Datasheet

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LOG101
7
SBOS242B
FIGURE 7. Precision Current Inverter/Current Source.
1.5kΩ
Photodiode
10nA to 1mA
10nA to 1mA
+5V
+5V
1.5kΩ
+3.3V
+3.3V
1/2 OPA2335
1/2 OPA2335
OPA2335
1
2
BSH203
TLV271 or
Back Bias
LOG101
Pin 1 or Pin 8
FIGURE 8. Precision Current Inverter/Current Source.
1.5kΩ
+5V
+5V
1.5kΩ
1.5kΩ
100kΩ
10nA to 1mA
LOG101
10nA to 1mA
100kΩ
100kΩ
+3.3V
+3.3V
1/2
OPA2335
1/2
OPA2335
Photodiode
Back Bias
Pin 1 or Pin 8
100kΩ
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 = logλ
1
´/ λ
1
(3)
If D
1
and D
2
are matched A ∝ (1V) logI
1
/I
2
(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.
FIGURE 9. Absorbance Measurement.
I
2
I
1
8
1
5
6
3
V
OUT
4
V+
V–
C
C
LOG101
D
2
D
1
Sample
λ
1
λ
1
´
λ
1
Light
Source
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.
8
1
3
6
4
5
1
2
35
4
–5V
1µF
LOG101
1µF
I
1
I
2
V
OUT
C
C
1µF
TPS
(1)
Single Supply +5V
NOTE: (1) TPS60402DBV negative charge pump.
FIGURE 10. Single +5V Power-Supply Operation.