, highest overall bandwidth can be achieved
by detecting the signal level at VOUT, then switching in
appropriate values of compensation capacitors.
VREF = 100mV
R1 R3
VOS
+
–
1
+5V
NEGATIVE INPUT CURRENTS
A1
IREF
R2
The LOG104 will function only with positive input currents
(conventional current flows into pins 1 and 8). In situations
where negative input currents are needed, the circuits in
Figures 6, 7, and 8 may be used.
R3 >> R2
FIGURE 4. T Network for Reference Current.
Figure 5 shows a low-level current source using a series
resistor. The low offset op-amp reduces the effect of the
LOG104’s input offset voltage.
QA
QB
IIN
National
LM394
FREQUENCY RESPONSE
The frequency response curves seen in the Typical Charac-
teristics Curves are shown for constant DC I1 and I2 with a
small-signal AC current on one input.
D1
D2
The 3dB frequency response of the LOG104 is a function of
the magnitude of the input current levels and of the value of the
frequency compensation capacitor. See Typical Characteristic
Curve “3dB Frequency Response” for details.
OPA703
IOUT
The transient response of the LOG104 is different for in-
creasing and decreasing signals. This is due to the fact that
a log amp is a nonlinear gain element and has different gains
FIGURE 6. Current Inverter/Current Source.
LOG104
6
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SBOS243B
VOLTAGE INPUTS
V+
The LOG104 gives the best performance with current inputs.
Voltage inputs may be handled directly with series resistors,
but the dynamic input range is limited to approximately three
decades of input voltage by voltage noise and offsets. The
transfer function of Equation (13) applies to this configuration.
4
I1
1
8
3
6
VOUT
D1
Sample
λ1´
LOG104
λ1
I2
+5V
λ1
Light
Source
5
D2
1
2
OPA2335
TLV271 or
+3.3V
CC
1/2 OPA2335
1.5kΩ
V–
1.5kΩ
+5V
FIGURE 9. Absorbance Measurement.
BSH203
1/2 OPA2335
OPERATION ON SINGLE SUPPLY
Back Bias
+3.3V
10nA to 1mA
LOG104
Many applications do not have the dual supplies required to
operate the LOG104. Figure 10 shows the LOG104 config-
ured for operation with a single +5V supply.
10nA to 1mA
Pin 1 or Pin 8
Photodiode
FIGURE 7. Precision Current Inverter/Current Source.
Single Supply +5V
4
3
1
VOUT
APPLICATION CIRCUITS
LOG RATIO
I1
LOG104
One of the more common uses of log ratio amplifiers is
to measure absorbance. A typical application is shown in
Figure 9.
6
8
5
I2
Absorbance of the sample is A = logλ1´/ λ1
(3)
CC
1µF
If D1 and D2 are matched A (0.5V) logI1/I2
(4)
3
5
2
1
TPS(1)
4
–5V
DATA COMPRESSION
1µF
1µF
In many applications the compressive effects of the logarith-
mic transfer function are useful. For example, a LOG104
preceding a 12-bit A/D converter can produce the dynamic
range equivalent to a 20-bit converter.
(1) TPS60402DBV negative charge pump.
FIGURE 10. Single +5V Power-Supply Operation.
1.5kΩ
100kΩ
100kΩ
+5V
10nA to 1mA
Back Bias
+3.3V
+3.3V
+5V
1.5kΩ
1.5kΩ
1/2 OPA2335
1/2 OPA2335
Photodiode
100kΩ
100kΩ
10nA to 1mA
LOG104
Pin 1 or Pin 8
FIGURE 8. Precision Current Inverter/Current Source.
LOG104
SBOS243B
7
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INSIDE THE LOG104
also
Using the base-emitter voltage relationship of matched
bipolar transistors, the LOG104 establishes a logarith-
mic function of input current ratios. Beginning with the
base-emitter voltage defined as:
R1 + R2
VOUT = VL
(9)
R1
R1 + R2
I1
I2
VOUT
=
n VT log
IC
IS
kT
q
(10)
R1
VBE = VT ln
where : VT =
(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
I1
I2
VOUT = 0.5V •log
or
(11)
IS = Reverse saturation current
I2
Q1
Q2
–
–
I1
From the circuit in Figure 11, we see that:
VOUT
+
+
A2
VBE VBE
1
2
I1
VL = VBE – VBE
1
2
(2)
(3)
A1
I1
I2
R2
VL
R1
VOUT = (0.5V) LOG
Substituting (1) into (2) yields:
I2
I2
I1
VL = VT1 ln
– VT2 ln
IS1
IS2
If the transistors are matched and isothermal and
VTI = VT2, then (3) becomes:
FIGURE 11. Simplified Model of a Log Amplifier.
I1
VL = VT1 ln –ln
IS
I2
(4)
(5)
IS
I1
VL = VT ln
and since
I2
ln x = 2.3log10
I1
x
(6)
(7)
VL = n VT log
I2
where n = 2.3
(8)
DEFINITION OF TERMS
TRANSFER FUNCTION
3.5
3.0
2.5
2.0
1.5
1.0
0.5
The ideal transfer function is:
VOUT = 0.5V • logI1/I2
(5)
See Figure 12 for the graphical representation of the transfer
over valid operating range for the LOG104.
0.0
–0.5
–1.0
–1.5
I1
ACCURACY
–2.0
VOUT = (0.5V) • LOG (I1/I2)
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.
–2.5
–3.0
–3.5
FIGURE 13. Transfer Function with Varying I2 and I1.
LOG104
8
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SBOS243B
TOTAL ERROR
MEASURING AVALANCHE PHOTODIODE CURRENT
The total error is the deviation (expressed in mV) of the
actual output from the ideal output of VOUT = 0.5V • log(I1/I2).
The wide dynamic range of the LOG104 is useful for measuring
avalanche photodiode current (APD), as shown in Figure 13.
Thus,
LOG CONFORMITY
(6)
VOUT(ACTUAL) = VOUT(IDEAL) ± Total Error.
For the LOG104, log conformity is calculated the same as
linearity and is plotted I1/I2 on a semi-log scale. In many
applications, log conformity is the most important specifica-
tion. This is true because bias current errors are negligible
(5pA compared to input currents of 100pA and above) and
the scale factor and offset errors may be trimmed to zero or
removed by system calibration. This leaves log conformity as
the major source of error.
It represents the sum of all the individual components of error
normally associated with the log amp when operated in the
current input mode. The worst-case error for any given ratio
of I1/I2 is the largest of the two errors when I1 and I2 are
considered separately. Temperature can affect total error.
ERRORS RTO AND RTI
As with any transfer function, errors generated by the func-
tion itself may be Referred-to-Output (RTO) or Referred-to-
Input (RTI). In this respect, log amps have a unique property:
Log conformity is defined as the peak deviation from the best
fit straight line of the VOUT versus log (I1/I2) curve. This is
expressed as a percent of ideal full-scale output. Thus, the
nonlinearity error expressed in volts over m decades is:
Given some error voltage at the log amp’s output, that error
corresponds to a constant percent of the input regardless of
the actual input level.
(7)
VOUT (NONLIN) = 0.5V/dec • 2Nm V
where N is the log conformity error, in percent.
ISHUNT
+15V to +60V
500Ω
Irx = 1µA to 1mA
Receiver
5kΩ
5kΩ
10Gbits/sec
+5V
APD
I to V
Converter
INA168
SOT23-5
IOUT = 0.1 • ISHUNT
1
2
IOUT
CC
1.2kΩ
1kΩ
+5V
4
3
1
Q1
Q2
OPA703
VOUT = 2.5V to 0V
A2
A1
100µA
25kΩ
8
LOG104
REF3025
2.5V
SO-8
5
6
–5V
FIGURE 14. High Side Shunt for Avalanche Photodiode (APD) Measures 3-Decades of APD Current.
LOG104
SBOS243B
9
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INDIVIDUAL ERROR COMPONENTS
Example: what is the error when
I1 = 1µA and I2 = 100nA
The ideal transfer function with current input is:
(10)
I1
VOUT = 0.5V • log
10−6 − 5 •10−12
10−7 − 5 •10−12
(
)
(8)
VOUT = 0.5 ± 0.0015 log
± 2 0.00015 ± 3.0mV
(
)
(
)
(
)
I2
The actual transfer function with the major components of
error is:
= 0.5055V
(11)
Since the ideal output is 0.5V, the error as a percent of
reading is
I1 – IB1
(9)
VOUT = 0.5V 1± ∆K log
± Nm ± VOS O
(
)
(
)
I2 – IB2
The individual component of error is:
0.5055
% error =
• 100% = 1.1%
(12)
0.5
For the case of voltage inputs, the actual transfer function is
∆K = gain accuracy (0.15%, typ), as specified in
specification table.
IB1 = bias current of A1 (5pA, typ)
IB2 = bias current of A2 (5pA, typ)
EOS
V
1
1
– IB1
– IB2
±
±
R1
V2
R1
EOS
VOUT = 0.5V 1± ∆K log
± Nm ± VOSO
(
)
(
)
(13)
N = log conformity error (0.01%, 0.06%, typ)
0.01% for n = 5, 0.06% for n = 7.5
2
R2
R2
VOSO = output offset voltage (3mV, typ)
m = number of decades over which N is specified:
E
EOS2
R2
OS1 and
R1
Where
are considered to be zero for large
values of resistance from external input current sources.
LOG104
10
www.ti.com
SBOS243B
PACKAGE OPTION ADDENDUM
www.ti.com
10-Jun-2004
PACKAGING INFORMATION
ORDERABLE DEVICE
STATUS(1)
PACKAGE TYPE
PACKAGE DRAWING
PINS
PACKAGE QTY
LOG104AID
ACTIVE
ACTIVE
SOIC
SOIC
D
D
8
8
100
LOG104AIDR
2500
(1) The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
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enhancements, improvements, and other changes to its products and services at any time and to discontinue
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TI warrants performance of its hardware products to the specifications applicable at the time of sale in
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Copyright 2004, Texas Instruments Incorporated
相关型号：
LOG104AIDRE4
LOG OR ANTILOG AMPLIFIER, 0.01MHz BAND WIDTH, PDSO8, ROHS COMPLIANT, PLASTIC, MS-012AA, SOIC-8
TI
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