參數(shù)資料
型號(hào)： LOG114AIRGVR
元件分類： 對(duì)數(shù)放大器
英文描述： Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER
中文描述： 單電源，高速，精密對(duì)數(shù)放大器
文件頁數(shù)： 13/30頁
文件大小： 616K
代理商： LOG114AIRGVR
"#$$%
SBOS301A MAY 2004 REVISED MARCH 2007
www.ti.com
13
DESIGN EXAMPLE FOR DUAL-SUPPLY
CONFIGURATION
Given these conditions:
V+ = 5V and V = 5V
100pA
≤
Input signal
The stage following the LOG114 is an analog-to-
digital converter (ADC) with +5V supply and
+2.5V reference voltage, so V
O4
swings from
+0.5V to +2.5V.
Due to LOG114 symmetry, you can choose either
I
1
or I
2
as the signal input pin. Choosing I
1
as the
reference makes the resistor network around A4
simpler. (Note: Current must flow into pins 3 (I
1
) and
pin 4 (I
2
).)
Select the magnitude of the reference current.
Since the signal (I
2
) spans eight decades, set I
1
to
1
μ
A four decades above the minimum I
2
value.
(Note that it does not have to be placed in the
middle. If I
2
spanned seven decades, I
1
could be set
three decades above the minimum and four
decades below the maximum I
2
value.) This
configuration results in more swing amplitude in the
negative direction, which provides more sensitivity
(
V
O4
per
I
2
) when the current signal decreases.
1.
2.
3.
Using Equation (1) calculate the expected range of
log outputs at V
LOGOUT
:
For I
2
10mA :
V
LOGOUT
0.375
log
I
1
I
2
0.375
log
1 A
10mA
1.5V
For I
2
100pA :
V
LOGOUT
0.375
log
I
1
I
2
0.375
log
1 A
100pA
1.5V
Therefore, the expected voltage range at the output
of amplifier A
3
is:
1.5V
V
LOGOUT
1.5V
4.
The A
4
amplifier scales and offsets the V
LOGOUT
signal for use by the ADC using the equation:
V
O4
S
FACTOR
V
LOGOUT
V
OFFSET
The A
4
amplifier is specified with a rated output swing
capability from (V) +0.5V to (V+) 0.5V.
Therefore, choose the final A
4
output:
0V
≤
V
O4
≤
+2.5V
This output results in a 2.5V range for the 3V V
LOGOUT
range, or 2.5V/3V scaling factor.
5.
When I
2
= 10mA, V
LOGOUT
= 1.5V. Using the
equation in step 5:
V
O4
0V
S
FACTOR
2.5V 3V(
V
LOGOUT
1.5V)
V
OFFSET
V
OFFSET
Therefore, V
OFFSET
= 0V
The A
4
amplifier configuration for V
O4
= 2.5/3(V
LOGOUT
)
+ 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 A4Output Amplifier
R
5
100k
V
+2.5V
V
LOGOUT
V
O4
=
2/3 (V
LOGOUT
)
I
2
100pA
10mA
A
4
amplifier used to scale and offset V
LOGOUT
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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