參數(shù)資料
型號： LOG114AIRGVR
元件分類： 對數(shù)放大器
英文描述： Single-Supply, High-Speed, Precision LOGARITHMIC AMPLIFIER
中文描述： 單電源，高速，精密對數(shù)放大器
文件頁數(shù)： 14/30頁
文件大?。?/td> 616K
代理商： LOG114AIRGVR
"#$$%
SBOS301A MAY 2004 REVISED MARCH 2007
www.ti.com
14
DESIGN EXAMPLE FOR SINGLE-SUPPLY
CONFIGURATION
Given these conditions:
V+ = 5V
V
= GND
100pA
≤
Input signal
≤
10mA
The stage following the LOG114 is an analog to
digital converter (ADC) with +5V supply and
+2.5V reference voltage
Choose either I
1
or I
2
as the signal input pin. For this
example, I
2
is used. Choosing I
1
as the reference
current makes the resistor network around A4
simpler. (Note: Current only flows into the I
1
and I
2
pins.)
1.
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,
and four decades below the maximum 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.
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
This result would be fine in a dualsupply system
(V+ = +5V, V = 5V) where the output can swing
below ground, but does not work in a single supply
+5V system. Therefore, an offset voltage must be
added to the system.
Select an offset voltage, V
Com
to use for centering
the output between (V) + 0.6V and (V+) 0.6V,
which is the full-scale output capability of the A
3
amplifier. Choosing V
Com
= 2.5V, and recalculating
the expected voltage output range for V
LOGOUT
using
Equation (2), results in:
4.
1V
V
LOGOUT
4V
5.
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 A4 output:
+0.5V
≤
V
O4
≤
+2.5V
This output results in a 2V range for the 3V V
LOGOUT
range, or 2V/3V scaling factor.
6.
When I
2
= 10mA, V
LOGOUT
= +1V, and V
O4
= 2.5V.
Using the equation in step 5:
V
O4
2.5V
S
FACTOR
2V 3V(1V)
V
LOGOUT
V
OFFSET
V
OFFSET
Therefore, V
OFFSET
= 3.16V
The A
4
amplifier configuration for V
O4
= 2/3(V
LOGOUT
) +
3.16 is seen in Figure 4a.
The overall transer function is:
V
O4
0.249
log
I
1
I
2
1.5V
A similar process can be used for configuring an
external rail-to-rail output op amp, such as the OPA335.
Because the OPA335 op amp can swing down to 0V
using a pulldown resistor, R
P
, connected to 5V (for
details, refer to the OPA335 data sheet, available for
download at www.ti.com), the scaling factor is 2.5V/3V
and the corresponding V
OFFSET
is 3.3V. This circuit
configuration is shown in Figure 4b.
(8)
(9)
(10)
(11)
(12)
(13)
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