參數(shù)資料
型號(hào)： LOG114AIRGVT
廠商： TEXAS INSTRUMENTS INC
元件分類： 模擬運(yùn)算功能
英文描述： LOG OR ANTILOG AMPLIFIER, PQCC16
封裝： 4 X 4 MM, GREEN, PLASTIC, QFN-16
文件頁數(shù)： 5/29頁
文件大?。?/td> 637K
代理商： LOG114AIRGVT
LOG114
SBOS301A MAY 2004 REVISED MARCH 2007
www.ti.com
13
DESIGN EXAMPLE FOR DUAL-SUPPLY
CONFIGURATION
Given these conditions:
D V+ = 5V and V = 5V
D 100pA ≤ Input signal
D The stage following the LOG114 is an analog-to-
digital converter (ADC) with +5V supply and
+2.5V reference voltage, so VO4 swings from
+0.5V to +2.5V.
1.
Due to LOG114 symmetry, you can choose either
I1 or I2 as the signal input pin. Choosing I1 as the
reference makes the resistor network around A4
simpler. (Note: Current must flow into pins 3 (I1) and
pin 4 (I2).)
2.
Select the magnitude of the reference current.
Since the signal (I2) spans eight decades, set I1 to
1
A four decades above the minimum I2 value.
(Note that it does not have to be placed in the
middle. If I2 spanned seven decades, I1 could be set
three decades above the minimum and four
decades below the maximum I2 value.) This
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(
VO4 per I2) when the current signal decreases.
3.
Using Equation (1) calculate the expected range of
log outputs at VLOGOUT:
For I
2 + 10mA :
V
LOGOUT + 0.375
log
I
1
I
2
+ 0.375
log
1
mA
10mA
+* 1.5V
For I
2 + 100pA :
V
LOGOUT + 0.375
log
I
1
I
2
+ 0.375
log
1
mA
100pA
+) 1.5V
Therefore, the expected voltage range at the output
of amplifier A3 is:
* 1.5V v V
LOGOUT v) 1.5V
4.
The A4 amplifier scales and offsets the VLOGOUT
signal for use by the ADC using the equation:
V
O4 +*SFACTOR
V
LOGOUT ) VOFFSET
The A4 amplifier is specified with a rated output swing
capability from (V) +0.5V to (V+) 0.5V.
Therefore, choose the final A4 output:
0V
≤ VO4 ≤ +2.5V
This output results in a 2.5V range for the 3V VLOGOUT
range, or 2.5V/3V scaling factor.
5.
When I2 = 10mA, VLOGOUT = 1.5V. Using the
equation in step 5:
V
O4 +*SFACTOR
V
LOGOUT ) VOFFSET
0V
+*2.5V 3V(*1.5V) ) V
OFFSET
Therefore, VOFFSET = 0V
The A4 amplifier configuration for VO4 = 2.5/3(VLOGOUT)
+ 0V is seen in Figure 3.
The overall transer function is:
V
O4 +*0.249
log
I
1
I
2
) 1.5V
A
4
Internal A4 Output Amplifier
R
5
100k
V
REF
+2.5V
V
LOGOUT
V
O4 = 2/3 (VLOGOUT)
I
2
100pA
10mA
A
4 amplifier usedtoscale andoffset VLOGOUT for 0V to 2.5V output.
R
6
82.5k
R
7
100k
R
8
37.4k
V
O4
0V
+2.5V
+5V
5V
Figure 3.
Operational
Amplifier
Configuration
for
Scaling the Output Going to ADC Stage.
(3)
(4)
(5)
(6)
(7)
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