Datasheet

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SBOS301A − MAY 2004 − REVISED MARCH 2007
www.ti.com
18
VOLTAGE INPUTS
The LOG114 provides the best performance with cur-
rent inputs. Voltage inputs may be handled directly by
using a low-impedance voltage source with series resis-
tors, but the dynamic input range is limited to approxi-
mately three decades of input voltage. This limitation
exists because of the magnitude of the required input
voltage and size of the corresponding series resistor.
For 10nA of input current, a 10V voltage source and a
1GΩ resistor would be required. Voltage and current
noise from these sources must be considered and can
limit the usefulness of this technique.
APPLICATION CIRCUITS
LOG RATIO
One of the more common uses of log ratio amplifiers is
to measure absorbance. See Figure 10 for a typical ap-
plication. Absorbance of the sample is A = log λ
1
′/λ
1
. If
D
1
and D
2
are matched, A ∝ (0.375V) log(I
1
/I
2
).
I
2
I
1
D
2
D
1
Sample
λ
1
′
λ
1
λ
1
Light
Source
Q
1
R
1
R
2
R
5
R
6
LOG114
R
3
R
4
V
O4
(2)
−
IN
4
+IN
4
V
LOGOUT
(1)
+IN
5
−
IN
5
V
O5
V
CM IN
I
1
4
1010
9
14
5
3
15
13
12
16
V
REF
2.5V
REF
V
−
V
REF GND
Q
2
A
3
A
1
A
2
A
4
A
5
I
2
V+
1
86 7
Com
+5V
NOTES: (1) V
LOGOUT
=0.375
×
log(I
1
/I
2
).
(2) V
O4
=0.375
×
K
×
log(I
1
/I
2
)
K=1+R
6
/R
5
.
Figure 10. Using the LOG114 to Measure Absorbance.