LOG104_14 Datasheet, PDF (7/15 Pages) Texas Instruments – LOGARITHMIC AND LOG RATIO AMPLIFIER

VOLTAGE INPUTS
The LOG104 gives the best performance with current inputs.
Voltage inputs may be handled directly with series resistors,
but the dynamic input range is limited to approximately three
decades of input voltage by voltage noise and offsets. The
transfer function of Equation (13) applies to this configuration.
+3.3V⁽¹⁾
+5V
TLV271
or
1
2
OPA2335
1/2 OPA2335
1.5kΩ
1.5kΩ
+5V
10nA to 1mA (Back Bias
+3.3V)
Photodiode
1/2 OPA2335
BSH203
LOG104
10nA to 1mA
Pin 1 or Pin 8
NOTE: (1) +3.3V bias is an arbitrary dc level < 5V that also
appears on the −IN through the op amp where it
applies a reverse bias to the photodiode.
FIGURE 7. Precision Current Inverter/Current 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 9.
Absorbance of the sample is A = logλ₁´/ λ₁
(3)
If D₁ and D₂ are matched A ∝ (0.5V) logI₁/ I₂ (4)
DATA COMPRESSION
In many applications the compressive effects of the logarith-
mic transfer function are useful. For example, a LOG104
preceding a 12-bit A/D converter can produce the dynamic
range equivalent to a 20-bit converter.
V+
I_{1 1}
4
3
Sample
D₁
λ₁
λ ₁´
LOG104
I_{2 8}
6
Light λ1
Source
D₂
5
C_C
V_OUT
V–
FIGURE 9. Absorbance Measurement.
OPERATION ON SINGLE SUPPLY
Many applications do not have the dual supplies required to
operate the LOG104. Figure 10 shows the LOG104 config-
ured for operation with a single +5V supply.
Single Supply +5V
1
I₁
8
I₂
4
3
LOG104
6
5
C_C
V_OUT
1µF
3
5
2
1
TPS⁽¹⁾
1µF
4
–5V
1µF
(1) TPS60402DBV negative charge pump.
FIGURE 10. Single +5V Power-Supply Operation.
10nA to 1mA
Back Bias
+3.3V⁽¹⁾
1.5kΩ
+5V
1/2 OPA2335
100kΩ
100kΩ
+5V
1.5kΩ
Photodiode
1/2 OPA2335
NOTE: (1) +3.3V bias is an arbitrary dc level < 5V that also
appears on the −IN through the op amp where it
applies a reverse bias to the photodiode.
FIGURE 8. Precision Current Inverter/Current Source.
100kΩ
1.5kΩ
100kΩ
10nA to 1mA
Pin 1 or Pin 8
LOG104
LOG104
SBOS243C
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