Datasheet

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LOG102
8
SBOS211B
FIGURE 10. Absorbance Measurement.
I
2
I
1
14
1
9
10
5
V
OUT
6
V+
V–
C
C
LOG102
D
2
D
1
Sample
λ
1
λ
1
´
λ
1
Light
Source
APPLICATION CIRCUITS
LOG RATIO
One of the more common uses of log ratio amplifiers is
to measure absorbance. A typical application is shown in
Figure 10.
Absorbance of the sample is A = logλ
1
´/ λ
1
(3)
If D
1
and D
2
are matched A ∝ (1V) logI
1
/I
2
(4)
FIGURE 9. Reverse Biased Photodiode Using Pin 10 on
LOG102.
FIGURE 8. Precision Current Inverter/Current Source.
1.5kΩ
+5V
+5V
1.5kΩ
1.5kΩ
100kΩ
10nA to 1mA
LOG102
10nA to 1mA
100kΩ
100kΩ
+3.3V
+3.3V
1/2 OPA2335
1/2 OPA2335
Photodiode
Back Bias
Pin 1 or Pin 14
100kΩ
I
1
Signal
C
C
(–V
RB
)
I
2
REF
(–V
RB
)
–V
RB
LOG102
6
9
10
14
15
V+
V
OUT
V–
DATA COMPRESSION
In many applications the compressive effects of the logarith-
mic transfer function are useful. For example, a LOG102
preceding a 12-bit Analog-to-Digital (A/D) converter can
produce the dynamic range equivalent to a 20-bit converter.
NOTES: (1) –V
RB
, must be 2.5V more positive than V–. Example, for
V
RB
= –9.5V, V– =12V. (2) Typically, –3.3V bias is used with ±12V supplies.