LOG102 Datasheet, PDF (8/15 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIER

10nA to 1mA
Back Bias
+3.3V
+3.3V
1.5kΩ
+5V
1/2 OPA2335
Photodiode
100kΩ
100kΩ
+5V
1/2 OPA2335
1.5kΩ
1.5kΩ
100kΩ
NOTE: OPA2335 Available Q2 2002
100kΩ
10nA to 1mA
Pin 1 or Pin 14
LOG102
FIGURE 8. Precision Current Inverter/Current Source.
V+
6
(–V_RB) ₁
5
I₁
Signal
(–V_RB) ₁₄
LOG102
I₂
REF
9 _CC 10
V_OUT
V–
–V_RB
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.
FIGURE 9. Reverse Biased Photodiode Using Pin 10 on
LOG102.
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 = logλ₁´/ λ₁ (3)
If D₁ and D₂ are matched A ∝ (1V) logI₁/ I₂
(4)
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.
V+
I_{1 1}
6
5
Sample
D₁
λ₁
λ ₁´
LOG102
I_{2 14}
10
Light λ1
Source
D₂
9
C_C
V–
V_OUT
FIGURE 10. Absorbance Measurement.
8
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LOG102
SBOS211A