LOG101AIDE4 Datasheet, PDF (7/16 Pages) Burr-Brown (TI) – Precision LOGARITHMIC AND LOG RATIO AMPLIFIER

+3.3V
+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
LOG101
10nA to 1mA
Pin 1 or Pin 8
FIGURE 7. Precision Current Inverter/Current Source.
VOLTAGE INPUTS
The LOG101 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.
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 ∝ (1V) logI₁/ I₂
(4)
DATA COMPRESSION
In many applications the compressive effects of the logarith-
mic transfer function are useful. For example, a LOG101
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}
4
3
Sample
D₁
λ₁
λ ₁´
LOG101
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 LOG101. Figure 10 shows the LOG101 config-
ured for operation with a single +5V supply.
Single Supply +5V
1
I₁
8
I₂
4
3
LOG101
6
5
C_C
V_OUT
1µF
3
5
2
1
TPS⁽¹⁾
1µF
4
–5V
1µF
NOTE: (1) TPS60402DBV negative charge pump.
FIGURE 10. Single +5V Power-Supply Operation.
10nA to 1mA
Back Bias
+3.3V
+3.3V
Photodiode
1.5kΩ
+5V
1/2
OPA2335
100kΩ
100kΩ
+5V
1/2
OPA2335
1.5kΩ
1.5kΩ
FIGURE 8. Precision Current Inverter/Current Source.
100kΩ
100kΩ
10nA to 1mA
Pin 1 or Pin 8
LOG101
LOG101
SBOS242B
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